diff --git a/.php-cs-fixer.php b/.php-cs-fixer.php
index a5f4be066..e2c430166 100644
--- a/.php-cs-fixer.php
+++ b/.php-cs-fixer.php
@@ -71,7 +71,6 @@ return $config->setRules([
         'random_api_migration' => true,
         'ternary_to_null_coalescing' => true,
         //'declare_strict_types' => true,
-        //'void_return' => true,
     ])
     ->setUsingCache(true)
     ->setRiskyAllowed(true)
diff --git a/phpstan/config.neon b/phpstan/config.neon
index 210f3f75d..e5469b6a7 100644
--- a/phpstan/config.neon
+++ b/phpstan/config.neon
@@ -31,15 +31,11 @@ parameters:
         - '~^Undefined variable: \$vendorDir$~'
         - '~^Undefined variable: \$baseDir$~'
 
-        # variable defined in eval
-        - '~^Undefined variable: \$res$~'
-
         # we don't have different constructors for parent/child
         - '~^Unsafe usage of new static\(\)\.$~'
 
         # Ignore some irrelevant errors in test files
-        - '~Method Composer\\Test\\[^:]+::(setUp(BeforeClass)?|tearDown(AfterClass)?|test[^(]+)\(\) has no return type specified.~'
-        - '~Method Composer\\Test\\[^:]+::(data\w+|provide\w+|\w+?Provider)\(\) has no return type specified.~'
+        - '~Method Composer\\Test\\[^:]+::(data\w+|provide\w+|\w+?Provider)\(\) (has no return type specified.|return type has no value type specified in iterable type array.)~'
 
         # PHPUnit assertions as instance methods
         - '~Dynamic call to static method PHPUnit\\Framework\\Assert::\w+\(\)~'
diff --git a/src/Composer/Autoload/AutoloadGenerator.php b/src/Composer/Autoload/AutoloadGenerator.php
index 73ec3d5e7..d3560bfcf 100644
--- a/src/Composer/Autoload/AutoloadGenerator.php
+++ b/src/Composer/Autoload/AutoloadGenerator.php
@@ -290,7 +290,7 @@ EOF;
         $mainAutoload = $rootPackage->getAutoload();
         if ($rootPackage->getTargetDir() && !empty($mainAutoload['psr-0'])) {
             $levels = substr_count($filesystem->normalizePath($rootPackage->getTargetDir()), '/') + 1;
-            $prefixes = implode(', ', array_map(function ($prefix) {
+            $prefixes = implode(', ', array_map(function ($prefix): string {
                 return var_export($prefix, true);
             }, array_keys($mainAutoload['psr-0'])));
             $baseDirFromTargetDirCode = $filesystem->findShortestPathCode($targetDir, $basePath, true);
@@ -441,7 +441,7 @@ EOF;
      * @param array<string, true> $scannedFiles
      * @return array<class-string, string>
      */
-    private function addClassMapCode(Filesystem $filesystem, $basePath, $vendorPath, $dir, $excluded, $namespaceFilter, $autoloadType, array $classMap, array &$ambiguousClasses, array &$scannedFiles)
+    private function addClassMapCode(Filesystem $filesystem, $basePath, $vendorPath, $dir, $excluded, $namespaceFilter, $autoloadType, array $classMap, array &$ambiguousClasses, array &$scannedFiles): array
     {
         foreach ($this->generateClassMap($dir, $excluded, $namespaceFilter, $autoloadType, true, $scannedFiles) as $class => $path) {
             $pathCode = $this->getPathCode($filesystem, $basePath, $vendorPath, $path).",\n";
@@ -464,7 +464,7 @@ EOF;
      * @param array<string, true> $scannedFiles
      * @return array<class-string, string>
      */
-    private function generateClassMap($dir, $excluded, $namespaceFilter, $autoloadType, $showAmbiguousWarning, array &$scannedFiles)
+    private function generateClassMap($dir, $excluded, $namespaceFilter, $autoloadType, $showAmbiguousWarning, array &$scannedFiles): array
     {
         if ($excluded) {
             // filter excluded patterns here to only use those matching $dir
@@ -554,7 +554,7 @@ EOF;
     {
         $rootPackageMap = array_shift($packageMap);
         if (is_array($filteredDevPackages)) {
-            $packageMap = array_filter($packageMap, function ($item) use ($filteredDevPackages) {
+            $packageMap = array_filter($packageMap, function ($item) use ($filteredDevPackages): bool {
                 return !in_array($item[0]->getName(), $filteredDevPackages, true);
             });
         } elseif ($filteredDevPackages) {
@@ -808,7 +808,7 @@ EOF;
 
         ksort($requiredExtensions);
 
-        $formatToPhpVersionId = function (Bound $bound) {
+        $formatToPhpVersionId = function (Bound $bound): int {
             if ($bound->isZero()) {
                 return 0;
             }
@@ -1229,7 +1229,7 @@ INITIALIZER;
                         $updir = null;
                         $path = Preg::replaceCallback(
                             '{^((?:(?:\\\\\\.){1,2}+/)+)}',
-                            function ($matches) use (&$updir) {
+                            function ($matches) use (&$updir): string {
                                 if (isset($matches[1])) {
                                     // undo preg_quote for the matched string
                                     $updir = str_replace('\\.', '.', $matches[1]);
@@ -1300,7 +1300,7 @@ INITIALIZER;
             }
         }
 
-        $add = function (PackageInterface $package) use (&$add, $packages, &$include, $replacedBy) {
+        $add = function (PackageInterface $package) use (&$add, $packages, &$include, $replacedBy): void {
             foreach ($package->getRequires() as $link) {
                 $target = $link->getTarget();
                 if (isset($replacedBy[$target])) {
@@ -1318,7 +1318,7 @@ INITIALIZER;
 
         return array_filter(
             $packageMap,
-            function ($item) use ($include) {
+            function ($item) use ($include): bool {
                 $package = $item[0];
                 foreach ($package->getNames() as $name) {
                     if (isset($include[$name])) {
diff --git a/src/Composer/Autoload/ClassMapGenerator.php b/src/Composer/Autoload/ClassMapGenerator.php
index cb948945f..8a2db5681 100644
--- a/src/Composer/Autoload/ClassMapGenerator.php
+++ b/src/Composer/Autoload/ClassMapGenerator.php
@@ -38,7 +38,7 @@ class ClassMapGenerator
      * @param string                     $file The name of the class map file
      * @return void
      */
-    public static function dump($dirs, $file)
+    public static function dump($dirs, $file): void
     {
         $maps = array();
 
@@ -61,7 +61,7 @@ class ClassMapGenerator
      * @return array<class-string, string> A class map array
      * @throws \RuntimeException When the path is neither an existing file nor directory
      */
-    public static function createMap($path, $excluded = null, IOInterface $io = null, $namespace = null, $autoloadType = null, &$scannedFiles = array())
+    public static function createMap($path, $excluded = null, IOInterface $io = null, $namespace = null, $autoloadType = null, &$scannedFiles = array()): array
     {
         $basePath = $path;
         if (is_string($path)) {
@@ -157,7 +157,7 @@ class ClassMapGenerator
      * @param  ?IOInterface             $io            IO object
      * @return array<int, class-string> valid classes
      */
-    private static function filterByNamespace($classes, $filePath, $baseNamespace, $namespaceType, $basePath, $io)
+    private static function filterByNamespace($classes, $filePath, $baseNamespace, $namespaceType, $basePath, $io): array
     {
         $validClasses = array();
         $rejectedClasses = array();
@@ -215,7 +215,7 @@ class ClassMapGenerator
      * @throws \RuntimeException
      * @return array<int, class-string> The found classes
      */
-    private static function findClasses($path)
+    private static function findClasses($path): array
     {
         $extraTypes = self::getExtraTypes();
 
@@ -296,7 +296,7 @@ class ClassMapGenerator
     /**
      * @return string
      */
-    private static function getExtraTypes()
+    private static function getExtraTypes(): string
     {
         static $extraTypes = null;
 
diff --git a/src/Composer/Autoload/PhpFileCleaner.php b/src/Composer/Autoload/PhpFileCleaner.php
index 7941f463c..8fc077418 100644
--- a/src/Composer/Autoload/PhpFileCleaner.php
+++ b/src/Composer/Autoload/PhpFileCleaner.php
@@ -51,7 +51,7 @@ class PhpFileCleaner
      * @param string[] $types
      * @return void
      */
-    public static function setTypeConfig($types)
+    public static function setTypeConfig($types): void
     {
         foreach ($types as $type) {
             self::$typeConfig[$type[0]] = array(
@@ -78,7 +78,7 @@ class PhpFileCleaner
     /**
      * @return string
      */
-    public function clean()
+    public function clean(): string
     {
         $clean = '';
 
@@ -152,7 +152,7 @@ class PhpFileCleaner
     /**
      * @return void
      */
-    private function skipToPhp()
+    private function skipToPhp(): void
     {
         while ($this->index < $this->len) {
             if ($this->contents[$this->index] === '<' && $this->peek('?')) {
@@ -168,7 +168,7 @@ class PhpFileCleaner
      * @param string $delimiter
      * @return void
      */
-    private function skipString($delimiter)
+    private function skipString($delimiter): void
     {
         $this->index += 1;
         while ($this->index < $this->len) {
@@ -187,7 +187,7 @@ class PhpFileCleaner
     /**
      * @return void
      */
-    private function skipComment()
+    private function skipComment(): void
     {
         $this->index += 2;
         while ($this->index < $this->len) {
@@ -203,7 +203,7 @@ class PhpFileCleaner
     /**
      * @return void
      */
-    private function skipToNewline()
+    private function skipToNewline(): void
     {
         while ($this->index < $this->len) {
             if ($this->contents[$this->index] === "\r" || $this->contents[$this->index] === "\n") {
@@ -217,7 +217,7 @@ class PhpFileCleaner
      * @param string $delimiter
      * @return void
      */
-    private function skipHeredoc($delimiter)
+    private function skipHeredoc($delimiter): void
     {
         $firstDelimiterChar = $delimiter[0];
         $delimiterLength = \strlen($delimiter);
@@ -260,7 +260,7 @@ class PhpFileCleaner
      * @param string $char
      * @return bool
      */
-    private function peek($char)
+    private function peek($char): bool
     {
         return $this->index + 1 < $this->len && $this->contents[$this->index + 1] === $char;
     }
@@ -270,7 +270,7 @@ class PhpFileCleaner
      * @param ?array<int, string> $match
      * @return bool
      */
-    private function match($regex, array &$match = null)
+    private function match($regex, array &$match = null): bool
     {
         return Preg::isMatch($regex, $this->contents, $match, 0, $this->index);
     }
diff --git a/src/Composer/Command/AboutCommand.php b/src/Composer/Command/AboutCommand.php
index 18b0db279..80d9a9caa 100644
--- a/src/Composer/Command/AboutCommand.php
+++ b/src/Composer/Command/AboutCommand.php
@@ -24,7 +24,7 @@ class AboutCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('about')
diff --git a/src/Composer/Command/ArchiveCommand.php b/src/Composer/Command/ArchiveCommand.php
index beaa65a77..b785f4ea4 100644
--- a/src/Composer/Command/ArchiveCommand.php
+++ b/src/Composer/Command/ArchiveCommand.php
@@ -41,7 +41,7 @@ class ArchiveCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('archive')
@@ -168,7 +168,7 @@ EOT
         if (count($packages) > 1) {
             $package = reset($packages);
             $io->writeError('<info>Found multiple matches, selected '.$package->getPrettyString().'.</info>');
-            $io->writeError('Alternatives were '.implode(', ', array_map(function ($p) {
+            $io->writeError('Alternatives were '.implode(', ', array_map(function ($p): string {
                 return $p->getPrettyString();
             }, $packages)).'.');
             $io->writeError('<comment>Please use a more specific constraint to pick a different package.</comment>');
diff --git a/src/Composer/Command/BaseDependencyCommand.php b/src/Composer/Command/BaseDependencyCommand.php
index 4b24202ed..224d2739e 100644
--- a/src/Composer/Command/BaseDependencyCommand.php
+++ b/src/Composer/Command/BaseDependencyCommand.php
@@ -50,7 +50,7 @@ class BaseDependencyCommand extends BaseCommand
      * @param  bool            $inverted Whether to invert matching process (why-not vs why behaviour)
      * @return int             Exit code of the operation.
      */
-    protected function doExecute(InputInterface $input, OutputInterface $output, $inverted = false)
+    protected function doExecute(InputInterface $input, OutputInterface $output, $inverted = false): int
     {
         // Emit command event on startup
         $composer = $this->requireComposer();
@@ -90,7 +90,7 @@ class BaseDependencyCommand extends BaseCommand
         $needles = array($needle);
         if ($inverted) {
             foreach ($packages as $package) {
-                $needles = array_merge($needles, array_map(function (Link $link) {
+                $needles = array_merge($needles, array_map(function (Link $link): string {
                     return $link->getTarget();
                 }, $package->getReplaces()));
             }
@@ -136,7 +136,7 @@ class BaseDependencyCommand extends BaseCommand
      *
      * @return void
      */
-    protected function printTable(OutputInterface $output, $results)
+    protected function printTable(OutputInterface $output, $results): void
     {
         $table = array();
         $doubles = array();
@@ -172,7 +172,7 @@ class BaseDependencyCommand extends BaseCommand
      *
      * @return void
      */
-    protected function initStyles(OutputInterface $output)
+    protected function initStyles(OutputInterface $output): void
     {
         $this->colors = array(
             'green',
@@ -197,7 +197,7 @@ class BaseDependencyCommand extends BaseCommand
      *
      * @return void
      */
-    protected function printTree($results, $prefix = '', $level = 1)
+    protected function printTree($results, $prefix = '', $level = 1): void
     {
         $count = count($results);
         $idx = 0;
@@ -223,7 +223,7 @@ class BaseDependencyCommand extends BaseCommand
      *
      * @return void
      */
-    private function writeTreeLine($line)
+    private function writeTreeLine($line): void
     {
         $io = $this->getIO();
         if (!$io->isDecorated()) {
diff --git a/src/Composer/Command/CheckPlatformReqsCommand.php b/src/Composer/Command/CheckPlatformReqsCommand.php
index d86dec5da..8e525e7bf 100644
--- a/src/Composer/Command/CheckPlatformReqsCommand.php
+++ b/src/Composer/Command/CheckPlatformReqsCommand.php
@@ -26,7 +26,7 @@ class CheckPlatformReqsCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this->setName('check-platform-reqs')
             ->setDescription('Check that platform requirements are satisfied.')
@@ -174,7 +174,7 @@ EOT
      *
      * @return void
      */
-    protected function printTable(OutputInterface $output, $results)
+    protected function printTable(OutputInterface $output, $results): void
     {
         $rows = array();
         foreach ($results as $result) {
diff --git a/src/Composer/Command/ClearCacheCommand.php b/src/Composer/Command/ClearCacheCommand.php
index 015a49b6f..e7c15de31 100644
--- a/src/Composer/Command/ClearCacheCommand.php
+++ b/src/Composer/Command/ClearCacheCommand.php
@@ -25,7 +25,7 @@ class ClearCacheCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('clear-cache')
diff --git a/src/Composer/Command/ConfigCommand.php b/src/Composer/Command/ConfigCommand.php
index 8b67042fe..3e51a4599 100644
--- a/src/Composer/Command/ConfigCommand.php
+++ b/src/Composer/Command/ConfigCommand.php
@@ -62,7 +62,7 @@ class ConfigCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('config')
@@ -154,7 +154,7 @@ EOT
      * @return void
      * @throws \Exception
      */
-    protected function initialize(InputInterface $input, OutputInterface $output)
+    protected function initialize(InputInterface $input, OutputInterface $output): void
     {
         parent::initialize($input, $output);
 
@@ -318,10 +318,10 @@ EOT
 
         $values = $input->getArgument('setting-value'); // what the user is trying to add/change
 
-        $booleanValidator = function ($val) {
+        $booleanValidator = function ($val): bool {
             return in_array($val, array('true', 'false', '1', '0'), true);
         };
-        $booleanNormalizer = function ($val) {
+        $booleanNormalizer = function ($val): bool {
             return $val !== 'false' && (bool) $val;
         };
 
@@ -331,7 +331,7 @@ EOT
             'use-include-path' => array($booleanValidator, $booleanNormalizer),
             'use-github-api' => array($booleanValidator, $booleanNormalizer),
             'preferred-install' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('auto', 'source', 'dist'), true);
                 },
                 function ($val) {
@@ -339,7 +339,7 @@ EOT
                 },
             ),
             'gitlab-protocol' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('git', 'http', 'https'), true);
                 },
                 function ($val) {
@@ -347,7 +347,7 @@ EOT
                 },
             ),
             'store-auths' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('true', 'false', 'prompt'), true);
                 },
                 function ($val) {
@@ -389,7 +389,7 @@ EOT
             'cache-ttl' => array('is_numeric', 'intval'),
             'cache-files-ttl' => array('is_numeric', 'intval'),
             'cache-files-maxsize' => array(
-                function ($val) {
+                function ($val): bool {
                     return Preg::isMatch('/^\s*([0-9.]+)\s*(?:([kmg])(?:i?b)?)?\s*$/i', $val);
                 },
                 function ($val) {
@@ -397,7 +397,7 @@ EOT
                 },
             ),
             'bin-compat' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('auto', 'full', 'symlink'));
                 },
                 function ($val) {
@@ -405,7 +405,7 @@ EOT
                 },
             ),
             'discard-changes' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('stash', 'true', 'false', '1', '0'), true);
                 },
                 function ($val) {
@@ -427,7 +427,7 @@ EOT
             'disable-tls' => array($booleanValidator, $booleanNormalizer),
             'secure-http' => array($booleanValidator, $booleanNormalizer),
             'cafile' => array(
-                function ($val) {
+                function ($val): bool {
                     return file_exists($val) && Filesystem::isReadable($val);
                 },
                 function ($val) {
@@ -435,7 +435,7 @@ EOT
                 },
             ),
             'capath' => array(
-                function ($val) {
+                function ($val): bool {
                     return is_dir($val) && Filesystem::isReadable($val);
                 },
                 function ($val) {
@@ -447,7 +447,7 @@ EOT
             'lock' => array($booleanValidator, $booleanNormalizer),
             'allow-plugins' => array($booleanValidator, $booleanNormalizer),
             'platform-check' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('php-only', 'true', 'false', '1', '0'), true);
                 },
                 function ($val) {
@@ -459,7 +459,7 @@ EOT
                 },
             ),
             'use-parent-dir' => array(
-                function ($val) {
+                function ($val): bool {
                     return in_array($val, array('true', 'false', 'prompt'), true);
                 },
                 function ($val) {
@@ -593,10 +593,10 @@ EOT
                 return $val;
             }),
             'minimum-stability' => array(
-                function ($val) {
+                function ($val): bool {
                     return isset(BasePackage::$stabilities[VersionParser::normalizeStability($val)]);
                 },
-                function ($val) {
+                function ($val): string {
                     return VersionParser::normalizeStability($val);
                 },
             ),
@@ -810,7 +810,7 @@ EOT
      *
      * @return void
      */
-    protected function handleSingleValue($key, array $callbacks, array $values, $method)
+    protected function handleSingleValue($key, array $callbacks, array $values, $method): void
     {
         list($validator, $normalizer) = $callbacks;
         if (1 !== count($values)) {
@@ -845,7 +845,7 @@ EOT
      *
      * @return void
      */
-    protected function handleMultiValue($key, array $callbacks, array $values, $method)
+    protected function handleMultiValue($key, array $callbacks, array $values, $method): void
     {
         list($validator, $normalizer) = $callbacks;
         if (true !== $validation = $validator($values)) {
@@ -868,7 +868,7 @@ EOT
      *
      * @return void
      */
-    protected function listConfiguration(array $contents, array $rawContents, OutputInterface $output, $k = null, $showSource = false)
+    protected function listConfiguration(array $contents, array $rawContents, OutputInterface $output, $k = null, $showSource = false): void
     {
         $origK = $k;
         $io = $this->getIO();
diff --git a/src/Composer/Command/CreateProjectCommand.php b/src/Composer/Command/CreateProjectCommand.php
index 5a270a81c..d7dcaeaae 100644
--- a/src/Composer/Command/CreateProjectCommand.php
+++ b/src/Composer/Command/CreateProjectCommand.php
@@ -63,7 +63,7 @@ class CreateProjectCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('create-project')
@@ -187,7 +187,7 @@ EOT
      * @return int
      * @throws \Exception
      */
-    public function installProject(IOInterface $io, Config $config, InputInterface $input, $packageName = null, $directory = null, $packageVersion = null, $stability = 'stable', $preferSource = false, $preferDist = false, $installDevPackages = false, $repositories = null, $disablePlugins = false, $disableScripts = false, $noProgress = false, $noInstall = false, PlatformRequirementFilterInterface $platformRequirementFilter = null, $secureHttp = true, $addRepository = false)
+    public function installProject(IOInterface $io, Config $config, InputInterface $input, $packageName = null, $directory = null, $packageVersion = null, $stability = 'stable', $preferSource = false, $preferDist = false, $installDevPackages = false, $repositories = null, $disablePlugins = false, $disableScripts = false, $noProgress = false, $noInstall = false, PlatformRequirementFilterInterface $platformRequirementFilter = null, $secureHttp = true, $addRepository = false): int
     {
         $oldCwd = getcwd();
 
@@ -350,7 +350,7 @@ EOT
      * @return bool
      * @throws \Exception
      */
-    protected function installRootPackage(IOInterface $io, Config $config, $packageName, PlatformRequirementFilterInterface $platformRequirementFilter, $directory = null, $packageVersion = null, $stability = 'stable', $preferSource = false, $preferDist = false, $installDevPackages = false, array $repositories = null, $disablePlugins = false, $disableScripts = false, $noProgress = false, $secureHttp = true)
+    protected function installRootPackage(IOInterface $io, Config $config, $packageName, PlatformRequirementFilterInterface $platformRequirementFilter, $directory = null, $packageVersion = null, $stability = 'stable', $preferSource = false, $preferDist = false, $installDevPackages = false, array $repositories = null, $disablePlugins = false, $disableScripts = false, $noProgress = false, $secureHttp = true): bool
     {
         if (!$secureHttp) {
             $config->merge(array('config' => array('secure-http' => false)), Config::SOURCE_COMMAND);
@@ -443,7 +443,7 @@ EOT
             @mkdir($directory, 0777, true);
             if ($realDir = realpath($directory)) {
                 pcntl_async_signals(true);
-                pcntl_signal(SIGINT, function () use ($realDir) {
+                pcntl_signal(SIGINT, function () use ($realDir): void {
                     $fs = new Filesystem();
                     $fs->removeDirectory($realDir);
                     exit(130);
@@ -454,7 +454,7 @@ EOT
         if (function_exists('sapi_windows_set_ctrl_handler') && PHP_SAPI === 'cli') {
             @mkdir($directory, 0777, true);
             if ($realDir = realpath($directory)) {
-                sapi_windows_set_ctrl_handler(function () use ($realDir) {
+                sapi_windows_set_ctrl_handler(function () use ($realDir): void {
                     $fs = new Filesystem();
                     $fs->removeDirectory($realDir);
                     exit(130);
diff --git a/src/Composer/Command/DependsCommand.php b/src/Composer/Command/DependsCommand.php
index dfd8d716b..54fda17a5 100644
--- a/src/Composer/Command/DependsCommand.php
+++ b/src/Composer/Command/DependsCommand.php
@@ -27,7 +27,7 @@ class DependsCommand extends BaseDependencyCommand
      *
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('depends')
diff --git a/src/Composer/Command/DiagnoseCommand.php b/src/Composer/Command/DiagnoseCommand.php
index a39ffd541..75bec936a 100644
--- a/src/Composer/Command/DiagnoseCommand.php
+++ b/src/Composer/Command/DiagnoseCommand.php
@@ -260,7 +260,8 @@ EOT
         try {
             $this->httpDownloader->get($proto . '://repo.packagist.org/packages.json');
         } catch (TransportException $e) {
-            if ($hints = HttpDownloader::getExceptionHints($e)) {
+            $hints = HttpDownloader::getExceptionHints($e);
+            if (null !== $hints && count($hints) > 0) {
                 foreach ($hints as $hint) {
                     $result[] = $hint;
                 }
@@ -436,7 +437,7 @@ EOT
     /**
      * @return string
      */
-    private function getCurlVersion()
+    private function getCurlVersion(): string
     {
         if (extension_loaded('curl')) {
             if (!HttpDownloader::isCurlEnabled()) {
@@ -458,7 +459,7 @@ EOT
      *
      * @return void
      */
-    private function outputResult($result)
+    private function outputResult($result): void
     {
         $io = $this->getIO();
         if (true === $result) {
@@ -510,7 +511,7 @@ EOT
     private function checkPlatform()
     {
         $output = '';
-        $out = function ($msg, $style) use (&$output) {
+        $out = function ($msg, $style) use (&$output): void {
             $output .= '<'.$style.'>'.$msg.'</'.$style.'>'.PHP_EOL;
         };
 
diff --git a/src/Composer/Command/FundCommand.php b/src/Composer/Command/FundCommand.php
index 6e551a66a..c6287dac9 100644
--- a/src/Composer/Command/FundCommand.php
+++ b/src/Composer/Command/FundCommand.php
@@ -33,7 +33,7 @@ class FundCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this->setName('fund')
             ->setDescription('Discover how to help fund the maintenance of your dependencies.')
@@ -134,7 +134,7 @@ class FundCommand extends BaseCommand
      * @param mixed[] $fundings
      * @return mixed[]
      */
-    private function insertFundingData(array $fundings, CompletePackageInterface $package)
+    private function insertFundingData(array $fundings, CompletePackageInterface $package): array
     {
         foreach ($package->getFunding() as $fundingOption) {
             list($vendor, $packageName) = explode('/', $package->getPrettyName());
diff --git a/src/Composer/Command/GlobalCommand.php b/src/Composer/Command/GlobalCommand.php
index 91aae03f0..0b5f442cc 100644
--- a/src/Composer/Command/GlobalCommand.php
+++ b/src/Composer/Command/GlobalCommand.php
@@ -29,7 +29,7 @@ class GlobalCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('global')
@@ -121,7 +121,7 @@ EOT
     /**
      * @inheritDoc
      */
-    public function isProxyCommand()
+    public function isProxyCommand(): bool
     {
         return true;
     }
diff --git a/src/Composer/Command/HomeCommand.php b/src/Composer/Command/HomeCommand.php
index a44546c55..b83f31e03 100644
--- a/src/Composer/Command/HomeCommand.php
+++ b/src/Composer/Command/HomeCommand.php
@@ -33,7 +33,7 @@ class HomeCommand extends BaseCommand
      *
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('browse')
@@ -101,7 +101,7 @@ EOT
      * @param bool $showOnly
      * @return bool
      */
-    private function handlePackage(CompletePackageInterface $package, $showHomepage, $showOnly)
+    private function handlePackage(CompletePackageInterface $package, $showHomepage, $showOnly): bool
     {
         $support = $package->getSupport();
         $url = $support['source'] ?? $package->getSourceUrl();
@@ -128,7 +128,7 @@ EOT
      * @param string $url
      * @return void
      */
-    private function openBrowser($url)
+    private function openBrowser($url): void
     {
         $url = ProcessExecutor::escape($url);
 
@@ -158,7 +158,7 @@ EOT
      *
      * @return RepositoryInterface[]
      */
-    private function initializeRepos()
+    private function initializeRepos(): array
     {
         $composer = $this->tryComposer();
 
diff --git a/src/Composer/Command/InitCommand.php b/src/Composer/Command/InitCommand.php
index 6869d4068..8b67c26ab 100644
--- a/src/Composer/Command/InitCommand.php
+++ b/src/Composer/Command/InitCommand.php
@@ -511,7 +511,7 @@ EOT
         }
 
         $namespace = array_map(
-            function ($part) {
+            function ($part): string {
                 $part = Preg::replace('/[^a-z0-9]/i', ' ', $part);
                 $part = ucwords($part);
 
@@ -623,7 +623,7 @@ EOT
     /**
      * @return void
      */
-    private function updateDependencies(OutputInterface $output)
+    private function updateDependencies(OutputInterface $output): void
     {
         try {
             $updateCommand = $this->getApplication()->find('update');
@@ -637,7 +637,7 @@ EOT
     /**
      * @return void
      */
-    private function runDumpAutoloadCommand(OutputInterface $output)
+    private function runDumpAutoloadCommand(OutputInterface $output): void
     {
         try {
             $command = $this->getApplication()->find('dump-autoload');
@@ -652,7 +652,7 @@ EOT
      * @param array<string, string|array<string>> $options
      * @return bool
      */
-    private function hasDependencies($options)
+    private function hasDependencies($options): bool
     {
         $requires = (array) $options['require'];
         $devRequires = isset($options['require-dev']) ? (array) $options['require-dev'] : array();
diff --git a/src/Composer/Command/InstallCommand.php b/src/Composer/Command/InstallCommand.php
index b992758bb..df1edf062 100644
--- a/src/Composer/Command/InstallCommand.php
+++ b/src/Composer/Command/InstallCommand.php
@@ -98,7 +98,7 @@ EOT
 
         $composer = $this->requireComposer();
 
-        if ((!$composer->getLocker() || !$composer->getLocker()->isLocked()) && !HttpDownloader::isCurlEnabled()) {
+        if (!$composer->getLocker()->isLocked() && !HttpDownloader::isCurlEnabled()) {
             $io->writeError('<warning>Composer is operating significantly slower than normal because you do not have the PHP curl extension enabled.</warning>');
         }
 
diff --git a/src/Composer/Command/LicensesCommand.php b/src/Composer/Command/LicensesCommand.php
index 5a9bfa16b..f59034282 100644
--- a/src/Composer/Command/LicensesCommand.php
+++ b/src/Composer/Command/LicensesCommand.php
@@ -34,7 +34,7 @@ class LicensesCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('licenses')
@@ -163,14 +163,14 @@ EOT
      * @param  array<string, PackageInterface> $bucket
      * @return array<string, PackageInterface>
      */
-    private function filterRequiredPackages(RepositoryInterface $repo, PackageInterface $package, $bucket = array())
+    private function filterRequiredPackages(RepositoryInterface $repo, PackageInterface $package, $bucket = array()): array
     {
         $requires = array_keys($package->getRequires());
 
         $packageListNames = array_keys($bucket);
         $packages = array_filter(
             $repo->getPackages(),
-            function ($package) use ($requires, $packageListNames) {
+            function ($package) use ($requires, $packageListNames): bool {
                 return in_array($package->getName(), $requires) && !in_array($package->getName(), $packageListNames);
             }
         );
@@ -191,7 +191,7 @@ EOT
      * @param  array<string, PackageInterface> $bucket   the list to add packages to
      * @return array<string, PackageInterface>
      */
-    public function appendPackages(array $packages, array $bucket)
+    public function appendPackages(array $packages, array $bucket): array
     {
         foreach ($packages as $package) {
             $bucket[$package->getName()] = $package;
diff --git a/src/Composer/Command/OutdatedCommand.php b/src/Composer/Command/OutdatedCommand.php
index bdf60f664..332a4c64e 100644
--- a/src/Composer/Command/OutdatedCommand.php
+++ b/src/Composer/Command/OutdatedCommand.php
@@ -26,7 +26,7 @@ class OutdatedCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('outdated')
@@ -105,7 +105,7 @@ EOT
     /**
      * @inheritDoc
      */
-    public function isProxyCommand()
+    public function isProxyCommand(): bool
     {
         return true;
     }
diff --git a/src/Composer/Command/PackageDiscoveryTrait.php b/src/Composer/Command/PackageDiscoveryTrait.php
index c55e97612..a9685ae13 100644
--- a/src/Composer/Command/PackageDiscoveryTrait.php
+++ b/src/Composer/Command/PackageDiscoveryTrait.php
@@ -42,7 +42,7 @@ trait PackageDiscoveryTrait
     /**
      * @return CompositeRepository
      */
-    protected function getRepos()
+    protected function getRepos(): CompositeRepository
     {
         if (null === $this->repos) {
             $this->repos = new CompositeRepository(array_merge(
diff --git a/src/Composer/Command/ProhibitsCommand.php b/src/Composer/Command/ProhibitsCommand.php
index 304b116e2..59165f4de 100644
--- a/src/Composer/Command/ProhibitsCommand.php
+++ b/src/Composer/Command/ProhibitsCommand.php
@@ -27,7 +27,7 @@ class ProhibitsCommand extends BaseDependencyCommand
      *
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('prohibits')
diff --git a/src/Composer/Command/ReinstallCommand.php b/src/Composer/Command/ReinstallCommand.php
index 7b8a00873..926ba9839 100644
--- a/src/Composer/Command/ReinstallCommand.php
+++ b/src/Composer/Command/ReinstallCommand.php
@@ -35,7 +35,7 @@ class ReinstallCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('reinstall')
@@ -123,7 +123,7 @@ EOT
                 $installOrder[$op->getPackage()->getName()] = $index;
             }
         }
-        usort($uninstallOperations, function ($a, $b) use ($installOrder) {
+        usort($uninstallOperations, function ($a, $b) use ($installOrder): int {
             return $installOrder[$b->getPackage()->getName()] - $installOrder[$a->getPackage()->getName()];
         });
 
diff --git a/src/Composer/Command/RemoveCommand.php b/src/Composer/Command/RemoveCommand.php
index e33675224..fdda37df1 100644
--- a/src/Composer/Command/RemoveCommand.php
+++ b/src/Composer/Command/RemoveCommand.php
@@ -116,7 +116,7 @@ EOT
 
             if (count($input->getArgument('packages')) === 0) {
                 $this->getIO()->writeError('<info>No unused packages to remove</info>');
-                $this->setCode(function () {
+                $this->setCode(function (): int {
                     return 0;
                 });
             }
diff --git a/src/Composer/Command/RequireCommand.php b/src/Composer/Command/RequireCommand.php
index c46a96518..5ad66a794 100644
--- a/src/Composer/Command/RequireCommand.php
+++ b/src/Composer/Command/RequireCommand.php
@@ -296,7 +296,7 @@ EOT
      * @param string $requireKey
      * @return string[]
      */
-    private function getInconsistentRequireKeys(array $newRequirements, $requireKey)
+    private function getInconsistentRequireKeys(array $newRequirements, $requireKey): array
     {
         $requireKeys = $this->getPackagesByRequireKey();
         $inconsistentRequirements = array();
@@ -315,7 +315,7 @@ EOT
     /**
      * @return array<string, string>
      */
-    private function getPackagesByRequireKey()
+    private function getPackagesByRequireKey(): array
     {
         $composerDefinition = $this->json->read();
         $require = array();
@@ -351,7 +351,7 @@ EOT
      * @return int
      * @throws \Exception
      */
-    private function doUpdate(InputInterface $input, OutputInterface $output, IOInterface $io, array $requirements, $requireKey, $removeKey)
+    private function doUpdate(InputInterface $input, OutputInterface $output, IOInterface $io, array $requirements, $requireKey, $removeKey): int
     {
         // Update packages
         $this->resetComposer();
@@ -449,7 +449,7 @@ EOT
      * @param bool $sortPackages
      * @return bool
      */
-    private function updateFileCleanly(JsonFile $json, array $new, $requireKey, $removeKey, $sortPackages)
+    private function updateFileCleanly(JsonFile $json, array $new, $requireKey, $removeKey, $sortPackages): bool
     {
         $contents = file_get_contents($json->getPath());
 
diff --git a/src/Composer/Command/RunScriptCommand.php b/src/Composer/Command/RunScriptCommand.php
index f35688c15..fc66a9d04 100644
--- a/src/Composer/Command/RunScriptCommand.php
+++ b/src/Composer/Command/RunScriptCommand.php
@@ -47,7 +47,7 @@ class RunScriptCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('run-script')
@@ -112,7 +112,7 @@ EOT
     /**
      * @return int
      */
-    protected function listScripts(OutputInterface $output)
+    protected function listScripts(OutputInterface $output): int
     {
         $scripts = $this->requireComposer()->getPackage()->getScripts();
 
diff --git a/src/Composer/Command/ScriptAliasCommand.php b/src/Composer/Command/ScriptAliasCommand.php
index 801068c9a..2c98c9a66 100644
--- a/src/Composer/Command/ScriptAliasCommand.php
+++ b/src/Composer/Command/ScriptAliasCommand.php
@@ -42,7 +42,7 @@ class ScriptAliasCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName($this->script)
diff --git a/src/Composer/Command/SearchCommand.php b/src/Composer/Command/SearchCommand.php
index 9a404482f..496394a30 100644
--- a/src/Composer/Command/SearchCommand.php
+++ b/src/Composer/Command/SearchCommand.php
@@ -33,7 +33,7 @@ class SearchCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('search')
diff --git a/src/Composer/Command/SelfUpdateCommand.php b/src/Composer/Command/SelfUpdateCommand.php
index af8d511e7..06b0d2d0d 100644
--- a/src/Composer/Command/SelfUpdateCommand.php
+++ b/src/Composer/Command/SelfUpdateCommand.php
@@ -42,7 +42,7 @@ class SelfUpdateCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('self-update')
@@ -346,7 +346,7 @@ TAGSPUBKEY
      * @return void
      * @throws \Exception
      */
-    protected function fetchKeys(IOInterface $io, Config $config)
+    protected function fetchKeys(IOInterface $io, Config $config): void
     {
         if (!$io->isInteractive()) {
             throw new \RuntimeException('Public keys can not be fetched in non-interactive mode, please run Composer interactively');
@@ -354,7 +354,7 @@ TAGSPUBKEY
 
         $io->write('Open <info>https://composer.github.io/pubkeys.html</info> to find the latest keys');
 
-        $validator = function ($value) {
+        $validator = function ($value): string {
             if (!Preg::isMatch('{^-----BEGIN PUBLIC KEY-----$}', trim($value))) {
                 throw new \UnexpectedValueException('Invalid input');
             }
@@ -397,7 +397,7 @@ TAGSPUBKEY
      * @return int
      * @throws FilesystemException
      */
-    protected function rollback(OutputInterface $output, $rollbackDir, $localFilename)
+    protected function rollback(OutputInterface $output, $rollbackDir, $localFilename): int
     {
         $rollbackVersion = $this->getLastBackupVersion($rollbackDir);
         if (!$rollbackVersion) {
@@ -431,7 +431,7 @@ TAGSPUBKEY
      * @throws FilesystemException If the file cannot be moved
      * @return bool                Whether the phar is valid and has been moved
      */
-    protected function setLocalPhar($localFilename, $newFilename, $backupTarget = null)
+    protected function setLocalPhar($localFilename, $newFilename, $backupTarget = null): bool
     {
         $io = $this->getIO();
         @chmod($newFilename, fileperms($localFilename));
@@ -483,7 +483,7 @@ TAGSPUBKEY
      *
      * @return void
      */
-    protected function cleanBackups($rollbackDir, $except = null)
+    protected function cleanBackups($rollbackDir, $except = null): void
     {
         $finder = $this->getOldInstallationFinder($rollbackDir);
         $io = $this->getIO();
@@ -520,7 +520,7 @@ TAGSPUBKEY
      * @param string $rollbackDir
      * @return Finder
      */
-    protected function getOldInstallationFinder($rollbackDir)
+    protected function getOldInstallationFinder($rollbackDir): Finder
     {
         return Finder::create()
             ->depth(0)
@@ -541,7 +541,7 @@ TAGSPUBKEY
      * @throws \Exception
      * @return bool       If the operation succeeded
      */
-    protected function validatePhar($pharFile, &$error)
+    protected function validatePhar($pharFile, &$error): bool
     {
         if (ini_get('phar.readonly')) {
             return true;
@@ -569,7 +569,7 @@ TAGSPUBKEY
      *
      * @return bool
      */
-    protected function isWindowsNonAdminUser()
+    protected function isWindowsNonAdminUser(): bool
     {
         if (!Platform::isWindows()) {
             return false;
@@ -590,7 +590,7 @@ TAGSPUBKEY
      * @param  string $newFilename   The downloaded or backup phar
      * @return bool   Whether composer.phar has been updated
      */
-    protected function tryAsWindowsAdmin($localFilename, $newFilename)
+    protected function tryAsWindowsAdmin($localFilename, $newFilename): bool
     {
         $io = $this->getIO();
 
diff --git a/src/Composer/Command/ShowCommand.php b/src/Composer/Command/ShowCommand.php
index 5a80f1db9..ae9350f1e 100644
--- a/src/Composer/Command/ShowCommand.php
+++ b/src/Composer/Command/ShowCommand.php
@@ -223,7 +223,7 @@ EOT
 
             if ($input->getOption('no-dev')) {
                 $packages = $this->filterRequiredPackages($installedRepo, $rootPkg);
-                $repos = $installedRepo = new InstalledRepository(array(new InstalledArrayRepository(array_map(function ($pkg) {
+                $repos = $installedRepo = new InstalledRepository(array(new InstalledArrayRepository(array_map(function ($pkg): \Composer\Package\PackageInterface {
                     return clone $pkg;
                 }, $packages))));
             }
@@ -320,7 +320,7 @@ EOT
         if ($input->getOption('tree')) {
             $rootRequires = $this->getRootRequires();
             $packages = $installedRepo->getPackages();
-            usort($packages, function (BasePackage $a, BasePackage $b) {
+            usort($packages, function (BasePackage $a, BasePackage $b): int {
                 return strcmp((string) $a, (string) $b);
             });
             $arrayTree = array();
@@ -923,7 +923,7 @@ EOT
      * @param array<string, string> $versions
      * @return array<string, string|string[]|null>
      */
-    private function appendVersions($json, array $versions)
+    private function appendVersions($json, array $versions): array
     {
         uasort($versions, 'version_compare');
         $versions = array_keys(array_reverse($versions));
@@ -936,7 +936,7 @@ EOT
      * @param array<string, string|string[]|null> $json
      * @return array<string, string|string[]|null>
      */
-    private function appendLicenses($json, CompletePackageInterface $package)
+    private function appendLicenses($json, CompletePackageInterface $package): array
     {
         if ($licenses = $package->getLicense()) {
             $spdxLicenses = new SpdxLicenses();
@@ -963,7 +963,7 @@ EOT
      * @param array<string, string|string[]|null> $json
      * @return array<string, string|string[]|null>
      */
-    private function appendAutoload($json, CompletePackageInterface $package)
+    private function appendAutoload($json, CompletePackageInterface $package): array
     {
         if ($package->getAutoload()) {
             $autoload = array();
@@ -996,7 +996,7 @@ EOT
      * @param array<string, string|string[]|null> $json
      * @return array<string, string|string[]|null>
      */
-    private function appendLinks($json, CompletePackageInterface $package)
+    private function appendLinks($json, CompletePackageInterface $package): array
     {
         foreach (Link::$TYPES as $linkType) {
             $json = $this->appendLink($json, $package, $linkType);
@@ -1010,7 +1010,7 @@ EOT
      * @param string $linkType
      * @return array<string, string|string[]|null>
      */
-    private function appendLink($json, CompletePackageInterface $package, $linkType)
+    private function appendLink($json, CompletePackageInterface $package, $linkType): array
     {
         $links = $package->{'get' . ucfirst($linkType)}();
 
@@ -1240,7 +1240,7 @@ EOT
      * @param string $updateStatus
      * @return string
      */
-    private function updateStatusToVersionStyle($updateStatus)
+    private function updateStatusToVersionStyle($updateStatus): string
     {
         // 'up-to-date' is printed green
         // 'semver-safe-update' is printed red
@@ -1251,7 +1251,7 @@ EOT
     /**
      * @return string
      */
-    private function getUpdateStatus(PackageInterface $latestPackage, PackageInterface $package)
+    private function getUpdateStatus(PackageInterface $latestPackage, PackageInterface $package): string
     {
         if ($latestPackage->getFullPrettyVersion() === $package->getFullPrettyVersion()) {
             return 'up-to-date';
@@ -1275,7 +1275,7 @@ EOT
      *
      * @return void
      */
-    private function writeTreeLine($line)
+    private function writeTreeLine($line): void
     {
         $io = $this->getIO();
         if (!$io->isDecorated()) {
@@ -1326,7 +1326,7 @@ EOT
     /**
      * @return RepositorySet
      */
-    private function getRepositorySet(Composer $composer)
+    private function getRepositorySet(Composer $composer): RepositorySet
     {
         if (!$this->repositorySet) {
             $this->repositorySet = new RepositorySet($composer->getPackage()->getMinimumStability(), $composer->getPackage()->getStabilityFlags());
@@ -1342,7 +1342,7 @@ EOT
      * @param  array<PackageInterface> $bucket
      * @return array<PackageInterface>
      */
-    private function filterRequiredPackages(RepositoryInterface $repo, PackageInterface $package, $bucket = array())
+    private function filterRequiredPackages(RepositoryInterface $repo, PackageInterface $package, $bucket = array()): array
     {
         $requires = $package->getRequires();
 
diff --git a/src/Composer/Command/StatusCommand.php b/src/Composer/Command/StatusCommand.php
index eed32b623..8aec0e70d 100644
--- a/src/Composer/Command/StatusCommand.php
+++ b/src/Composer/Command/StatusCommand.php
@@ -40,7 +40,7 @@ class StatusCommand extends BaseCommand
      * @return void
      * @throws \Symfony\Component\Console\Exception\InvalidArgumentException
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('status')
@@ -80,7 +80,7 @@ EOT
     /**
      * @return int
      */
-    private function doExecute(InputInterface $input)
+    private function doExecute(InputInterface $input): int
     {
         // init repos
         $composer = $this->requireComposer();
@@ -163,7 +163,7 @@ EOT
 
             foreach ($errors as $path => $changes) {
                 if ($input->getOption('verbose')) {
-                    $indentedChanges = implode("\n", array_map(function ($line) {
+                    $indentedChanges = implode("\n", array_map(function ($line): string {
                         return '    ' . ltrim($line);
                     }, explode("\n", $changes)));
                     $io->write('<info>'.$path.'</info>:');
@@ -179,7 +179,7 @@ EOT
 
             foreach ($unpushedChanges as $path => $changes) {
                 if ($input->getOption('verbose')) {
-                    $indentedChanges = implode("\n", array_map(function ($line) {
+                    $indentedChanges = implode("\n", array_map(function ($line): string {
                         return '    ' . ltrim($line);
                     }, explode("\n", $changes)));
                     $io->write('<info>'.$path.'</info>:');
diff --git a/src/Composer/Command/SuggestsCommand.php b/src/Composer/Command/SuggestsCommand.php
index 8c72ba045..eae73a05c 100644
--- a/src/Composer/Command/SuggestsCommand.php
+++ b/src/Composer/Command/SuggestsCommand.php
@@ -26,7 +26,7 @@ class SuggestsCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('suggests')
diff --git a/src/Composer/Command/UpdateCommand.php b/src/Composer/Command/UpdateCommand.php
index c2efd014b..39decf788 100644
--- a/src/Composer/Command/UpdateCommand.php
+++ b/src/Composer/Command/UpdateCommand.php
@@ -129,7 +129,7 @@ EOT
 
         // extract --with shorthands from the allowlist
         if (count($packages) > 0) {
-            $allowlistPackagesWithRequirements = array_filter($packages, function ($pkg) {
+            $allowlistPackagesWithRequirements = array_filter($packages, function ($pkg): bool {
                 return Preg::isMatch('{\S+[ =:]\S+}', $pkg);
             });
             foreach ($this->formatRequirements($allowlistPackagesWithRequirements) as $package => $constraint) {
@@ -180,7 +180,7 @@ EOT
 
         // the arguments lock/nothing/mirrors are not package names but trigger a mirror update instead
         // they are further mutually exclusive with listing actual package names
-        $filteredPackages = array_filter($packages, function ($package) {
+        $filteredPackages = array_filter($packages, function ($package): bool {
             return !in_array($package, array('lock', 'nothing', 'mirrors'), true);
         });
         $updateMirrors = $input->getOption('lock') || count($filteredPackages) != count($packages);
@@ -245,7 +245,7 @@ EOT
      * @param array<string> $packages
      * @return array<string>
      */
-    private function getPackagesInteractively(IOInterface $io, InputInterface $input, OutputInterface $output, Composer $composer, array $packages)
+    private function getPackagesInteractively(IOInterface $io, InputInterface $input, OutputInterface $output, Composer $composer, array $packages): array
     {
         if (!$input->isInteractive()) {
             throw new \InvalidArgumentException('--interactive cannot be used in non-interactive terminals.');
@@ -312,7 +312,7 @@ EOT
      * @param string $constraint
      * @return Link
      */
-    private function appendConstraintToLink(Link $link, $constraint)
+    private function appendConstraintToLink(Link $link, $constraint): Link
     {
         $parser = new VersionParser;
         $oldPrettyString = $link->getConstraint()->getPrettyString();
diff --git a/src/Composer/Command/ValidateCommand.php b/src/Composer/Command/ValidateCommand.php
index 700461bcf..8c5ad5aea 100644
--- a/src/Composer/Command/ValidateCommand.php
+++ b/src/Composer/Command/ValidateCommand.php
@@ -38,7 +38,7 @@ class ValidateCommand extends BaseCommand
      * configure
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('validate')
@@ -169,7 +169,7 @@ EOT
      *
      * @return void
      */
-    private function outputResult(IOInterface $io, $name, &$errors, &$warnings, $checkPublish = false, $publishErrors = array(), $checkLock = false, $lockErrors = array(), $printSchemaUrl = false)
+    private function outputResult(IOInterface $io, $name, &$errors, &$warnings, $checkPublish = false, $publishErrors = array(), $checkLock = false, $lockErrors = array(), $printSchemaUrl = false): void
     {
         $doPrintSchemaUrl = false;
 
@@ -193,13 +193,13 @@ EOT
         }
 
         if ($errors) {
-            $errors = array_map(function ($err) {
+            $errors = array_map(function ($err): string {
                 return '- ' . $err;
             }, $errors);
             array_unshift($errors, '# General errors');
         }
         if ($warnings) {
-            $warnings = array_map(function ($err) {
+            $warnings = array_map(function ($err): string {
                 return '- ' . $err;
             }, $warnings);
             array_unshift($warnings, '# General warnings');
@@ -210,7 +210,7 @@ EOT
 
         // If checking publish errors, display them as errors, otherwise just show them as warnings
         if ($publishErrors) {
-            $publishErrors = array_map(function ($err) {
+            $publishErrors = array_map(function ($err): string {
                 return '- ' . $err;
             }, $publishErrors);
 
diff --git a/src/Composer/Compiler.php b/src/Composer/Compiler.php
index 362deeb34..6bf250468 100644
--- a/src/Composer/Compiler.php
+++ b/src/Composer/Compiler.php
@@ -44,7 +44,7 @@ class Compiler
      *
      * @throws \RuntimeException
      */
-    public function compile($pharFile = 'composer.phar')
+    public function compile($pharFile = 'composer.phar'): void
     {
         if (file_exists($pharFile)) {
             unlink($pharFile);
@@ -82,7 +82,7 @@ class Compiler
 
         $phar->startBuffering();
 
-        $finderSort = function ($a, $b) {
+        $finderSort = function ($a, $b): int {
             return strcmp(strtr($a->getRealPath(), '\\', '/'), strtr($b->getRealPath(), '\\', '/'));
         };
 
@@ -201,7 +201,7 @@ class Compiler
      * @param  \SplFileInfo $file
      * @return string
      */
-    private function getRelativeFilePath($file)
+    private function getRelativeFilePath($file): string
     {
         $realPath = $file->getRealPath();
         $pathPrefix = dirname(__DIR__, 2).DIRECTORY_SEPARATOR;
@@ -217,7 +217,7 @@ class Compiler
      *
      * @return void
      */
-    private function addFile(\Phar $phar, \SplFileInfo $file, $strip = true)
+    private function addFile(\Phar $phar, \SplFileInfo $file, $strip = true): void
     {
         $path = $this->getRelativeFilePath($file);
         $content = file_get_contents($file);
@@ -245,7 +245,7 @@ class Compiler
     /**
      * @return void
      */
-    private function addComposerBin(\Phar $phar)
+    private function addComposerBin(\Phar $phar): void
     {
         $content = file_get_contents(__DIR__.'/../../bin/composer');
         $content = Preg::replace('{^#!/usr/bin/env php\s*}', '', $content);
@@ -258,7 +258,7 @@ class Compiler
      * @param  string $source A PHP string
      * @return string The PHP string with the whitespace removed
      */
-    private function stripWhitespace($source)
+    private function stripWhitespace($source): string
     {
         if (!function_exists('token_get_all')) {
             return $source;
@@ -289,7 +289,7 @@ class Compiler
     /**
      * @return string
      */
-    private function getStub()
+    private function getStub(): string
     {
         $stub = <<<'EOF'
 #!/usr/bin/env php
diff --git a/src/Composer/Composer.php b/src/Composer/Composer.php
index 749c9e35d..8913397b6 100644
--- a/src/Composer/Composer.php
+++ b/src/Composer/Composer.php
@@ -12,15 +12,10 @@
 
 namespace Composer;
 
-use Composer\Package\RootPackageInterface;
 use Composer\Package\Locker;
 use Composer\Pcre\Preg;
-use Composer\Util\Loop;
-use Composer\Repository\RepositoryManager;
-use Composer\Installer\InstallationManager;
 use Composer\Plugin\PluginManager;
 use Composer\Downloader\DownloadManager;
-use Composer\EventDispatcher\EventDispatcher;
 use Composer\Autoload\AutoloadGenerator;
 use Composer\Package\Archiver\ArchiveManager;
 
@@ -29,7 +24,7 @@ use Composer\Package\Archiver\ArchiveManager;
  * @author Konstantin Kudryashiv <ever.zet@gmail.com>
  * @author Nils Adermann <naderman@naderman.de>
  */
-class Composer
+class Composer extends PartialComposer
 {
     /*
      * Examples of the following constants in the various configurations they can be in
@@ -71,7 +66,7 @@ class Composer
     /**
      * @return string
      */
-    public static function getVersion()
+    public static function getVersion(): string
     {
         // no replacement done, this must be a source checkout
         if (self::VERSION === '@package_version'.'@') {
@@ -87,50 +82,20 @@ class Composer
     }
 
     /**
-     * @var RootPackageInterface
+     * @var Locker
      */
-    private $package;
-
-    /**
-     * @var ?Locker
-     */
-    private $locker = null;
-
-    /**
-     * @var Loop
-     */
-    private $loop;
-
-    /**
-     * @var Repository\RepositoryManager
-     */
-    private $repositoryManager;
+    private $locker;
 
     /**
      * @var Downloader\DownloadManager
      */
     private $downloadManager;
 
-    /**
-     * @var Installer\InstallationManager
-     */
-    private $installationManager;
-
     /**
      * @var Plugin\PluginManager
      */
     private $pluginManager;
 
-    /**
-     * @var Config
-     */
-    private $config;
-
-    /**
-     * @var EventDispatcher
-     */
-    private $eventDispatcher;
-
     /**
      * @var Autoload\AutoloadGenerator
      */
@@ -141,178 +106,52 @@ class Composer
      */
     private $archiveManager;
 
-    /**
-     * @return void
-     */
-    public function setPackage(RootPackageInterface $package)
-    {
-        $this->package = $package;
-    }
-
-    /**
-     * @return RootPackageInterface
-     */
-    public function getPackage()
-    {
-        return $this->package;
-    }
-
-    /**
-     * @return void
-     */
-    public function setConfig(Config $config)
-    {
-        $this->config = $config;
-    }
-
-    /**
-     * @return Config
-     */
-    public function getConfig()
-    {
-        return $this->config;
-    }
-
-    /**
-     * @return void
-     */
-    public function setLocker(Locker $locker)
+    public function setLocker(Locker $locker): void
     {
         $this->locker = $locker;
     }
 
-    /**
-     * @return ?Locker
-     */
-    public function getLocker()
+    public function getLocker(): Locker
     {
         return $this->locker;
     }
 
-    /**
-     * @return void
-     */
-    public function setLoop(Loop $loop)
-    {
-        $this->loop = $loop;
-    }
-
-    /**
-     * @return Loop
-     */
-    public function getLoop()
-    {
-        return $this->loop;
-    }
-
-    /**
-     * @return void
-     */
-    public function setRepositoryManager(RepositoryManager $manager)
-    {
-        $this->repositoryManager = $manager;
-    }
-
-    /**
-     * @return RepositoryManager
-     */
-    public function getRepositoryManager()
-    {
-        return $this->repositoryManager;
-    }
-
-    /**
-     * @return void
-     */
-    public function setDownloadManager(DownloadManager $manager)
+    public function setDownloadManager(DownloadManager $manager): void
     {
         $this->downloadManager = $manager;
     }
 
-    /**
-     * @return DownloadManager
-     */
-    public function getDownloadManager()
+    public function getDownloadManager(): DownloadManager
     {
         return $this->downloadManager;
     }
 
-    /**
-     * @return void
-     */
-    public function setArchiveManager(ArchiveManager $manager)
+    public function setArchiveManager(ArchiveManager $manager): void
     {
         $this->archiveManager = $manager;
     }
 
-    /**
-     * @return ArchiveManager
-     */
-    public function getArchiveManager()
+    public function getArchiveManager(): ArchiveManager
     {
         return $this->archiveManager;
     }
 
-    /**
-     * @return void
-     */
-    public function setInstallationManager(InstallationManager $manager)
-    {
-        $this->installationManager = $manager;
-    }
-
-    /**
-     * @return InstallationManager
-     */
-    public function getInstallationManager()
-    {
-        return $this->installationManager;
-    }
-
-    /**
-     * @return void
-     */
-    public function setPluginManager(PluginManager $manager)
+    public function setPluginManager(PluginManager $manager): void
     {
         $this->pluginManager = $manager;
     }
 
-    /**
-     * @return PluginManager
-     */
-    public function getPluginManager()
+    public function getPluginManager(): PluginManager
     {
         return $this->pluginManager;
     }
 
-    /**
-     * @return void
-     */
-    public function setEventDispatcher(EventDispatcher $eventDispatcher)
-    {
-        $this->eventDispatcher = $eventDispatcher;
-    }
-
-    /**
-     * @return EventDispatcher
-     */
-    public function getEventDispatcher()
-    {
-        return $this->eventDispatcher;
-    }
-
-    /**
-     * @return void
-     */
-    public function setAutoloadGenerator(AutoloadGenerator $autoloadGenerator)
+    public function setAutoloadGenerator(AutoloadGenerator $autoloadGenerator): void
     {
         $this->autoloadGenerator = $autoloadGenerator;
     }
 
-    /**
-     * @return AutoloadGenerator
-     */
-    public function getAutoloadGenerator()
+    public function getAutoloadGenerator(): AutoloadGenerator
     {
         return $this->autoloadGenerator;
     }
diff --git a/src/Composer/Config.php b/src/Composer/Config.php
index a83f70175..24423a0a4 100644
--- a/src/Composer/Config.php
+++ b/src/Composer/Config.php
@@ -140,7 +140,7 @@ class Config
     /**
      * @return void
      */
-    public function setConfigSource(ConfigSourceInterface $source)
+    public function setConfigSource(ConfigSourceInterface $source): void
     {
         $this->configSource = $source;
     }
@@ -148,7 +148,7 @@ class Config
     /**
      * @return ConfigSourceInterface
      */
-    public function getConfigSource()
+    public function getConfigSource(): ConfigSourceInterface
     {
         return $this->configSource;
     }
@@ -156,7 +156,7 @@ class Config
     /**
      * @return void
      */
-    public function setAuthConfigSource(ConfigSourceInterface $source)
+    public function setAuthConfigSource(ConfigSourceInterface $source): void
     {
         $this->authConfigSource = $source;
     }
@@ -164,7 +164,7 @@ class Config
     /**
      * @return ConfigSourceInterface
      */
-    public function getAuthConfigSource()
+    public function getAuthConfigSource(): ConfigSourceInterface
     {
         return $this->authConfigSource;
     }
@@ -177,7 +177,7 @@ class Config
      *
      * @return void
      */
-    public function merge($config, $source = self::SOURCE_UNKNOWN)
+    public function merge($config, $source = self::SOURCE_UNKNOWN): void
     {
         // override defaults with given config
         if (!empty($config['config']) && is_array($config['config'])) {
@@ -263,7 +263,7 @@ class Config
     /**
      * @return array<int|string, mixed>
      */
-    public function getRepositories()
+    public function getRepositories(): array
     {
         return $this->repositories;
     }
@@ -441,7 +441,7 @@ class Config
      *
      * @return array<string, mixed[]>
      */
-    public function all($flags = 0)
+    public function all($flags = 0): array
     {
         $all = array(
             'repositories' => $this->getRepositories(),
@@ -457,7 +457,7 @@ class Config
      * @param string $key
      * @return string
      */
-    public function getSourceOfValue($key)
+    public function getSourceOfValue($key): string
     {
         $this->get($key);
 
@@ -471,7 +471,7 @@ class Config
      *
      * @return void
      */
-    private function setSourceOfConfigValue($configValue, $path, $source)
+    private function setSourceOfConfigValue($configValue, $path, $source): void
     {
         $this->sourceOfConfigValue[$path] = $source;
 
@@ -485,7 +485,7 @@ class Config
     /**
      * @return array<string, mixed[]>
      */
-    public function raw()
+    public function raw(): array
     {
         return array(
             'repositories' => $this->getRepositories(),
@@ -499,7 +499,7 @@ class Config
      * @param  string $key
      * @return bool
      */
-    public function has($key)
+    public function has($key): bool
     {
         return array_key_exists($key, $this->config);
     }
@@ -531,7 +531,7 @@ class Config
      * @param  string $path
      * @return string
      */
-    private function realpath($path)
+    private function realpath($path): string
     {
         if (Preg::isMatch('{^(?:/|[a-z]:|[a-z0-9.]+://)}i', $path)) {
             return $path;
@@ -563,7 +563,7 @@ class Config
      *
      * @return void
      */
-    private function disableRepoByName($name)
+    private function disableRepoByName($name): void
     {
         if (isset($this->repositories[$name])) {
             unset($this->repositories[$name]);
@@ -580,7 +580,7 @@ class Config
      *
      * @return void
      */
-    public function prohibitUrlByConfig($url, IOInterface $io = null)
+    public function prohibitUrlByConfig($url, IOInterface $io = null): void
     {
         // Return right away if the URL is malformed or custom (see issue #5173)
         if (false === filter_var($url, FILTER_VALIDATE_URL)) {
@@ -626,7 +626,7 @@ class Config
      *
      * @return void
      */
-    public static function disableProcessTimeout()
+    public static function disableProcessTimeout(): void
     {
         // Override global timeout set earlier by environment or config
         ProcessExecutor::setTimeout(0);
diff --git a/src/Composer/Config/JsonConfigSource.php b/src/Composer/Config/JsonConfigSource.php
index 604c41f9a..84de4f32c 100644
--- a/src/Composer/Config/JsonConfigSource.php
+++ b/src/Composer/Config/JsonConfigSource.php
@@ -62,7 +62,7 @@ class JsonConfigSource implements ConfigSourceInterface
      */
     public function addRepository($name, $config, $append = true)
     {
-        $this->manipulateJson('addRepository', function (&$config, $repo, $repoConfig) use ($append) {
+        $this->manipulateJson('addRepository', function (&$config, $repo, $repoConfig) use ($append): void {
             // if converting from an array format to hashmap format, and there is a {"packagist.org":false} repo, we have
             // to convert it to "packagist.org": false key on the hashmap otherwise it fails schema validation
             if (isset($config['repositories'])) {
@@ -91,7 +91,7 @@ class JsonConfigSource implements ConfigSourceInterface
      */
     public function removeRepository($name)
     {
-        $this->manipulateJson('removeRepository', function (&$config, $repo) {
+        $this->manipulateJson('removeRepository', function (&$config, $repo): void {
             unset($config['repositories'][$repo]);
         }, $name);
     }
@@ -102,7 +102,7 @@ class JsonConfigSource implements ConfigSourceInterface
     public function addConfigSetting($name, $value)
     {
         $authConfig = $this->authConfig;
-        $this->manipulateJson('addConfigSetting', function (&$config, $key, $val) use ($authConfig) {
+        $this->manipulateJson('addConfigSetting', function (&$config, $key, $val) use ($authConfig): void {
             if (Preg::isMatch('{^(bitbucket-oauth|github-oauth|gitlab-oauth|gitlab-token|bearer|http-basic|platform)\.}', $key)) {
                 list($key, $host) = explode('.', $key, 2);
                 if ($authConfig) {
@@ -122,7 +122,7 @@ class JsonConfigSource implements ConfigSourceInterface
     public function removeConfigSetting($name)
     {
         $authConfig = $this->authConfig;
-        $this->manipulateJson('removeConfigSetting', function (&$config, $key) use ($authConfig) {
+        $this->manipulateJson('removeConfigSetting', function (&$config, $key) use ($authConfig): void {
             if (Preg::isMatch('{^(bitbucket-oauth|github-oauth|gitlab-oauth|gitlab-token|bearer|http-basic|platform)\.}', $key)) {
                 list($key, $host) = explode('.', $key, 2);
                 if ($authConfig) {
@@ -141,7 +141,7 @@ class JsonConfigSource implements ConfigSourceInterface
      */
     public function addProperty($name, $value)
     {
-        $this->manipulateJson('addProperty', function (&$config, $key, $val) {
+        $this->manipulateJson('addProperty', function (&$config, $key, $val): void {
             if (strpos($key, 'extra.') === 0 || strpos($key, 'scripts.') === 0) {
                 $bits = explode('.', $key);
                 $last = array_pop($bits);
@@ -164,7 +164,7 @@ class JsonConfigSource implements ConfigSourceInterface
      */
     public function removeProperty($name)
     {
-        $this->manipulateJson('removeProperty', function (&$config, $key) {
+        $this->manipulateJson('removeProperty', function (&$config, $key): void {
             if (strpos($key, 'extra.') === 0 || strpos($key, 'scripts.') === 0) {
                 $bits = explode('.', $key);
                 $last = array_pop($bits);
@@ -187,7 +187,7 @@ class JsonConfigSource implements ConfigSourceInterface
      */
     public function addLink($type, $name, $value)
     {
-        $this->manipulateJson('addLink', function (&$config, $type, $name, $value) {
+        $this->manipulateJson('addLink', function (&$config, $type, $name, $value): void {
             $config[$type][$name] = $value;
         }, $type, $name, $value);
     }
@@ -197,10 +197,10 @@ class JsonConfigSource implements ConfigSourceInterface
      */
     public function removeLink($type, $name)
     {
-        $this->manipulateJson('removeSubNode', function (&$config, $type, $name) {
+        $this->manipulateJson('removeSubNode', function (&$config, $type, $name): void {
             unset($config[$type][$name]);
         }, $type, $name);
-        $this->manipulateJson('removeMainKeyIfEmpty', function (&$config, $type) {
+        $this->manipulateJson('removeMainKeyIfEmpty', function (&$config, $type): void {
             if (0 === count($config[$type])) {
                 unset($config[$type]);
             }
@@ -214,7 +214,7 @@ class JsonConfigSource implements ConfigSourceInterface
      *
      * @return void
      */
-    private function manipulateJson($method, $fallback, ...$args)
+    private function manipulateJson($method, $fallback, ...$args): void
     {
         if ($this->file->exists()) {
             if (!is_writable($this->file->getPath())) {
@@ -297,7 +297,7 @@ class JsonConfigSource implements ConfigSourceInterface
      * @param  mixed $value
      * @return int
      */
-    private function arrayUnshiftRef(&$array, &$value)
+    private function arrayUnshiftRef(&$array, &$value): int
     {
         $return = array_unshift($array, '');
         $array[0] = &$value;
diff --git a/src/Composer/Console/Application.php b/src/Composer/Console/Application.php
index 4762396af..5bf8f6ffc 100644
--- a/src/Composer/Console/Application.php
+++ b/src/Composer/Console/Application.php
@@ -94,14 +94,14 @@ class Application extends BaseApplication
         if (!$shutdownRegistered) {
             if (function_exists('pcntl_async_signals') && function_exists('pcntl_signal')) {
                 pcntl_async_signals(true);
-                pcntl_signal(SIGINT, function ($sig) {
+                pcntl_signal(SIGINT, function ($sig): void {
                     exit(130);
                 });
             }
 
             $shutdownRegistered = true;
 
-            register_shutdown_function(function () {
+            register_shutdown_function(function (): void {
                 $lastError = error_get_last();
 
                 if ($lastError && $lastError['message'] &&
@@ -291,7 +291,7 @@ class Application extends BaseApplication
             }
 
             // Check system temp folder for usability as it can cause weird runtime issues otherwise
-            Silencer::call(function () use ($io) {
+            Silencer::call(function () use ($io): void {
                 $tempfile = sys_get_temp_dir() . '/temp-' . md5(microtime());
                 if (!(file_put_contents($tempfile, __FILE__) && (file_get_contents($tempfile) == __FILE__) && unlink($tempfile) && !file_exists($tempfile))) {
                     $io->writeError(sprintf('<error>PHP temp directory (%s) does not exist or is not writable to Composer. Set sys_temp_dir in your php.ini</error>', sys_get_temp_dir()));
@@ -358,7 +358,7 @@ class Application extends BaseApplication
      * @throws \RuntimeException
      * @return string
      */
-    private function getNewWorkingDir(InputInterface $input)
+    private function getNewWorkingDir(InputInterface $input): string
     {
         $workingDir = $input->getParameterOption(array('--working-dir', '-d'));
         if (false !== $workingDir && !is_dir($workingDir)) {
@@ -371,7 +371,7 @@ class Application extends BaseApplication
     /**
      * @return void
      */
-    private function hintCommonErrors(\Exception $exception)
+    private function hintCommonErrors(\Exception $exception): void
     {
         $io = $this->getIO();
 
@@ -408,7 +408,8 @@ class Application extends BaseApplication
             $io->writeError('<error>Check https://getcomposer.org/doc/06-config.md#process-timeout for details</error>', true, IOInterface::QUIET);
         }
 
-        if ($hints = HttpDownloader::getExceptionHints($exception)) {
+        $hints = HttpDownloader::getExceptionHints($exception);
+        if (null !== $hints && count($hints) > 0) {
             foreach ($hints as $hint) {
                 $io->writeError($hint, true, IOInterface::QUIET);
             }
@@ -553,13 +554,13 @@ class Application extends BaseApplication
     /**
      * @return Command\BaseCommand[]
      */
-    private function getPluginCommands()
+    private function getPluginCommands(): array
     {
         $commands = array();
 
         $composer = $this->getComposer(false, false);
         if (null === $composer) {
-            $composer = Factory::createGlobal($this->io);
+            $composer = Factory::createGlobal($this->io, $this->disablePluginsByDefault, $this->disableScriptsByDefault);
         }
 
         if (null !== $composer) {
diff --git a/src/Composer/Console/GithubActionError.php b/src/Composer/Console/GithubActionError.php
index 6cd49c20d..578ac8eca 100644
--- a/src/Composer/Console/GithubActionError.php
+++ b/src/Composer/Console/GithubActionError.php
@@ -34,7 +34,7 @@ final class GithubActionError
      *
      * @return void
      */
-    public function emit($message, $file = null, $line = null)
+    public function emit($message, $file = null, $line = null): void
     {
         if (Platform::getEnv('GITHUB_ACTIONS') && !Platform::getEnv('COMPOSER_TESTS_ARE_RUNNING')) {
             $message = $this->escapeData($message);
@@ -55,7 +55,7 @@ final class GithubActionError
      * @param string $data
      * @return string
      */
-    private function escapeData($data)
+    private function escapeData($data): string
     {
         // see https://github.com/actions/toolkit/blob/4f7fb6513a355689f69f0849edeb369a4dc81729/packages/core/src/command.ts#L80-L85
         $data = str_replace("%", '%25', $data);
@@ -69,7 +69,7 @@ final class GithubActionError
      * @param string $property
      * @return string
      */
-    private function escapeProperty($property)
+    private function escapeProperty($property): string
     {
         // see https://github.com/actions/toolkit/blob/4f7fb6513a355689f69f0849edeb369a4dc81729/packages/core/src/command.ts#L87-L94
         $property = str_replace("%", '%25', $property);
diff --git a/src/Composer/DependencyResolver/Decisions.php b/src/Composer/DependencyResolver/Decisions.php
index 2df464bc1..8740cf7bf 100644
--- a/src/Composer/DependencyResolver/Decisions.php
+++ b/src/Composer/DependencyResolver/Decisions.php
@@ -43,7 +43,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $level
      * @return void
      */
-    public function decide($literal, $level, Rule $why)
+    public function decide($literal, $level, Rule $why): void
     {
         $this->addDecision($literal, $level);
         $this->decisionQueue[] = array(
@@ -56,7 +56,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literal
      * @return bool
      */
-    public function satisfy($literal)
+    public function satisfy($literal): bool
     {
         $packageId = abs($literal);
 
@@ -70,7 +70,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literal
      * @return bool
      */
-    public function conflict($literal)
+    public function conflict($literal): bool
     {
         $packageId = abs($literal);
 
@@ -84,7 +84,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literalOrPackageId
      * @return bool
      */
-    public function decided($literalOrPackageId)
+    public function decided($literalOrPackageId): bool
     {
         return !empty($this->decisionMap[abs($literalOrPackageId)]);
     }
@@ -93,7 +93,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literalOrPackageId
      * @return bool
      */
-    public function undecided($literalOrPackageId)
+    public function undecided($literalOrPackageId): bool
     {
         return empty($this->decisionMap[abs($literalOrPackageId)]);
     }
@@ -102,7 +102,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literalOrPackageId
      * @return bool
      */
-    public function decidedInstall($literalOrPackageId)
+    public function decidedInstall($literalOrPackageId): bool
     {
         $packageId = abs($literalOrPackageId);
 
@@ -113,7 +113,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literalOrPackageId
      * @return int
      */
-    public function decisionLevel($literalOrPackageId)
+    public function decisionLevel($literalOrPackageId): int
     {
         $packageId = abs($literalOrPackageId);
         if (isset($this->decisionMap[$packageId])) {
@@ -127,7 +127,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $literalOrPackageId
      * @return Rule|null
      */
-    public function decisionRule($literalOrPackageId)
+    public function decisionRule($literalOrPackageId): ?Rule
     {
         $packageId = abs($literalOrPackageId);
 
@@ -144,7 +144,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $queueOffset
      * @return array{0: int, 1: Rule} a literal and decision reason
      */
-    public function atOffset($queueOffset)
+    public function atOffset($queueOffset): array
     {
         return $this->decisionQueue[$queueOffset];
     }
@@ -153,7 +153,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $queueOffset
      * @return bool
      */
-    public function validOffset($queueOffset)
+    public function validOffset($queueOffset): bool
     {
         return $queueOffset >= 0 && $queueOffset < \count($this->decisionQueue);
     }
@@ -161,7 +161,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return Rule
      */
-    public function lastReason()
+    public function lastReason(): Rule
     {
         return $this->decisionQueue[\count($this->decisionQueue) - 1][self::DECISION_REASON];
     }
@@ -169,7 +169,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return int
      */
-    public function lastLiteral()
+    public function lastLiteral(): int
     {
         return $this->decisionQueue[\count($this->decisionQueue) - 1][self::DECISION_LITERAL];
     }
@@ -177,7 +177,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return void
      */
-    public function reset()
+    public function reset(): void
     {
         while ($decision = array_pop($this->decisionQueue)) {
             $this->decisionMap[abs($decision[self::DECISION_LITERAL])] = 0;
@@ -188,7 +188,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $offset
      * @return void
      */
-    public function resetToOffset($offset)
+    public function resetToOffset($offset): void
     {
         while (\count($this->decisionQueue) > $offset + 1) {
             $decision = array_pop($this->decisionQueue);
@@ -199,7 +199,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return void
      */
-    public function revertLast()
+    public function revertLast(): void
     {
         $this->decisionMap[abs($this->lastLiteral())] = 0;
         array_pop($this->decisionQueue);
@@ -242,7 +242,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return bool
      */
-    public function isEmpty()
+    public function isEmpty(): bool
     {
         return \count($this->decisionQueue) === 0;
     }
@@ -252,7 +252,7 @@ class Decisions implements \Iterator, \Countable
      * @param int $level
      * @return void
      */
-    protected function addDecision($literal, $level)
+    protected function addDecision($literal, $level): void
     {
         $packageId = abs($literal);
 
@@ -275,7 +275,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return string
      */
-    public function toString(Pool $pool = null)
+    public function toString(Pool $pool = null): string
     {
         $decisionMap = $this->decisionMap;
         ksort($decisionMap);
diff --git a/src/Composer/DependencyResolver/DefaultPolicy.php b/src/Composer/DependencyResolver/DefaultPolicy.php
index 74e60d31a..e9add445f 100644
--- a/src/Composer/DependencyResolver/DefaultPolicy.php
+++ b/src/Composer/DependencyResolver/DefaultPolicy.php
@@ -44,7 +44,7 @@ class DefaultPolicy implements PolicyInterface
      *
      * @phpstan-param Constraint::STR_OP_* $operator
      */
-    public function versionCompare(PackageInterface $a, PackageInterface $b, $operator)
+    public function versionCompare(PackageInterface $a, PackageInterface $b, $operator): bool
     {
         if ($this->preferStable && ($stabA = $a->getStability()) !== ($stabB = $b->getStability())) {
             return BasePackage::$stabilities[$stabA] < BasePackage::$stabilities[$stabB];
@@ -61,12 +61,12 @@ class DefaultPolicy implements PolicyInterface
      * @param  string $requiredPackage
      * @return int[]
      */
-    public function selectPreferredPackages(Pool $pool, array $literals, $requiredPackage = null)
+    public function selectPreferredPackages(Pool $pool, array $literals, $requiredPackage = null): array
     {
         $packages = $this->groupLiteralsByName($pool, $literals);
 
         foreach ($packages as &$nameLiterals) {
-            usort($nameLiterals, function ($a, $b) use ($pool, $requiredPackage) {
+            usort($nameLiterals, function ($a, $b) use ($pool, $requiredPackage): int {
                 return $this->compareByPriority($pool, $pool->literalToPackage($a), $pool->literalToPackage($b), $requiredPackage, true);
             });
         }
@@ -79,7 +79,7 @@ class DefaultPolicy implements PolicyInterface
         $selected = \call_user_func_array('array_merge', array_values($packages));
 
         // now sort the result across all packages to respect replaces across packages
-        usort($selected, function ($a, $b) use ($pool, $requiredPackage) {
+        usort($selected, function ($a, $b) use ($pool, $requiredPackage): int {
             return $this->compareByPriority($pool, $pool->literalToPackage($a), $pool->literalToPackage($b), $requiredPackage);
         });
 
@@ -90,7 +90,7 @@ class DefaultPolicy implements PolicyInterface
      * @param  int[] $literals
      * @return array<string, int[]>
      */
-    protected function groupLiteralsByName(Pool $pool, $literals)
+    protected function groupLiteralsByName(Pool $pool, $literals): array
     {
         $packages = array();
         foreach ($literals as $literal) {
@@ -111,7 +111,7 @@ class DefaultPolicy implements PolicyInterface
      * @param bool $ignoreReplace
      * @return int
      */
-    public function compareByPriority(Pool $pool, BasePackage $a, BasePackage $b, $requiredPackage = null, $ignoreReplace = false)
+    public function compareByPriority(Pool $pool, BasePackage $a, BasePackage $b, $requiredPackage = null, $ignoreReplace = false): int
     {
         // prefer aliases to the original package
         if ($a->getName() === $b->getName()) {
@@ -164,7 +164,7 @@ class DefaultPolicy implements PolicyInterface
      *
      * @return bool
      */
-    protected function replaces(BasePackage $source, BasePackage $target)
+    protected function replaces(BasePackage $source, BasePackage $target): bool
     {
         foreach ($source->getReplaces() as $link) {
             if ($link->getTarget() === $target->getName()
@@ -182,7 +182,7 @@ class DefaultPolicy implements PolicyInterface
      * @param  int[] $literals
      * @return int[]
      */
-    protected function pruneToBestVersion(Pool $pool, $literals)
+    protected function pruneToBestVersion(Pool $pool, $literals): array
     {
         $operator = $this->preferLowest ? '<' : '>';
         $bestLiterals = array($literals[0]);
@@ -213,7 +213,7 @@ class DefaultPolicy implements PolicyInterface
      * @param  int[] $literals
      * @return int[]
      */
-    protected function pruneRemoteAliases(Pool $pool, array $literals)
+    protected function pruneRemoteAliases(Pool $pool, array $literals): array
     {
         $hasLocalAlias = false;
 
diff --git a/src/Composer/DependencyResolver/GenericRule.php b/src/Composer/DependencyResolver/GenericRule.php
index 00abfafd3..c86aa19cb 100644
--- a/src/Composer/DependencyResolver/GenericRule.php
+++ b/src/Composer/DependencyResolver/GenericRule.php
@@ -36,7 +36,7 @@ class GenericRule extends Rule
     /**
      * @return int[]
      */
-    public function getLiterals()
+    public function getLiterals(): array
     {
         return $this->literals;
     }
@@ -59,7 +59,7 @@ class GenericRule extends Rule
      * @param  Rule $rule The rule to check against
      * @return bool Whether the rules are equal
      */
-    public function equals(Rule $rule)
+    public function equals(Rule $rule): bool
     {
         return $this->literals === $rule->getLiterals();
     }
@@ -67,7 +67,7 @@ class GenericRule extends Rule
     /**
      * @return bool
      */
-    public function isAssertion()
+    public function isAssertion(): bool
     {
         return 1 === \count($this->literals);
     }
diff --git a/src/Composer/DependencyResolver/LockTransaction.php b/src/Composer/DependencyResolver/LockTransaction.php
index a3abe1e2a..4d04a5cd6 100644
--- a/src/Composer/DependencyResolver/LockTransaction.php
+++ b/src/Composer/DependencyResolver/LockTransaction.php
@@ -62,7 +62,7 @@ class LockTransaction extends Transaction
     /**
      * @return void
      */
-    public function setResultPackages(Pool $pool, Decisions $decisions)
+    public function setResultPackages(Pool $pool, Decisions $decisions): void
     {
         $this->resultPackages = array('all' => array(), 'non-dev' => array(), 'dev' => array());
         foreach ($decisions as $i => $decision) {
@@ -82,7 +82,7 @@ class LockTransaction extends Transaction
     /**
      * @return void
      */
-    public function setNonDevPackages(LockTransaction $extractionResult)
+    public function setNonDevPackages(LockTransaction $extractionResult): void
     {
         $packages = $extractionResult->getNewLockPackages(false);
 
@@ -106,7 +106,7 @@ class LockTransaction extends Transaction
      * @param bool $updateMirrors
      * @return BasePackage[]
      */
-    public function getNewLockPackages($devMode, $updateMirrors = false)
+    public function getNewLockPackages($devMode, $updateMirrors = false): array
     {
         $packages = array();
         foreach ($this->resultPackages[$devMode ? 'dev' : 'non-dev'] as $package) {
@@ -137,7 +137,7 @@ class LockTransaction extends Transaction
      * @param array<array{package: string, version: string, alias: string, alias_normalized: string}> $aliases
      * @return array<array{package: string, version: string, alias: string, alias_normalized: string}>
      */
-    public function getAliases($aliases)
+    public function getAliases($aliases): array
     {
         $usedAliases = array();
 
@@ -152,7 +152,7 @@ class LockTransaction extends Transaction
             }
         }
 
-        usort($usedAliases, function ($a, $b) {
+        usort($usedAliases, function ($a, $b): int {
             return strcmp($a['package'], $b['package']);
         });
 
diff --git a/src/Composer/DependencyResolver/MultiConflictRule.php b/src/Composer/DependencyResolver/MultiConflictRule.php
index deda73010..a7fcb0036 100644
--- a/src/Composer/DependencyResolver/MultiConflictRule.php
+++ b/src/Composer/DependencyResolver/MultiConflictRule.php
@@ -42,7 +42,7 @@ class MultiConflictRule extends Rule
     /**
      * @return int[]
      */
-    public function getLiterals()
+    public function getLiterals(): array
     {
         return $this->literals;
     }
@@ -65,7 +65,7 @@ class MultiConflictRule extends Rule
      * @param  Rule $rule The rule to check against
      * @return bool Whether the rules are equal
      */
-    public function equals(Rule $rule)
+    public function equals(Rule $rule): bool
     {
         if ($rule instanceof MultiConflictRule) {
             return $this->literals === $rule->getLiterals();
@@ -77,7 +77,7 @@ class MultiConflictRule extends Rule
     /**
      * @return bool
      */
-    public function isAssertion()
+    public function isAssertion(): bool
     {
         return false;
     }
@@ -86,7 +86,7 @@ class MultiConflictRule extends Rule
      * @return never
      * @throws \RuntimeException
      */
-    public function disable()
+    public function disable(): void
     {
         throw new \RuntimeException("Disabling multi conflict rules is not possible. Please contact composer at https://github.com/composer/composer to let us debug what lead to this situation.");
     }
diff --git a/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php b/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php
index 320fc54c0..e1c21008b 100644
--- a/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php
+++ b/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php
@@ -38,7 +38,7 @@ class MarkAliasInstalledOperation extends SolverOperation implements OperationIn
      *
      * @return AliasPackage
      */
-    public function getPackage()
+    public function getPackage(): AliasPackage
     {
         return $this->package;
     }
@@ -46,7 +46,7 @@ class MarkAliasInstalledOperation extends SolverOperation implements OperationIn
     /**
      * @inheritDoc
      */
-    public function show($lock)
+    public function show($lock): string
     {
         return 'Marking <info>'.$this->package->getPrettyName().'</info> (<comment>'.$this->package->getFullPrettyVersion().'</comment>) as installed, alias of <info>'.$this->package->getAliasOf()->getPrettyName().'</info> (<comment>'.$this->package->getAliasOf()->getFullPrettyVersion().'</comment>)';
     }
diff --git a/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php b/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php
index f447047d4..6cdd02837 100644
--- a/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php
+++ b/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php
@@ -38,7 +38,7 @@ class MarkAliasUninstalledOperation extends SolverOperation implements Operation
      *
      * @return AliasPackage
      */
-    public function getPackage()
+    public function getPackage(): AliasPackage
     {
         return $this->package;
     }
@@ -46,7 +46,7 @@ class MarkAliasUninstalledOperation extends SolverOperation implements Operation
     /**
      * @inheritDoc
      */
-    public function show($lock)
+    public function show($lock): string
     {
         return 'Marking <info>'.$this->package->getPrettyName().'</info> (<comment>'.$this->package->getFullPrettyVersion().'</comment>) as uninstalled, alias of <info>'.$this->package->getAliasOf()->getPrettyName().'</info> (<comment>'.$this->package->getAliasOf()->getFullPrettyVersion().'</comment>)';
     }
diff --git a/src/Composer/DependencyResolver/Operation/UninstallOperation.php b/src/Composer/DependencyResolver/Operation/UninstallOperation.php
index ebfe98df8..35e7ed753 100644
--- a/src/Composer/DependencyResolver/Operation/UninstallOperation.php
+++ b/src/Composer/DependencyResolver/Operation/UninstallOperation.php
@@ -38,7 +38,7 @@ class UninstallOperation extends SolverOperation implements OperationInterface
      *
      * @return PackageInterface
      */
-    public function getPackage()
+    public function getPackage(): PackageInterface
     {
         return $this->package;
     }
@@ -46,7 +46,7 @@ class UninstallOperation extends SolverOperation implements OperationInterface
     /**
      * @inheritDoc
      */
-    public function show($lock)
+    public function show($lock): string
     {
         return self::format($this->package, $lock);
     }
@@ -55,7 +55,7 @@ class UninstallOperation extends SolverOperation implements OperationInterface
      * @param bool $lock
      * @return string
      */
-    public static function format(PackageInterface $package, $lock = false)
+    public static function format(PackageInterface $package, $lock = false): string
     {
         return 'Removing <info>'.$package->getPrettyName().'</info> (<comment>'.$package->getFullPrettyVersion().'</comment>)';
     }
diff --git a/src/Composer/DependencyResolver/Operation/UpdateOperation.php b/src/Composer/DependencyResolver/Operation/UpdateOperation.php
index be9a49c22..dbfb3d310 100644
--- a/src/Composer/DependencyResolver/Operation/UpdateOperation.php
+++ b/src/Composer/DependencyResolver/Operation/UpdateOperation.php
@@ -49,7 +49,7 @@ class UpdateOperation extends SolverOperation implements OperationInterface
      *
      * @return PackageInterface
      */
-    public function getInitialPackage()
+    public function getInitialPackage(): PackageInterface
     {
         return $this->initialPackage;
     }
@@ -59,7 +59,7 @@ class UpdateOperation extends SolverOperation implements OperationInterface
      *
      * @return PackageInterface
      */
-    public function getTargetPackage()
+    public function getTargetPackage(): PackageInterface
     {
         return $this->targetPackage;
     }
@@ -67,7 +67,7 @@ class UpdateOperation extends SolverOperation implements OperationInterface
     /**
      * @inheritDoc
      */
-    public function show($lock)
+    public function show($lock): string
     {
         return self::format($this->initialPackage, $this->targetPackage, $lock);
     }
@@ -76,7 +76,7 @@ class UpdateOperation extends SolverOperation implements OperationInterface
      * @param bool $lock
      * @return string
      */
-    public static function format(PackageInterface $initialPackage, PackageInterface $targetPackage, $lock = false)
+    public static function format(PackageInterface $initialPackage, PackageInterface $targetPackage, $lock = false): string
     {
         $fromVersion = $initialPackage->getFullPrettyVersion();
         $toVersion = $targetPackage->getFullPrettyVersion();
diff --git a/src/Composer/DependencyResolver/PolicyInterface.php b/src/Composer/DependencyResolver/PolicyInterface.php
index 48ce24e16..4651ceb4d 100644
--- a/src/Composer/DependencyResolver/PolicyInterface.php
+++ b/src/Composer/DependencyResolver/PolicyInterface.php
@@ -26,12 +26,12 @@ interface PolicyInterface
      *
      * @phpstan-param Constraint::STR_OP_* $operator
      */
-    public function versionCompare(PackageInterface $a, PackageInterface $b, $operator);
+    public function versionCompare(PackageInterface $a, PackageInterface $b, $operator): bool;
 
     /**
      * @param  int[]   $literals
      * @param  ?string $requiredPackage
      * @return int[]
      */
-    public function selectPreferredPackages(Pool $pool, array $literals, $requiredPackage = null);
+    public function selectPreferredPackages(Pool $pool, array $literals, $requiredPackage = null): array;
 }
diff --git a/src/Composer/DependencyResolver/Pool.php b/src/Composer/DependencyResolver/Pool.php
index 0fa5783e4..9a1a73b3c 100644
--- a/src/Composer/DependencyResolver/Pool.php
+++ b/src/Composer/DependencyResolver/Pool.php
@@ -60,7 +60,7 @@ class Pool implements \Countable
      * @param  string $name
      * @return array<string, string>
      */
-    public function getRemovedVersions($name, ConstraintInterface $constraint)
+    public function getRemovedVersions($name, ConstraintInterface $constraint): array
     {
         if (!isset($this->removedVersions[$name])) {
             return array();
@@ -80,7 +80,7 @@ class Pool implements \Countable
      * @param  string $objectHash
      * @return array<string, string>
      */
-    public function getRemovedVersionsByPackage($objectHash)
+    public function getRemovedVersionsByPackage($objectHash): array
     {
         if (!isset($this->removedVersionsByPackage[$objectHash])) {
             return array();
@@ -93,7 +93,7 @@ class Pool implements \Countable
      * @param BasePackage[] $packages
      * @return void
      */
-    private function setPackages(array $packages)
+    private function setPackages(array $packages): void
     {
         $id = 1;
 
@@ -111,7 +111,7 @@ class Pool implements \Countable
     /**
      * @return BasePackage[]
      */
-    public function getPackages()
+    public function getPackages(): array
     {
         return $this->packages;
     }
@@ -122,7 +122,7 @@ class Pool implements \Countable
      * @param  int         $id
      * @return BasePackage
      */
-    public function packageById($id)
+    public function packageById($id): BasePackage
     {
         return $this->packages[$id - 1];
     }
@@ -143,7 +143,7 @@ class Pool implements \Countable
      *                                         packages must match or null to return all
      * @return BasePackage[] A set of packages
      */
-    public function whatProvides($name, ConstraintInterface $constraint = null)
+    public function whatProvides($name, ConstraintInterface $constraint = null): array
     {
         $key = (string) $constraint;
         if (isset($this->providerCache[$name][$key])) {
@@ -159,7 +159,7 @@ class Pool implements \Countable
      *                                          packages must match or null to return all
      * @return BasePackage[]
      */
-    private function computeWhatProvides($name, ConstraintInterface $constraint = null)
+    private function computeWhatProvides($name, ConstraintInterface $constraint = null): array
     {
         if (!isset($this->packageByName[$name])) {
             return array();
@@ -180,7 +180,7 @@ class Pool implements \Countable
      * @param int $literal
      * @return BasePackage
      */
-    public function literalToPackage($literal)
+    public function literalToPackage($literal): BasePackage
     {
         $packageId = abs($literal);
 
@@ -192,7 +192,7 @@ class Pool implements \Countable
      * @param array<int, BasePackage> $installedMap
      * @return string
      */
-    public function literalToPrettyString($literal, $installedMap)
+    public function literalToPrettyString($literal, $installedMap): string
     {
         $package = $this->literalToPackage($literal);
 
@@ -212,7 +212,7 @@ class Pool implements \Countable
      * @param  string              $name       Name of the package to be matched
      * @return bool
      */
-    public function match(BasePackage $candidate, $name, ConstraintInterface $constraint = null)
+    public function match(BasePackage $candidate, $name, ConstraintInterface $constraint = null): bool
     {
         $candidateName = $candidate->getName();
         $candidateVersion = $candidate->getVersion();
@@ -255,7 +255,7 @@ class Pool implements \Countable
     /**
      * @return bool
      */
-    public function isUnacceptableFixedOrLockedPackage(BasePackage $package)
+    public function isUnacceptableFixedOrLockedPackage(BasePackage $package): bool
     {
         return \in_array($package, $this->unacceptableFixedOrLockedPackages, true);
     }
@@ -263,7 +263,7 @@ class Pool implements \Countable
     /**
      * @return BasePackage[]
      */
-    public function getUnacceptableFixedOrLockedPackages()
+    public function getUnacceptableFixedOrLockedPackages(): array
     {
         return $this->unacceptableFixedOrLockedPackages;
     }
diff --git a/src/Composer/DependencyResolver/PoolBuilder.php b/src/Composer/DependencyResolver/PoolBuilder.php
index 69769ca7f..9766557e6 100644
--- a/src/Composer/DependencyResolver/PoolBuilder.php
+++ b/src/Composer/DependencyResolver/PoolBuilder.php
@@ -158,7 +158,7 @@ class PoolBuilder
      * @param RepositoryInterface[] $repositories
      * @return Pool
      */
-    public function buildPool(array $repositories, Request $request)
+    public function buildPool(array $repositories, Request $request): Pool
     {
         if ($request->getUpdateAllowList()) {
             $this->updateAllowList = $request->getUpdateAllowList();
@@ -300,7 +300,7 @@ class PoolBuilder
      * @param string $name
      * @return void
      */
-    private function markPackageNameForLoading(Request $request, $name, ConstraintInterface $constraint)
+    private function markPackageNameForLoading(Request $request, $name, ConstraintInterface $constraint): void
     {
         // Skip platform requires at this stage
         if (PlatformRepository::isPlatformPackage($name)) {
@@ -359,7 +359,7 @@ class PoolBuilder
      * @param RepositoryInterface[] $repositories
      * @return void
      */
-    private function loadPackagesMarkedForLoading(Request $request, array $repositories)
+    private function loadPackagesMarkedForLoading(Request $request, array $repositories): void
     {
         foreach ($this->packagesToLoad as $name => $constraint) {
             $this->loadedPackages[$name] = $constraint;
@@ -396,7 +396,7 @@ class PoolBuilder
      * @param RepositoryInterface[] $repositories
      * @return void
      */
-    private function loadPackage(Request $request, array $repositories, BasePackage $package, $propagateUpdate)
+    private function loadPackage(Request $request, array $repositories, BasePackage $package, $propagateUpdate): void
     {
         $index = $this->indexCounter++;
         $this->packages[$index] = $package;
@@ -501,7 +501,7 @@ class PoolBuilder
      * @param string $name packageName
      * @return bool
      */
-    private function isRootRequire(Request $request, $name)
+    private function isRootRequire(Request $request, $name): bool
     {
         $rootRequires = $request->getRequires();
 
@@ -512,7 +512,7 @@ class PoolBuilder
      * @param  string $name
      * @return string[]
      */
-    private function getSkippedRootRequires(Request $request, $name)
+    private function getSkippedRootRequires(Request $request, $name): array
     {
         if (!isset($this->skippedLoad[$name])) {
             return array();
@@ -522,7 +522,7 @@ class PoolBuilder
         $matches = array();
 
         if (isset($rootRequires[$name])) {
-            return array_map(function (PackageInterface $package) use ($name) {
+            return array_map(function (PackageInterface $package) use ($name): string {
                 if ($name !== $package->getName()) {
                     return $package->getName() .' (via replace of '.$name.')';
                 }
@@ -555,7 +555,7 @@ class PoolBuilder
      *
      * @return bool
      */
-    private function isUpdateAllowed(BasePackage $package)
+    private function isUpdateAllowed(BasePackage $package): bool
     {
         foreach ($this->updateAllowList as $pattern => $void) {
             $patternRegexp = BasePackage::packageNameToRegexp($pattern);
@@ -570,7 +570,7 @@ class PoolBuilder
     /**
      * @return void
      */
-    private function warnAboutNonMatchingUpdateAllowList(Request $request)
+    private function warnAboutNonMatchingUpdateAllowList(Request $request): void
     {
         foreach ($this->updateAllowList as $pattern => $void) {
             $patternRegexp = BasePackage::packageNameToRegexp($pattern);
@@ -602,7 +602,7 @@ class PoolBuilder
      * @param string $name
      * @return void
      */
-    private function unlockPackage(Request $request, array $repositories, $name)
+    private function unlockPackage(Request $request, array $repositories, $name): void
     {
         foreach ($this->skippedLoad[$name] as $packageOrReplacer) {
             // if we unfixed a replaced package name, we also need to unfix the replacer itself
@@ -668,7 +668,7 @@ class PoolBuilder
      * @param int $index
      * @return void
      */
-    private function removeLoadedPackage(Request $request, array $repositories, BasePackage $package, $index)
+    private function removeLoadedPackage(Request $request, array $repositories, BasePackage $package, $index): void
     {
         $repoIndex = array_search($package->getRepository(), $repositories, true);
 
@@ -686,7 +686,7 @@ class PoolBuilder
     /**
      * @return Pool
      */
-    private function runOptimizer(Request $request, Pool $pool)
+    private function runOptimizer(Request $request, Pool $pool): Pool
     {
         if (null === $this->poolOptimizer) {
             return $pool;
diff --git a/src/Composer/DependencyResolver/PoolOptimizer.php b/src/Composer/DependencyResolver/PoolOptimizer.php
index 7e1e32fdc..5c77eda79 100644
--- a/src/Composer/DependencyResolver/PoolOptimizer.php
+++ b/src/Composer/DependencyResolver/PoolOptimizer.php
@@ -71,7 +71,7 @@ class PoolOptimizer
     /**
      * @return Pool
      */
-    public function optimize(Request $request, Pool $pool)
+    public function optimize(Request $request, Pool $pool): Pool
     {
         $this->prepare($request, $pool);
 
@@ -99,7 +99,7 @@ class PoolOptimizer
     /**
      * @return void
      */
-    private function prepare(Request $request, Pool $pool)
+    private function prepare(Request $request, Pool $pool): void
     {
         $irremovablePackageConstraintGroups = array();
 
@@ -155,7 +155,7 @@ class PoolOptimizer
     /**
      * @return void
      */
-    private function markPackageIrremovable(BasePackage $package)
+    private function markPackageIrremovable(BasePackage $package): void
     {
         $this->irremovablePackages[$package->id] = true;
         if ($package instanceof AliasPackage) {
@@ -173,7 +173,7 @@ class PoolOptimizer
     /**
      * @return Pool Optimized pool
      */
-    private function applyRemovalsToPool(Pool $pool)
+    private function applyRemovalsToPool(Pool $pool): Pool
     {
         $packages = array();
         $removedVersions = array();
@@ -193,7 +193,7 @@ class PoolOptimizer
     /**
      * @return void
      */
-    private function optimizeByIdenticalDependencies(Request $request, Pool $pool)
+    private function optimizeByIdenticalDependencies(Request $request, Pool $pool): void
     {
         $identicalDefinitionsPerPackage = array();
         $packageIdenticalDefinitionLookup = array();
@@ -278,7 +278,7 @@ class PoolOptimizer
     /**
      * @return string
      */
-    private function calculateDependencyHash(BasePackage $package)
+    private function calculateDependencyHash(BasePackage $package): string
     {
         $hash = '';
 
@@ -322,7 +322,7 @@ class PoolOptimizer
      * @param int $id
      * @return void
      */
-    private function markPackageForRemoval($id)
+    private function markPackageForRemoval($id): void
     {
         // We are not allowed to remove packages if they have been marked as irremovable
         if (isset($this->irremovablePackages[$id])) {
@@ -337,7 +337,7 @@ class PoolOptimizer
      * @param array<int, array<string, array{groupHash: string, dependencyHash: string}>> $packageIdenticalDefinitionLookup
      * @return void
      */
-    private function keepPackage(BasePackage $package, $identicalDefinitionsPerPackage, $packageIdenticalDefinitionLookup)
+    private function keepPackage(BasePackage $package, $identicalDefinitionsPerPackage, $packageIdenticalDefinitionLookup): void
     {
         unset($this->packagesToRemove[$package->id]);
 
@@ -389,7 +389,7 @@ class PoolOptimizer
      *
      * @return void
      */
-    private function optimizeImpossiblePackagesAway(Request $request, Pool $pool)
+    private function optimizeImpossiblePackagesAway(Request $request, Pool $pool): void
     {
         if (count($request->getLockedPackages()) === 0) {
             return;
diff --git a/src/Composer/DependencyResolver/Problem.php b/src/Composer/DependencyResolver/Problem.php
index 124a544cb..77e2975c8 100644
--- a/src/Composer/DependencyResolver/Problem.php
+++ b/src/Composer/DependencyResolver/Problem.php
@@ -54,7 +54,7 @@ class Problem
      * @param Rule $rule A rule which is a reason for this problem
      * @return void
      */
-    public function addRule(Rule $rule)
+    public function addRule(Rule $rule): void
     {
         $this->addReason(spl_object_hash($rule), $rule);
     }
@@ -64,7 +64,7 @@ class Problem
      *
      * @return array<int, array<int, Rule>> The problem's reasons
      */
-    public function getReasons()
+    public function getReasons(): array
     {
         return $this->reasons;
     }
@@ -77,7 +77,7 @@ class Problem
      * @param array<Rule[]> $learnedPool
      * @return string
      */
-    public function getPrettyString(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, array $installedMap = array(), array $learnedPool = array())
+    public function getPrettyString(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, array $installedMap = array(), array $learnedPool = array()): string
     {
         // TODO doesn't this entirely defeat the purpose of the problem sections? what's the point of sections?
         $reasons = call_user_func_array('array_merge', array_reverse($this->reasons));
@@ -117,7 +117,7 @@ class Problem
      * @return string
      * @internal
      */
-    public static function formatDeduplicatedRules($rules, $indent, RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, array $installedMap = array(), array $learnedPool = array())
+    public static function formatDeduplicatedRules($rules, $indent, RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, array $installedMap = array(), array $learnedPool = array()): string
     {
         $messages = array();
         $templates = array();
@@ -165,7 +165,7 @@ class Problem
     /**
      * @return bool
      */
-    public function isCausedByLock(RepositorySet $repositorySet, Request $request, Pool $pool)
+    public function isCausedByLock(RepositorySet $repositorySet, Request $request, Pool $pool): bool
     {
         foreach ($this->reasons as $sectionRules) {
             foreach ($sectionRules as $rule) {
@@ -185,7 +185,7 @@ class Problem
      * @param Rule   $reason The reason descriptor
      * @return void
      */
-    protected function addReason($id, Rule $reason)
+    protected function addReason($id, Rule $reason): void
     {
         // TODO: if a rule is part of a problem description in two sections, isn't this going to remove a message
         // that is important to understand the issue?
@@ -199,7 +199,7 @@ class Problem
     /**
      * @return void
      */
-    public function nextSection()
+    public function nextSection(): void
     {
         $this->section++;
     }
@@ -210,7 +210,7 @@ class Problem
      * @param string $packageName
      * @return array{0: string, 1: string}
      */
-    public static function getMissingPackageReason(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, $packageName, ConstraintInterface $constraint = null)
+    public static function getMissingPackageReason(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, $packageName, ConstraintInterface $constraint = null): array
     {
         if (PlatformRepository::isPlatformPackage($packageName)) {
             // handle php/php-*/hhvm
@@ -286,7 +286,7 @@ class Problem
         if ($packages = $repositorySet->findPackages($packageName, $constraint)) {
             $rootReqs = $repositorySet->getRootRequires();
             if (isset($rootReqs[$packageName])) {
-                $filtered = array_filter($packages, function ($p) use ($rootReqs, $packageName) {
+                $filtered = array_filter($packages, function ($p) use ($rootReqs, $packageName): bool {
                     return $rootReqs[$packageName]->matches(new Constraint('==', $p->getVersion()));
                 });
                 if (0 === count($filtered)) {
@@ -296,7 +296,7 @@ class Problem
 
             if ($lockedPackage) {
                 $fixedConstraint = new Constraint('==', $lockedPackage->getVersion());
-                $filtered = array_filter($packages, function ($p) use ($fixedConstraint) {
+                $filtered = array_filter($packages, function ($p) use ($fixedConstraint): bool {
                     return $fixedConstraint->matches(new Constraint('==', $p->getVersion()));
                 });
                 if (0 === count($filtered)) {
@@ -304,7 +304,7 @@ class Problem
                 }
             }
 
-            $nonLockedPackages = array_filter($packages, function ($p) {
+            $nonLockedPackages = array_filter($packages, function ($p): bool {
                 return !$p->getRepository() instanceof LockArrayRepository;
             });
 
@@ -360,7 +360,7 @@ class Problem
 
         if ($providers = $repositorySet->getProviders($packageName)) {
             $maxProviders = 20;
-            $providersStr = implode(array_map(function ($p) {
+            $providersStr = implode(array_map(function ($p): string {
                 $description = $p['description'] ? ' '.substr($p['description'], 0, 100) : '';
 
                 return "      - ${p['name']}".$description."\n";
@@ -382,7 +382,7 @@ class Problem
      * @param bool $useRemovedVersionGroup
      * @return string
      */
-    public static function getPackageList(array $packages, $isVerbose, Pool $pool = null, ConstraintInterface $constraint = null, $useRemovedVersionGroup = false)
+    public static function getPackageList(array $packages, $isVerbose, Pool $pool = null, ConstraintInterface $constraint = null, $useRemovedVersionGroup = false): string
     {
         $prepared = array();
         $hasDefaultBranch = array();
@@ -427,7 +427,7 @@ class Problem
      * @param  string $version the effective runtime version of the platform package
      * @return ?string a version string or null if it appears the package was artificially disabled
      */
-    private static function getPlatformPackageVersion(Pool $pool, $packageName, $version)
+    private static function getPlatformPackageVersion(Pool $pool, $packageName, $version): ?string
     {
         $available = $pool->whatProvides($packageName);
 
@@ -471,7 +471,7 @@ class Problem
      * @param int $maxDev
      * @return list<string> a list of pretty versions and '...' where versions were removed
      */
-    private static function condenseVersionList(array $versions, $max, $maxDev = 16)
+    private static function condenseVersionList(array $versions, $max, $maxDev = 16): array
     {
         if (count($versions) <= $max) {
             return $versions;
@@ -505,7 +505,7 @@ class Problem
      * @param PackageInterface[] $packages
      * @return bool
      */
-    private static function hasMultipleNames(array $packages)
+    private static function hasMultipleNames(array $packages): bool
     {
         $name = null;
         foreach ($packages as $package) {
@@ -527,7 +527,7 @@ class Problem
      * @param string $reason
      * @return array{0: string, 1: string}
      */
-    private static function computeCheckForLowerPrioRepo(Pool $pool, $isVerbose, $packageName, array $higherRepoPackages, array $allReposPackages, $reason, ConstraintInterface $constraint = null)
+    private static function computeCheckForLowerPrioRepo(Pool $pool, $isVerbose, $packageName, array $higherRepoPackages, array $allReposPackages, $reason, ConstraintInterface $constraint = null): array
     {
         $nextRepoPackages = array();
         $nextRepo = null;
@@ -576,7 +576,7 @@ class Problem
      *
      * @return string
      */
-    protected static function constraintToText(ConstraintInterface $constraint = null)
+    protected static function constraintToText(ConstraintInterface $constraint = null): string
     {
         return $constraint ? ' '.$constraint->getPrettyString() : '';
     }
diff --git a/src/Composer/DependencyResolver/Request.php b/src/Composer/DependencyResolver/Request.php
index 3b680ad0b..508a8f0b2 100644
--- a/src/Composer/DependencyResolver/Request.php
+++ b/src/Composer/DependencyResolver/Request.php
@@ -64,7 +64,7 @@ class Request
      * @param string $packageName
      * @return void
      */
-    public function requireName($packageName, ConstraintInterface $constraint = null)
+    public function requireName($packageName, ConstraintInterface $constraint = null): void
     {
         $packageName = strtolower($packageName);
 
@@ -85,7 +85,7 @@ class Request
      *
      * @return void
      */
-    public function fixPackage(BasePackage $package)
+    public function fixPackage(BasePackage $package): void
     {
         $this->fixedPackages[spl_object_hash($package)] = $package;
     }
@@ -102,7 +102,7 @@ class Request
      *
      * @return void
      */
-    public function lockPackage(BasePackage $package)
+    public function lockPackage(BasePackage $package): void
     {
         $this->lockedPackages[spl_object_hash($package)] = $package;
     }
@@ -116,7 +116,7 @@ class Request
      *
      * @return void
      */
-    public function fixLockedPackage(BasePackage $package)
+    public function fixLockedPackage(BasePackage $package): void
     {
         $this->fixedPackages[spl_object_hash($package)] = $package;
         $this->fixedLockedPackages[spl_object_hash($package)] = $package;
@@ -125,7 +125,7 @@ class Request
     /**
      * @return void
      */
-    public function unlockPackage(BasePackage $package)
+    public function unlockPackage(BasePackage $package): void
     {
         unset($this->lockedPackages[spl_object_hash($package)]);
     }
@@ -135,7 +135,7 @@ class Request
      * @param false|self::UPDATE_* $updateAllowTransitiveDependencies
      * @return void
      */
-    public function setUpdateAllowList($updateAllowList, $updateAllowTransitiveDependencies)
+    public function setUpdateAllowList($updateAllowList, $updateAllowTransitiveDependencies): void
     {
         $this->updateAllowList = $updateAllowList;
         $this->updateAllowTransitiveDependencies = $updateAllowTransitiveDependencies;
@@ -144,7 +144,7 @@ class Request
     /**
      * @return string[]
      */
-    public function getUpdateAllowList()
+    public function getUpdateAllowList(): array
     {
         return $this->updateAllowList;
     }
@@ -152,7 +152,7 @@ class Request
     /**
      * @return bool
      */
-    public function getUpdateAllowTransitiveDependencies()
+    public function getUpdateAllowTransitiveDependencies(): bool
     {
         return $this->updateAllowTransitiveDependencies !== self::UPDATE_ONLY_LISTED;
     }
@@ -160,7 +160,7 @@ class Request
     /**
      * @return bool
      */
-    public function getUpdateAllowTransitiveRootDependencies()
+    public function getUpdateAllowTransitiveRootDependencies(): bool
     {
         return $this->updateAllowTransitiveDependencies === self::UPDATE_LISTED_WITH_TRANSITIVE_DEPS;
     }
@@ -168,7 +168,7 @@ class Request
     /**
      * @return array<string, ConstraintInterface>
      */
-    public function getRequires()
+    public function getRequires(): array
     {
         return $this->requires;
     }
@@ -176,7 +176,7 @@ class Request
     /**
      * @return array<string, BasePackage>
      */
-    public function getFixedPackages()
+    public function getFixedPackages(): array
     {
         return $this->fixedPackages;
     }
@@ -184,7 +184,7 @@ class Request
     /**
      * @return bool
      */
-    public function isFixedPackage(BasePackage $package)
+    public function isFixedPackage(BasePackage $package): bool
     {
         return isset($this->fixedPackages[spl_object_hash($package)]);
     }
@@ -192,7 +192,7 @@ class Request
     /**
      * @return array<string, BasePackage>
      */
-    public function getLockedPackages()
+    public function getLockedPackages(): array
     {
         return $this->lockedPackages;
     }
@@ -200,7 +200,7 @@ class Request
     /**
      * @return bool
      */
-    public function isLockedPackage(PackageInterface $package)
+    public function isLockedPackage(PackageInterface $package): bool
     {
         return isset($this->lockedPackages[spl_object_hash($package)]) || isset($this->fixedLockedPackages[spl_object_hash($package)]);
     }
@@ -208,7 +208,7 @@ class Request
     /**
      * @return array<string, BasePackage>
      */
-    public function getFixedOrLockedPackages()
+    public function getFixedOrLockedPackages(): array
     {
         return array_merge($this->fixedPackages, $this->lockedPackages);
     }
@@ -222,7 +222,7 @@ class Request
      *       Some locked packages may not be in the pool,
      *       so they have a package->id of -1
      */
-    public function getPresentMap($packageIds = false)
+    public function getPresentMap($packageIds = false): array
     {
         $presentMap = array();
 
@@ -242,7 +242,7 @@ class Request
     /**
      * @return BasePackage[]
      */
-    public function getFixedPackagesMap()
+    public function getFixedPackagesMap(): array
     {
         $fixedPackagesMap = array();
 
@@ -256,7 +256,7 @@ class Request
     /**
      * @return ?LockArrayRepository
      */
-    public function getLockedRepository()
+    public function getLockedRepository(): ?LockArrayRepository
     {
         return $this->lockedRepository;
     }
diff --git a/src/Composer/DependencyResolver/Rule.php b/src/Composer/DependencyResolver/Rule.php
index 133e7f7a0..1e2d08daa 100644
--- a/src/Composer/DependencyResolver/Rule.php
+++ b/src/Composer/DependencyResolver/Rule.php
@@ -71,7 +71,7 @@ abstract class Rule
     /**
      * @return int[]
      */
-    abstract public function getLiterals();
+    abstract public function getLiterals(): array;
 
     /**
      * @return int|string
@@ -84,12 +84,12 @@ abstract class Rule
      * @param Rule $rule
      * @return bool
      */
-    abstract public function equals(Rule $rule);
+    abstract public function equals(Rule $rule): bool;
 
     /**
      * @return int
      */
-    public function getReason()
+    public function getReason(): int
     {
         return ($this->bitfield & (255 << self::BITFIELD_REASON)) >> self::BITFIELD_REASON;
     }
@@ -105,7 +105,7 @@ abstract class Rule
     /**
      * @return string|null
      */
-    public function getRequiredPackage()
+    public function getRequiredPackage(): ?string
     {
         $reason = $this->getReason();
 
@@ -128,7 +128,7 @@ abstract class Rule
      * @param RuleSet::TYPE_* $type
      * @return void
      */
-    public function setType($type)
+    public function setType($type): void
     {
         $this->bitfield = ($this->bitfield & ~(255 << self::BITFIELD_TYPE)) | ((255 & $type) << self::BITFIELD_TYPE);
     }
@@ -136,7 +136,7 @@ abstract class Rule
     /**
      * @return int
      */
-    public function getType()
+    public function getType(): int
     {
         return ($this->bitfield & (255 << self::BITFIELD_TYPE)) >> self::BITFIELD_TYPE;
     }
@@ -144,7 +144,7 @@ abstract class Rule
     /**
      * @return void
      */
-    public function disable()
+    public function disable(): void
     {
         $this->bitfield = ($this->bitfield & ~(255 << self::BITFIELD_DISABLED)) | (1 << self::BITFIELD_DISABLED);
     }
@@ -152,7 +152,7 @@ abstract class Rule
     /**
      * @return void
      */
-    public function enable()
+    public function enable(): void
     {
         $this->bitfield &= ~(255 << self::BITFIELD_DISABLED);
     }
@@ -160,7 +160,7 @@ abstract class Rule
     /**
      * @return bool
      */
-    public function isDisabled()
+    public function isDisabled(): bool
     {
         return (bool) (($this->bitfield & (255 << self::BITFIELD_DISABLED)) >> self::BITFIELD_DISABLED);
     }
@@ -168,7 +168,7 @@ abstract class Rule
     /**
      * @return bool
      */
-    public function isEnabled()
+    public function isEnabled(): bool
     {
         return !(($this->bitfield & (255 << self::BITFIELD_DISABLED)) >> self::BITFIELD_DISABLED);
     }
@@ -176,12 +176,12 @@ abstract class Rule
     /**
      * @return bool
      */
-    abstract public function isAssertion();
+    abstract public function isAssertion(): bool;
 
     /**
      * @return bool
      */
-    public function isCausedByLock(RepositorySet $repositorySet, Request $request, Pool $pool)
+    public function isCausedByLock(RepositorySet $repositorySet, Request $request, Pool $pool): bool
     {
         if ($this->getReason() === self::RULE_PACKAGE_REQUIRES) {
             if (PlatformRepository::isPlatformPackage($this->reasonData->getTarget())) {
@@ -232,7 +232,7 @@ abstract class Rule
      * @internal
      * @return BasePackage
      */
-    public function getSourcePackage(Pool $pool)
+    public function getSourcePackage(Pool $pool): BasePackage
     {
         $literals = $this->getLiterals();
 
@@ -267,7 +267,7 @@ abstract class Rule
      * @param array<Rule[]> $learnedPool
      * @return string
      */
-    public function getPrettyString(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, array $installedMap = array(), array $learnedPool = array())
+    public function getPrettyString(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, array $installedMap = array(), array $learnedPool = array()): string
     {
         $literals = $this->getLiterals();
 
@@ -281,7 +281,7 @@ abstract class Rule
                     return 'No package found to satisfy root composer.json require '.$packageName.($constraint ? ' '.$constraint->getPrettyString() : '');
                 }
 
-                $packagesNonAlias = array_values(array_filter($packages, function ($p) {
+                $packagesNonAlias = array_values(array_filter($packages, function ($p): bool {
                     return !($p instanceof AliasPackage);
                 }));
                 if (count($packagesNonAlias) === 1) {
@@ -484,7 +484,7 @@ abstract class Rule
      * @param bool $useRemovedVersionGroup
      * @return string
      */
-    protected function formatPackagesUnique(Pool $pool, array $packages, $isVerbose, ConstraintInterface $constraint = null, $useRemovedVersionGroup = false)
+    protected function formatPackagesUnique(Pool $pool, array $packages, $isVerbose, ConstraintInterface $constraint = null, $useRemovedVersionGroup = false): string
     {
         foreach ($packages as $index => $package) {
             if (!\is_object($package)) {
@@ -498,7 +498,7 @@ abstract class Rule
     /**
      * @return BasePackage
      */
-    private function deduplicateDefaultBranchAlias(BasePackage $package)
+    private function deduplicateDefaultBranchAlias(BasePackage $package): BasePackage
     {
         if ($package instanceof AliasPackage && $package->getPrettyVersion() === VersionParser::DEFAULT_BRANCH_ALIAS) {
             $package = $package->getAliasOf();
diff --git a/src/Composer/DependencyResolver/Rule2Literals.php b/src/Composer/DependencyResolver/Rule2Literals.php
index cf5e20792..f07d011fc 100644
--- a/src/Composer/DependencyResolver/Rule2Literals.php
+++ b/src/Composer/DependencyResolver/Rule2Literals.php
@@ -45,7 +45,7 @@ class Rule2Literals extends Rule
     }
 
     /** @return int[] */
-    public function getLiterals()
+    public function getLiterals(): array
     {
         return array($this->literal1, $this->literal2);
     }
@@ -66,7 +66,7 @@ class Rule2Literals extends Rule
      * @param  Rule $rule The rule to check against
      * @return bool Whether the rules are equal
      */
-    public function equals(Rule $rule)
+    public function equals(Rule $rule): bool
     {
         // specialized fast-case
         if ($rule instanceof self) {
@@ -98,7 +98,7 @@ class Rule2Literals extends Rule
     }
 
     /** @return false */
-    public function isAssertion()
+    public function isAssertion(): bool
     {
         return false;
     }
diff --git a/src/Composer/DependencyResolver/RuleSet.php b/src/Composer/DependencyResolver/RuleSet.php
index d83320c52..68c2b7905 100644
--- a/src/Composer/DependencyResolver/RuleSet.php
+++ b/src/Composer/DependencyResolver/RuleSet.php
@@ -59,7 +59,7 @@ class RuleSet implements \IteratorAggregate, \Countable
      * @param self::TYPE_* $type
      * @return void
      */
-    public function add(Rule $rule, $type)
+    public function add(Rule $rule, $type): void
     {
         if (!isset(self::$types[$type])) {
             throw new \OutOfBoundsException('Unknown rule type: ' . $type);
@@ -112,13 +112,13 @@ class RuleSet implements \IteratorAggregate, \Countable
      * @param int $id
      * @return Rule
      */
-    public function ruleById($id)
+    public function ruleById($id): Rule
     {
         return $this->ruleById[$id];
     }
 
     /** @return array<self::TYPE_*, Rule[]> */
-    public function getRules()
+    public function getRules(): array
     {
         return $this->rules;
     }
@@ -132,7 +132,7 @@ class RuleSet implements \IteratorAggregate, \Countable
      * @param  self::TYPE_*|array<self::TYPE_*> $types
      * @return RuleSetIterator
      */
-    public function getIteratorFor($types)
+    public function getIteratorFor($types): RuleSetIterator
     {
         if (!\is_array($types)) {
             $types = array($types);
@@ -154,7 +154,7 @@ class RuleSet implements \IteratorAggregate, \Countable
      * @param array<self::TYPE_*>|self::TYPE_* $types
      * @return RuleSetIterator
      */
-    public function getIteratorWithout($types)
+    public function getIteratorWithout($types): RuleSetIterator
     {
         if (!\is_array($types)) {
             $types = array($types);
@@ -170,7 +170,7 @@ class RuleSet implements \IteratorAggregate, \Countable
     }
 
     /** @return array{0: 0, 1: 1, 2: 4} */
-    public function getTypes()
+    public function getTypes(): array
     {
         $types = self::$types;
 
@@ -181,7 +181,7 @@ class RuleSet implements \IteratorAggregate, \Countable
      * @param bool $isVerbose
      * @return string
      */
-    public function getPrettyString(RepositorySet $repositorySet = null, Request $request = null, Pool $pool = null, $isVerbose = false)
+    public function getPrettyString(RepositorySet $repositorySet = null, Request $request = null, Pool $pool = null, $isVerbose = false): string
     {
         $string = "\n";
         foreach ($this->rules as $type => $rules) {
diff --git a/src/Composer/DependencyResolver/RuleSetGenerator.php b/src/Composer/DependencyResolver/RuleSetGenerator.php
index fef50b0eb..71f86864b 100644
--- a/src/Composer/DependencyResolver/RuleSetGenerator.php
+++ b/src/Composer/DependencyResolver/RuleSetGenerator.php
@@ -56,7 +56,7 @@ class RuleSetGenerator
      *
      * @phpstan-param ReasonData $reasonData
      */
-    protected function createRequireRule(BasePackage $package, array $providers, $reason, $reasonData = null)
+    protected function createRequireRule(BasePackage $package, array $providers, $reason, $reasonData = null): ?Rule
     {
         $literals = array(-$package->id);
 
@@ -85,7 +85,7 @@ class RuleSetGenerator
      *
      * @phpstan-param ReasonData $reasonData
      */
-    protected function createInstallOneOfRule(array $packages, $reason, $reasonData)
+    protected function createInstallOneOfRule(array $packages, $reason, $reasonData): Rule
     {
         $literals = array();
         foreach ($packages as $package) {
@@ -109,7 +109,7 @@ class RuleSetGenerator
      *
      * @phpstan-param ReasonData $reasonData
      */
-    protected function createRule2Literals(BasePackage $issuer, BasePackage $provider, $reason, $reasonData = null)
+    protected function createRule2Literals(BasePackage $issuer, BasePackage $provider, $reason, $reasonData = null): ?Rule
     {
         // ignore self conflict
         if ($issuer === $provider) {
@@ -127,7 +127,7 @@ class RuleSetGenerator
      *
      * @phpstan-param ReasonData $reasonData
      */
-    protected function createMultiConflictRule(array $packages, $reason, $reasonData)
+    protected function createMultiConflictRule(array $packages, $reason, $reasonData): Rule
     {
         $literals = array();
         foreach ($packages as $package) {
@@ -152,7 +152,7 @@ class RuleSetGenerator
      *
      * @return void
      */
-    private function addRule($type, Rule $newRule = null)
+    private function addRule($type, Rule $newRule = null): void
     {
         if (!$newRule) {
             return;
@@ -164,7 +164,7 @@ class RuleSetGenerator
     /**
      * @return void
      */
-    protected function addRulesForPackage(BasePackage $package, PlatformRequirementFilterInterface $platformRequirementFilter)
+    protected function addRulesForPackage(BasePackage $package, PlatformRequirementFilterInterface $platformRequirementFilter): void
     {
         /** @var \SplQueue<BasePackage> */
         $workQueue = new \SplQueue;
@@ -218,7 +218,7 @@ class RuleSetGenerator
     /**
      * @return void
      */
-    protected function addConflictRules(PlatformRequirementFilterInterface $platformRequirementFilter)
+    protected function addConflictRules(PlatformRequirementFilterInterface $platformRequirementFilter): void
     {
         /** @var BasePackage $package */
         foreach ($this->addedMap as $package) {
@@ -259,7 +259,7 @@ class RuleSetGenerator
     /**
      * @return void
      */
-    protected function addRulesForRequest(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter)
+    protected function addRulesForRequest(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter): void
     {
         foreach ($request->getFixedPackages() as $package) {
             if ($package->id == -1) {
@@ -305,7 +305,7 @@ class RuleSetGenerator
     /**
      * @return void
      */
-    protected function addRulesForRootAliases(PlatformRequirementFilterInterface $platformRequirementFilter)
+    protected function addRulesForRootAliases(PlatformRequirementFilterInterface $platformRequirementFilter): void
     {
         foreach ($this->pool->getPackages() as $package) {
             // ensure that rules for root alias packages and aliases of packages which were loaded are also loaded
@@ -323,7 +323,7 @@ class RuleSetGenerator
     /**
      * @return RuleSet
      */
-    public function getRulesFor(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter = null)
+    public function getRulesFor(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter = null): RuleSet
     {
         $platformRequirementFilter = $platformRequirementFilter ?: PlatformRequirementFilterFactory::ignoreNothing();
 
diff --git a/src/Composer/DependencyResolver/RuleWatchChain.php b/src/Composer/DependencyResolver/RuleWatchChain.php
index a03aaf2f6..a54032686 100644
--- a/src/Composer/DependencyResolver/RuleWatchChain.php
+++ b/src/Composer/DependencyResolver/RuleWatchChain.php
@@ -29,7 +29,7 @@ class RuleWatchChain extends \SplDoublyLinkedList
      * @param int $offset The offset to seek to.
      * @return void
      */
-    public function seek($offset)
+    public function seek($offset): void
     {
         $this->rewind();
         for ($i = 0; $i < $offset; $i++, $this->next());
@@ -45,7 +45,7 @@ class RuleWatchChain extends \SplDoublyLinkedList
      *
      * @return void
      */
-    public function remove()
+    public function remove(): void
     {
         $offset = $this->key();
         $this->offsetUnset($offset);
diff --git a/src/Composer/DependencyResolver/RuleWatchGraph.php b/src/Composer/DependencyResolver/RuleWatchGraph.php
index e9e3caa17..03da05ce2 100644
--- a/src/Composer/DependencyResolver/RuleWatchGraph.php
+++ b/src/Composer/DependencyResolver/RuleWatchGraph.php
@@ -40,7 +40,7 @@ class RuleWatchGraph
      * @param RuleWatchNode $node The rule node to be inserted into the graph
      * @return void
      */
-    public function insert(RuleWatchNode $node)
+    public function insert(RuleWatchNode $node): void
     {
         if ($node->getRule()->isAssertion()) {
             return;
@@ -88,7 +88,7 @@ class RuleWatchGraph
      *                                   register decisions resulting from propagation
      * @return Rule|null If a conflict is found the conflicting rule is returned
      */
-    public function propagateLiteral($decidedLiteral, $level, Decisions $decisions)
+    public function propagateLiteral($decidedLiteral, $level, Decisions $decisions): ?Rule
     {
         // we invert the decided literal here, example:
         // A was decided => (-A|B) now requires B to be true, so we look for
@@ -110,7 +110,7 @@ class RuleWatchGraph
                 if (!$node->getRule()->isDisabled() && !$decisions->satisfy($otherWatch)) {
                     $ruleLiterals = $node->getRule()->getLiterals();
 
-                    $alternativeLiterals = array_filter($ruleLiterals, function ($ruleLiteral) use ($literal, $otherWatch, $decisions) {
+                    $alternativeLiterals = array_filter($ruleLiterals, function ($ruleLiteral) use ($literal, $otherWatch, $decisions): bool {
                         return $literal !== $ruleLiteral &&
                             $otherWatch !== $ruleLiteral &&
                             !$decisions->conflict($ruleLiteral);
@@ -156,7 +156,7 @@ class RuleWatchGraph
      * @param RuleWatchNode $node        The rule node to be moved
      * @return void
      */
-    protected function moveWatch($fromLiteral, $toLiteral, RuleWatchNode $node)
+    protected function moveWatch($fromLiteral, $toLiteral, RuleWatchNode $node): void
     {
         if (!isset($this->watchChains[$toLiteral])) {
             $this->watchChains[$toLiteral] = new RuleWatchChain;
diff --git a/src/Composer/DependencyResolver/RuleWatchNode.php b/src/Composer/DependencyResolver/RuleWatchNode.php
index a0abf254d..136ac4a86 100644
--- a/src/Composer/DependencyResolver/RuleWatchNode.php
+++ b/src/Composer/DependencyResolver/RuleWatchNode.php
@@ -54,7 +54,7 @@ class RuleWatchNode
      * @param Decisions $decisions The decisions made so far by the solver
      * @return void
      */
-    public function watch2OnHighest(Decisions $decisions)
+    public function watch2OnHighest(Decisions $decisions): void
     {
         $literals = $this->rule->getLiterals();
 
@@ -80,7 +80,7 @@ class RuleWatchNode
      *
      * @return Rule
      */
-    public function getRule()
+    public function getRule(): Rule
     {
         return $this->rule;
     }
@@ -91,7 +91,7 @@ class RuleWatchNode
      * @param  int $literal The watched literal that should not be returned
      * @return int A literal
      */
-    public function getOtherWatch($literal)
+    public function getOtherWatch($literal): int
     {
         if ($this->watch1 == $literal) {
             return $this->watch2;
@@ -107,7 +107,7 @@ class RuleWatchNode
      * @param int $to   The literal to be watched now
      * @return void
      */
-    public function moveWatch($from, $to)
+    public function moveWatch($from, $to): void
     {
         if ($this->watch1 == $from) {
             $this->watch1 = $to;
diff --git a/src/Composer/DependencyResolver/Solver.php b/src/Composer/DependencyResolver/Solver.php
index b0a3eb00f..43f253969 100644
--- a/src/Composer/DependencyResolver/Solver.php
+++ b/src/Composer/DependencyResolver/Solver.php
@@ -68,7 +68,7 @@ class Solver
     /**
      * @return int
      */
-    public function getRuleSetSize()
+    public function getRuleSetSize(): int
     {
         return \count($this->rules);
     }
@@ -76,7 +76,7 @@ class Solver
     /**
      * @return Pool
      */
-    public function getPool()
+    public function getPool(): Pool
     {
         return $this->pool;
     }
@@ -86,7 +86,7 @@ class Solver
     /**
      * @return void
      */
-    private function makeAssertionRuleDecisions()
+    private function makeAssertionRuleDecisions(): void
     {
         $decisionStart = \count($this->decisions) - 1;
 
@@ -159,7 +159,7 @@ class Solver
     /**
      * @return void
      */
-    protected function setupFixedMap(Request $request)
+    protected function setupFixedMap(Request $request): void
     {
         $this->fixedMap = array();
         foreach ($request->getFixedPackages() as $package) {
@@ -170,7 +170,7 @@ class Solver
     /**
      * @return void
      */
-    protected function checkForRootRequireProblems(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter)
+    protected function checkForRootRequireProblems(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter): void
     {
         foreach ($request->getRequires() as $packageName => $constraint) {
             if ($platformRequirementFilter->isIgnored($packageName)) {
@@ -190,7 +190,7 @@ class Solver
     /**
      * @return LockTransaction
      */
-    public function solve(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter = null)
+    public function solve(Request $request, PlatformRequirementFilterInterface $platformRequirementFilter = null): LockTransaction
     {
         $platformRequirementFilter = $platformRequirementFilter ?: PlatformRequirementFilterFactory::ignoreNothing();
 
@@ -233,7 +233,7 @@ class Solver
      * @param  int       $level
      * @return Rule|null A rule on conflict, otherwise null.
      */
-    protected function propagate($level)
+    protected function propagate($level): ?Rule
     {
         while ($this->decisions->validOffset($this->propagateIndex)) {
             $decision = $this->decisions->atOffset($this->propagateIndex);
@@ -261,7 +261,7 @@ class Solver
      *
      * @return void
      */
-    private function revert($level)
+    private function revert($level): void
     {
         while (!$this->decisions->isEmpty()) {
             $literal = $this->decisions->lastLiteral();
@@ -302,7 +302,7 @@ class Solver
      * @param  string|int $literal
      * @return int
      */
-    private function setPropagateLearn($level, $literal, Rule $rule)
+    private function setPropagateLearn($level, $literal, Rule $rule): int
     {
         $level++;
 
@@ -351,7 +351,7 @@ class Solver
      * @param  int[] $decisionQueue
      * @return int
      */
-    private function selectAndInstall($level, array $decisionQueue, Rule $rule)
+    private function selectAndInstall($level, array $decisionQueue, Rule $rule): int
     {
         // choose best package to install from decisionQueue
         $literals = $this->policy->selectPreferredPackages($this->pool, $decisionQueue, $rule->getRequiredPackage());
@@ -370,7 +370,7 @@ class Solver
      * @param  int   $level
      * @return array{int, int, GenericRule, int}
      */
-    protected function analyze($level, Rule $rule)
+    protected function analyze($level, Rule $rule): array
     {
         $analyzedRule = $rule;
         $ruleLevel = 1;
@@ -518,7 +518,7 @@ class Solver
      * @param array<string, true> $ruleSeen
      * @return void
      */
-    private function analyzeUnsolvableRule(Problem $problem, Rule $conflictRule, array &$ruleSeen)
+    private function analyzeUnsolvableRule(Problem $problem, Rule $conflictRule, array &$ruleSeen): void
     {
         $why = spl_object_hash($conflictRule);
         $ruleSeen[$why] = true;
@@ -548,7 +548,7 @@ class Solver
     /**
      * @return int
      */
-    private function analyzeUnsolvable(Rule $conflictRule)
+    private function analyzeUnsolvable(Rule $conflictRule): int
     {
         $problem = new Problem();
         $problem->addRule($conflictRule);
@@ -606,7 +606,7 @@ class Solver
      *
      * @return void
      */
-    private function enableDisableLearnedRules()
+    private function enableDisableLearnedRules(): void
     {
         foreach ($this->rules->getIteratorFor(RuleSet::TYPE_LEARNED) as $rule) {
             $why = $this->learnedWhy[spl_object_hash($rule)];
@@ -631,7 +631,7 @@ class Solver
     /**
      * @return void
      */
-    private function runSat()
+    private function runSat(): void
     {
         $this->propagateIndex = 0;
 
diff --git a/src/Composer/DependencyResolver/SolverProblemsException.php b/src/Composer/DependencyResolver/SolverProblemsException.php
index deaec7d24..fb5dfdde8 100644
--- a/src/Composer/DependencyResolver/SolverProblemsException.php
+++ b/src/Composer/DependencyResolver/SolverProblemsException.php
@@ -46,7 +46,7 @@ class SolverProblemsException extends \RuntimeException
      * @param bool $isDevExtraction
      * @return string
      */
-    public function getPrettyString(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, $isDevExtraction = false)
+    public function getPrettyString(RepositorySet $repositorySet, Request $request, Pool $pool, $isVerbose, $isDevExtraction = false): string
     {
         $installedMap = $request->getPresentMap(true);
         $missingExtensions = array();
@@ -100,7 +100,7 @@ class SolverProblemsException extends \RuntimeException
     /**
      * @return Problem[]
      */
-    public function getProblems()
+    public function getProblems(): array
     {
         return $this->problems;
     }
@@ -109,7 +109,7 @@ class SolverProblemsException extends \RuntimeException
      * @param string[] $missingExtensions
      * @return string
      */
-    private function createExtensionHint(array $missingExtensions)
+    private function createExtensionHint(array $missingExtensions): string
     {
         $paths = IniHelper::getAll();
 
@@ -133,7 +133,7 @@ class SolverProblemsException extends \RuntimeException
      * @param Rule[][] $reasonSets
      * @return string[]
      */
-    private function getExtensionProblems(array $reasonSets)
+    private function getExtensionProblems(array $reasonSets): array
     {
         $missingExtensions = array();
         foreach ($reasonSets as $reasonSet) {
diff --git a/src/Composer/DependencyResolver/Transaction.php b/src/Composer/DependencyResolver/Transaction.php
index ef9c74174..2d3272c51 100644
--- a/src/Composer/DependencyResolver/Transaction.php
+++ b/src/Composer/DependencyResolver/Transaction.php
@@ -60,7 +60,7 @@ class Transaction
     /**
      * @return OperationInterface[]
      */
-    public function getOperations()
+    public function getOperations(): array
     {
         return $this->operations;
     }
@@ -69,9 +69,9 @@ class Transaction
      * @param PackageInterface[] $resultPackages
      * @return void
      */
-    private function setResultPackageMaps($resultPackages)
+    private function setResultPackageMaps($resultPackages): void
     {
-        $packageSort = function (PackageInterface $a, PackageInterface $b) {
+        $packageSort = function (PackageInterface $a, PackageInterface $b): int {
             // sort alias packages by the same name behind their non alias version
             if ($a->getName() == $b->getName()) {
                 if ($a instanceof AliasPackage != $b instanceof AliasPackage) {
@@ -101,7 +101,7 @@ class Transaction
     /**
      * @return OperationInterface[]
      */
-    protected function calculateOperations()
+    protected function calculateOperations(): array
     {
         $operations = array();
 
@@ -218,7 +218,7 @@ class Transaction
      *
      * @return array<string, PackageInterface>
      */
-    protected function getRootPackages()
+    protected function getRootPackages(): array
     {
         $roots = $this->resultPackageMap;
 
@@ -244,7 +244,7 @@ class Transaction
     /**
      * @return PackageInterface[]
      */
-    protected function getProvidersInResult(Link $link)
+    protected function getProvidersInResult(Link $link): array
     {
         if (!isset($this->resultPackagesByName[$link->getTarget()])) {
             return array();
@@ -266,7 +266,7 @@ class Transaction
      * @param  OperationInterface[] $operations
      * @return OperationInterface[] reordered operation list
      */
-    private function movePluginsToFront(array $operations)
+    private function movePluginsToFront(array $operations): array
     {
         $dlModifyingPluginsNoDeps = array();
         $dlModifyingPluginsWithDeps = array();
@@ -289,7 +289,7 @@ class Transaction
             // is this a downloads modifying plugin or a dependency of one?
             if ($isDownloadsModifyingPlugin || count(array_intersect($package->getNames(), $dlModifyingPluginRequires))) {
                 // get the package's requires, but filter out any platform requirements
-                $requires = array_filter(array_keys($package->getRequires()), function ($req) {
+                $requires = array_filter(array_keys($package->getRequires()), function ($req): bool {
                     return !PlatformRepository::isPlatformPackage($req);
                 });
 
@@ -314,7 +314,7 @@ class Transaction
             // is this a plugin or a dependency of a plugin?
             if ($isPlugin || count(array_intersect($package->getNames(), $pluginRequires))) {
                 // get the package's requires, but filter out any platform requirements
-                $requires = array_filter(array_keys($package->getRequires()), function ($req) {
+                $requires = array_filter(array_keys($package->getRequires()), function ($req): bool {
                     return !PlatformRepository::isPlatformPackage($req);
                 });
 
@@ -343,7 +343,7 @@ class Transaction
      * @param  OperationInterface[] $operations
      * @return OperationInterface[] reordered operation list
      */
-    private function moveUninstallsToFront(array $operations)
+    private function moveUninstallsToFront(array $operations): array
     {
         $uninstOps = array();
         foreach ($operations as $idx => $op) {
diff --git a/src/Composer/Downloader/ArchiveDownloader.php b/src/Composer/Downloader/ArchiveDownloader.php
index ddbea79c0..152dfb9c4 100644
--- a/src/Composer/Downloader/ArchiveDownloader.php
+++ b/src/Composer/Downloader/ArchiveDownloader.php
@@ -116,7 +116,7 @@ abstract class ArchiveDownloader extends FileDownloader
             $promise = \React\Promise\resolve();
         }
 
-        return $promise->then(function () use ($package, $filesystem, $fileName, $temporaryDir, $path) {
+        return $promise->then(function () use ($package, $filesystem, $fileName, $temporaryDir, $path): \React\Promise\PromiseInterface {
             $filesystem->unlink($fileName);
 
             /**
@@ -125,7 +125,7 @@ abstract class ArchiveDownloader extends FileDownloader
              * @param  string         $dir Directory
              * @return \SplFileInfo[]
              */
-            $getFolderContent = function ($dir) {
+            $getFolderContent = function ($dir): array {
                 $finder = Finder::create()
                     ->ignoreVCS(false)
                     ->ignoreDotFiles(false)
diff --git a/src/Composer/Downloader/DownloadManager.php b/src/Composer/Downloader/DownloadManager.php
index 71eb87ee6..41e61e0ad 100644
--- a/src/Composer/Downloader/DownloadManager.php
+++ b/src/Composer/Downloader/DownloadManager.php
@@ -59,7 +59,7 @@ class DownloadManager
      * @param  bool            $preferSource prefer downloading from source
      * @return DownloadManager
      */
-    public function setPreferSource($preferSource)
+    public function setPreferSource($preferSource): DownloadManager
     {
         $this->preferSource = $preferSource;
 
@@ -72,7 +72,7 @@ class DownloadManager
      * @param  bool            $preferDist prefer downloading from dist
      * @return DownloadManager
      */
-    public function setPreferDist($preferDist)
+    public function setPreferDist($preferDist): DownloadManager
     {
         $this->preferDist = $preferDist;
 
@@ -86,7 +86,7 @@ class DownloadManager
      *
      * @return DownloadManager
      */
-    public function setPreferences(array $preferences)
+    public function setPreferences(array $preferences): DownloadManager
     {
         $this->packagePreferences = $preferences;
 
@@ -100,7 +100,7 @@ class DownloadManager
      * @param  DownloaderInterface $downloader downloader instance
      * @return DownloadManager
      */
-    public function setDownloader($type, DownloaderInterface $downloader)
+    public function setDownloader($type, DownloaderInterface $downloader): DownloadManager
     {
         $type = strtolower($type);
         $this->downloaders[$type] = $downloader;
@@ -115,7 +115,7 @@ class DownloadManager
      * @throws \InvalidArgumentException if downloader for provided type is not registered
      * @return DownloaderInterface
      */
-    public function getDownloader($type)
+    public function getDownloader($type): DownloaderInterface
     {
         $type = strtolower($type);
         if (!isset($this->downloaders[$type])) {
@@ -134,7 +134,7 @@ class DownloadManager
      *                                           wrong type
      * @return DownloaderInterface|null
      */
-    public function getDownloaderForPackage(PackageInterface $package)
+    public function getDownloaderForPackage(PackageInterface $package): ?DownloaderInterface
     {
         $installationSource = $package->getInstallationSource();
 
@@ -168,7 +168,7 @@ class DownloadManager
     /**
      * @return string
      */
-    public function getDownloaderType(DownloaderInterface $downloader)
+    public function getDownloaderType(DownloaderInterface $downloader): string
     {
         return array_search($downloader, $this->downloaders);
     }
@@ -184,7 +184,7 @@ class DownloadManager
      * @throws \RuntimeException
      * @return PromiseInterface
      */
-    public function download(PackageInterface $package, $targetDir, PackageInterface $prevPackage = null)
+    public function download(PackageInterface $package, $targetDir, PackageInterface $prevPackage = null): PromiseInterface
     {
         $targetDir = $this->normalizeTargetDir($targetDir);
         $this->filesystem->ensureDirectoryExists(dirname($targetDir));
@@ -253,7 +253,7 @@ class DownloadManager
      *
      * @return PromiseInterface|null
      */
-    public function prepare($type, PackageInterface $package, $targetDir, PackageInterface $prevPackage = null)
+    public function prepare($type, PackageInterface $package, $targetDir, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         $targetDir = $this->normalizeTargetDir($targetDir);
         $downloader = $this->getDownloaderForPackage($package);
@@ -274,7 +274,7 @@ class DownloadManager
      * @throws \RuntimeException
      * @return PromiseInterface|null
      */
-    public function install(PackageInterface $package, $targetDir)
+    public function install(PackageInterface $package, $targetDir): ?PromiseInterface
     {
         $targetDir = $this->normalizeTargetDir($targetDir);
         $downloader = $this->getDownloaderForPackage($package);
@@ -295,7 +295,7 @@ class DownloadManager
      * @throws \InvalidArgumentException if initial package is not installed
      * @return PromiseInterface|null
      */
-    public function update(PackageInterface $initial, PackageInterface $target, $targetDir)
+    public function update(PackageInterface $initial, PackageInterface $target, $targetDir): ?PromiseInterface
     {
         $targetDir = $this->normalizeTargetDir($targetDir);
         $downloader = $this->getDownloaderForPackage($target);
@@ -330,9 +330,14 @@ class DownloadManager
         // if downloader type changed, or update failed and user asks for reinstall,
         // we wipe the dir and do a new install instead of updating it
         $promise = $initialDownloader->remove($initial, $targetDir);
-        if ($promise) {
-            return $promise->then(function ($res) use ($target, $targetDir) {
-                return $this->install($target, $targetDir);
+        if ($promise instanceof PromiseInterface) {
+            return $promise->then(function ($res) use ($target, $targetDir): PromiseInterface {
+                $promise = $this->install($target, $targetDir);
+                if ($promise instanceof PromiseInterface) {
+                    return $promise;
+                }
+                
+                return \React\Promise\resolve();
             });
         }
 
@@ -347,7 +352,7 @@ class DownloadManager
      *
      * @return PromiseInterface|null
      */
-    public function remove(PackageInterface $package, $targetDir)
+    public function remove(PackageInterface $package, $targetDir): ?PromiseInterface
     {
         $targetDir = $this->normalizeTargetDir($targetDir);
         $downloader = $this->getDownloaderForPackage($package);
@@ -368,7 +373,7 @@ class DownloadManager
      *
      * @return PromiseInterface|null
      */
-    public function cleanup($type, PackageInterface $package, $targetDir, PackageInterface $prevPackage = null)
+    public function cleanup($type, PackageInterface $package, $targetDir, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         $targetDir = $this->normalizeTargetDir($targetDir);
         $downloader = $this->getDownloaderForPackage($package);
@@ -386,7 +391,7 @@ class DownloadManager
      *
      * @return string
      */
-    protected function resolvePackageInstallPreference(PackageInterface $package)
+    protected function resolvePackageInstallPreference(PackageInterface $package): string
     {
         foreach ($this->packagePreferences as $pattern => $preference) {
             $pattern = '{^'.str_replace('\\*', '.*', preg_quote($pattern)).'$}i';
@@ -406,7 +411,7 @@ class DownloadManager
      * @return string[]
      * @phpstan-return array<'dist'|'source'>&non-empty-array
      */
-    private function getAvailableSources(PackageInterface $package, PackageInterface $prevPackage = null)
+    private function getAvailableSources(PackageInterface $package, PackageInterface $prevPackage = null): array
     {
         $sourceType = $package->getSourceType();
         $distType = $package->getDistType();
@@ -432,7 +437,7 @@ class DownloadManager
             && !(!$prevPackage->isDev() && $prevPackage->getInstallationSource() === 'dist' && $package->isDev())
         ) {
             $prevSource = $prevPackage->getInstallationSource();
-            usort($sources, function ($a, $b) use ($prevSource) {
+            usort($sources, function ($a, $b) use ($prevSource): int {
                 return $a === $prevSource ? -1 : 1;
             });
 
@@ -456,7 +461,7 @@ class DownloadManager
      *
      * @return string
      */
-    private function normalizeTargetDir($dir)
+    private function normalizeTargetDir($dir): string
     {
         if ($dir === '\\' || $dir === '/') {
             return $dir;
diff --git a/src/Composer/Downloader/FileDownloader.php b/src/Composer/Downloader/FileDownloader.php
index f4bd777df..ab347d23d 100644
--- a/src/Composer/Downloader/FileDownloader.php
+++ b/src/Composer/Downloader/FileDownloader.php
@@ -118,7 +118,7 @@ class FileDownloader implements DownloaderInterface, ChangeReportInterface
             throw new \InvalidArgumentException('The given package is missing url information');
         }
 
-        $cacheKeyGenerator = function (PackageInterface $package, $key) {
+        $cacheKeyGenerator = function (PackageInterface $package, $key): string {
             $cacheKey = sha1($key);
 
             return $package->getName().'/'.$cacheKey.'.'.$package->getDistType();
@@ -195,7 +195,7 @@ class FileDownloader implements DownloaderInterface, ChangeReportInterface
                     ->then($accept, $reject);
             }
 
-            return $result->then(function ($result) use ($fileName, $checksum, $url, $package, $eventDispatcher) {
+            return $result->then(function ($result) use ($fileName, $checksum, $url, $package, $eventDispatcher): string {
                 // in case of retry, the first call's Promise chain finally calls this twice at the end,
                 // once with $result being the returned $fileName from $accept, and then once for every
                 // failed request with a null result, which can be skipped.
@@ -221,7 +221,7 @@ class FileDownloader implements DownloaderInterface, ChangeReportInterface
             });
         };
 
-        $accept = function ($response) use ($cache, $package, $fileName, &$urls) {
+        $accept = function ($response) use ($cache, $package, $fileName, &$urls): string {
             $url = reset($urls);
             $cacheKey = $url['cacheKey'];
             FileDownloader::$downloadMetadata[$package->getName()] = @filesize($fileName) ?: $response->getHeader('Content-Length') ?: '?';
@@ -407,10 +407,13 @@ class FileDownloader implements DownloaderInterface, ChangeReportInterface
             $promise = \React\Promise\resolve();
         }
 
-        return $promise->then(function () use ($target, $path) {
+        return $promise->then(function () use ($target, $path): PromiseInterface {
             $promise = $this->install($target, $path, false);
+            if ($promise instanceof PromiseInterface) {
+                return $promise;
+            }
 
-            return $promise;
+            return \React\Promise\resolve();
         });
     }
 
@@ -426,7 +429,7 @@ class FileDownloader implements DownloaderInterface, ChangeReportInterface
         }
         $promise = $this->filesystem->removeDirectoryAsync($path);
 
-        return $promise->then(function ($result) use ($path) {
+        return $promise->then(function ($result) use ($path): void {
             if (!$result) {
                 throw new \RuntimeException('Could not completely delete '.$path.', aborting.');
             }
diff --git a/src/Composer/Downloader/FossilDownloader.php b/src/Composer/Downloader/FossilDownloader.php
index f5215d966..662828b84 100644
--- a/src/Composer/Downloader/FossilDownloader.php
+++ b/src/Composer/Downloader/FossilDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 use Composer\Pcre\Preg;
 use Composer\Util\ProcessExecutor;
@@ -24,7 +25,7 @@ class FossilDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doDownload(PackageInterface $package, $path, $url, PackageInterface $prevPackage = null)
+    protected function doDownload(PackageInterface $package, $path, $url, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         return \React\Promise\resolve();
     }
@@ -32,7 +33,7 @@ class FossilDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doInstall(PackageInterface $package, $path, $url)
+    protected function doInstall(PackageInterface $package, $path, $url): ?PromiseInterface
     {
         // Ensure we are allowed to use this URL by config
         $this->config->prohibitUrlByConfig($url, $this->io);
@@ -60,7 +61,7 @@ class FossilDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doUpdate(PackageInterface $initial, PackageInterface $target, $path, $url)
+    protected function doUpdate(PackageInterface $initial, PackageInterface $target, $path, $url): ?PromiseInterface
     {
         // Ensure we are allowed to use this URL by config
         $this->config->prohibitUrlByConfig($url, $this->io);
@@ -83,7 +84,7 @@ class FossilDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    public function getLocalChanges(PackageInterface $package, $path)
+    public function getLocalChanges(PackageInterface $package, $path): ?string
     {
         if (!$this->hasMetadataRepository($path)) {
             return null;
@@ -97,7 +98,7 @@ class FossilDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function getCommitLogs($fromReference, $toReference, $path)
+    protected function getCommitLogs($fromReference, $toReference, $path): string
     {
         $command = sprintf('fossil timeline -t ci -W 0 -n 0 before %s', ProcessExecutor::escape($toReference));
 
@@ -121,7 +122,7 @@ class FossilDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function hasMetadataRepository($path)
+    protected function hasMetadataRepository($path): bool
     {
         return is_file($path . '/.fslckout') || is_file($path . '/_FOSSIL_');
     }
diff --git a/src/Composer/Downloader/GitDownloader.php b/src/Composer/Downloader/GitDownloader.php
index 33ff7922a..7f7d74c9f 100644
--- a/src/Composer/Downloader/GitDownloader.php
+++ b/src/Composer/Downloader/GitDownloader.php
@@ -114,7 +114,7 @@ class GitDownloader extends VcsDownloader implements DvcsDownloaderInterface
 
         $this->io->writeError($msg);
 
-        $commandCallable = function ($url) use ($path, $command, $cachePath) {
+        $commandCallable = function ($url) use ($path, $command, $cachePath): string {
             return str_replace(
                 array('%url%', '%path%', '%cachePath%', '%sanitizedUrl%'),
                 array(
@@ -172,7 +172,7 @@ class GitDownloader extends VcsDownloader implements DvcsDownloaderInterface
 
         $this->io->writeError($msg);
 
-        $commandCallable = function ($url) use ($ref, $command, $cachePath) {
+        $commandCallable = function ($url) use ($ref, $command, $cachePath): string {
             return str_replace(
                 array('%url%', '%ref%', '%cachePath%', '%sanitizedUrl%'),
                 array(
@@ -358,7 +358,7 @@ class GitDownloader extends VcsDownloader implements DvcsDownloaderInterface
             return parent::cleanChanges($package, $path, $update);
         }
 
-        $changes = array_map(function ($elem) {
+        $changes = array_map(function ($elem): string {
             return '    '.$elem;
         }, Preg::split('{\s*\r?\n\s*}', $changes));
         $this->io->writeError('    <error>'.$package->getPrettyName().' has modified files:</error>');
diff --git a/src/Composer/Downloader/GzipDownloader.php b/src/Composer/Downloader/GzipDownloader.php
index 8b5caf474..5826078a0 100644
--- a/src/Composer/Downloader/GzipDownloader.php
+++ b/src/Composer/Downloader/GzipDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
@@ -23,7 +24,7 @@ use Composer\Util\ProcessExecutor;
  */
 class GzipDownloader extends ArchiveDownloader
 {
-    protected function extract(PackageInterface $package, $file, $path)
+    protected function extract(PackageInterface $package, $file, $path): ?PromiseInterface
     {
         $filename = pathinfo(parse_url($package->getDistUrl(), PHP_URL_PATH), PATHINFO_FILENAME);
         $targetFilepath = $path . DIRECTORY_SEPARATOR . $filename;
@@ -59,7 +60,7 @@ class GzipDownloader extends ArchiveDownloader
      *
      * @return void
      */
-    private function extractUsingExt($file, $targetFilepath)
+    private function extractUsingExt($file, $targetFilepath): void
     {
         $archiveFile = gzopen($file, 'rb');
         $targetFile = fopen($targetFilepath, 'wb');
diff --git a/src/Composer/Downloader/HgDownloader.php b/src/Composer/Downloader/HgDownloader.php
index 45b0e2874..d7db15a23 100644
--- a/src/Composer/Downloader/HgDownloader.php
+++ b/src/Composer/Downloader/HgDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\Hg as HgUtils;
@@ -24,7 +25,7 @@ class HgDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doDownload(PackageInterface $package, $path, $url, PackageInterface $prevPackage = null)
+    protected function doDownload(PackageInterface $package, $path, $url, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         if (null === HgUtils::getVersion($this->process)) {
             throw new \RuntimeException('hg was not found in your PATH, skipping source download');
@@ -36,11 +37,11 @@ class HgDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doInstall(PackageInterface $package, $path, $url)
+    protected function doInstall(PackageInterface $package, $path, $url): ?PromiseInterface
     {
         $hgUtils = new HgUtils($this->io, $this->config, $this->process);
 
-        $cloneCommand = function ($url) use ($path) {
+        $cloneCommand = function ($url) use ($path): string {
             return sprintf('hg clone -- %s %s', ProcessExecutor::escape($url), ProcessExecutor::escape($path));
         };
 
@@ -58,7 +59,7 @@ class HgDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doUpdate(PackageInterface $initial, PackageInterface $target, $path, $url)
+    protected function doUpdate(PackageInterface $initial, PackageInterface $target, $path, $url): ?PromiseInterface
     {
         $hgUtils = new HgUtils($this->io, $this->config, $this->process);
 
@@ -69,7 +70,7 @@ class HgDownloader extends VcsDownloader
             throw new \RuntimeException('The .hg directory is missing from '.$path.', see https://getcomposer.org/commit-deps for more information');
         }
 
-        $command = function ($url) use ($ref) {
+        $command = function ($url) use ($ref): string {
             return sprintf('hg pull -- %s && hg up -- %s', ProcessExecutor::escape($url), ProcessExecutor::escape($ref));
         };
 
@@ -81,7 +82,7 @@ class HgDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    public function getLocalChanges(PackageInterface $package, $path)
+    public function getLocalChanges(PackageInterface $package, $path): ?string
     {
         if (!is_dir($path.'/.hg')) {
             return null;
@@ -95,7 +96,7 @@ class HgDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function getCommitLogs($fromReference, $toReference, $path)
+    protected function getCommitLogs($fromReference, $toReference, $path): string
     {
         $command = sprintf('hg log -r %s:%s --style compact', ProcessExecutor::escape($fromReference), ProcessExecutor::escape($toReference));
 
@@ -109,7 +110,7 @@ class HgDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function hasMetadataRepository($path)
+    protected function hasMetadataRepository($path): bool
     {
         return is_dir($path . '/.hg');
     }
diff --git a/src/Composer/Downloader/PathDownloader.php b/src/Composer/Downloader/PathDownloader.php
index 74c36f136..9c24383c7 100644
--- a/src/Composer/Downloader/PathDownloader.php
+++ b/src/Composer/Downloader/PathDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\Archiver\ArchivableFilesFinder;
 use Composer\Package\Dumper\ArrayDumper;
 use Composer\Package\PackageInterface;
@@ -38,7 +39,7 @@ class PathDownloader extends FileDownloader implements VcsCapableDownloaderInter
     /**
      * @inheritDoc
      */
-    public function download(PackageInterface $package, $path, PackageInterface $prevPackage = null, $output = true)
+    public function download(PackageInterface $package, $path, PackageInterface $prevPackage = null, $output = true): ?PromiseInterface
     {
         $path = Filesystem::trimTrailingSlash($path);
         $url = $package->getDistUrl();
@@ -74,7 +75,7 @@ class PathDownloader extends FileDownloader implements VcsCapableDownloaderInter
     /**
      * @inheritDoc
      */
-    public function install(PackageInterface $package, $path, $output = true)
+    public function install(PackageInterface $package, $path, $output = true): ?PromiseInterface
     {
         $path = Filesystem::trimTrailingSlash($path);
         $url = $package->getDistUrl();
@@ -160,7 +161,7 @@ class PathDownloader extends FileDownloader implements VcsCapableDownloaderInter
     /**
      * @inheritDoc
      */
-    public function remove(PackageInterface $package, $path, $output = true)
+    public function remove(PackageInterface $package, $path, $output = true): ?PromiseInterface
     {
         $path = Filesystem::trimTrailingSlash($path);
         /**
@@ -205,7 +206,7 @@ class PathDownloader extends FileDownloader implements VcsCapableDownloaderInter
     /**
      * @inheritDoc
      */
-    public function getVcsReference(PackageInterface $package, $path)
+    public function getVcsReference(PackageInterface $package, $path): ?string
     {
         $path = Filesystem::trimTrailingSlash($path);
         $parser = new VersionParser;
@@ -223,7 +224,7 @@ class PathDownloader extends FileDownloader implements VcsCapableDownloaderInter
     /**
      * @inheritDoc
      */
-    protected function getInstallOperationAppendix(PackageInterface $package, $path)
+    protected function getInstallOperationAppendix(PackageInterface $package, $path): string
     {
         $realUrl = realpath($package->getDistUrl());
 
@@ -295,7 +296,7 @@ class PathDownloader extends FileDownloader implements VcsCapableDownloaderInter
      *
      * @return bool
      */
-    private function safeJunctions()
+    private function safeJunctions(): bool
     {
         // We need to call mklink, and rmdir on Windows 7 (version 6.1)
         return function_exists('proc_open') &&
diff --git a/src/Composer/Downloader/PerforceDownloader.php b/src/Composer/Downloader/PerforceDownloader.php
index c7d0e7985..ae603a832 100644
--- a/src/Composer/Downloader/PerforceDownloader.php
+++ b/src/Composer/Downloader/PerforceDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 use Composer\Repository\VcsRepository;
 use Composer\Util\Perforce;
@@ -27,7 +28,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doDownload(PackageInterface $package, $path, $url, PackageInterface $prevPackage = null)
+    protected function doDownload(PackageInterface $package, $path, $url, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         return \React\Promise\resolve();
     }
@@ -35,7 +36,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    public function doInstall(PackageInterface $package, $path, $url)
+    public function doInstall(PackageInterface $package, $path, $url): ?PromiseInterface
     {
         $ref = $package->getSourceReference();
         $label = $this->getLabelFromSourceReference((string) $ref);
@@ -57,7 +58,7 @@ class PerforceDownloader extends VcsDownloader
      *
      * @return string|null
      */
-    private function getLabelFromSourceReference($ref)
+    private function getLabelFromSourceReference($ref): ?string
     {
         $pos = strpos($ref, '@');
         if (false !== $pos) {
@@ -73,7 +74,7 @@ class PerforceDownloader extends VcsDownloader
      *
      * @return void
      */
-    public function initPerforce(PackageInterface $package, $path, $url)
+    public function initPerforce(PackageInterface $package, $path, $url): void
     {
         if (!empty($this->perforce)) {
             $this->perforce->initializePath($path);
@@ -92,7 +93,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @return array<string, mixed>
      */
-    private function getRepoConfig(VcsRepository $repository)
+    private function getRepoConfig(VcsRepository $repository): array
     {
         return $repository->getRepoConfig();
     }
@@ -100,7 +101,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function doUpdate(PackageInterface $initial, PackageInterface $target, $path, $url)
+    protected function doUpdate(PackageInterface $initial, PackageInterface $target, $path, $url): ?PromiseInterface
     {
         return $this->doInstall($target, $path, $url);
     }
@@ -108,7 +109,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    public function getLocalChanges(PackageInterface $package, $path)
+    public function getLocalChanges(PackageInterface $package, $path): ?string
     {
         $this->io->writeError('Perforce driver does not check for local changes before overriding');
 
@@ -118,7 +119,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function getCommitLogs($fromReference, $toReference, $path)
+    protected function getCommitLogs($fromReference, $toReference, $path): string
     {
         return $this->perforce->getCommitLogs($fromReference, $toReference);
     }
@@ -126,7 +127,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @return void
      */
-    public function setPerforce(Perforce $perforce)
+    public function setPerforce(Perforce $perforce): void
     {
         $this->perforce = $perforce;
     }
@@ -134,7 +135,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @inheritDoc
      */
-    protected function hasMetadataRepository($path)
+    protected function hasMetadataRepository($path): bool
     {
         return true;
     }
diff --git a/src/Composer/Downloader/PharDownloader.php b/src/Composer/Downloader/PharDownloader.php
index 2bfd70a72..b91b2ec1e 100644
--- a/src/Composer/Downloader/PharDownloader.php
+++ b/src/Composer/Downloader/PharDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 
 /**
@@ -24,7 +25,7 @@ class PharDownloader extends ArchiveDownloader
     /**
      * @inheritDoc
      */
-    protected function extract(PackageInterface $package, $file, $path)
+    protected function extract(PackageInterface $package, $file, $path): ?PromiseInterface
     {
         // Can throw an UnexpectedValueException
         $archive = new \Phar($file);
diff --git a/src/Composer/Downloader/RarDownloader.php b/src/Composer/Downloader/RarDownloader.php
index e11366421..ea9c63604 100644
--- a/src/Composer/Downloader/RarDownloader.php
+++ b/src/Composer/Downloader/RarDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Util\IniHelper;
 use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
@@ -27,7 +28,7 @@ use RarArchive;
  */
 class RarDownloader extends ArchiveDownloader
 {
-    protected function extract(PackageInterface $package, $file, $path)
+    protected function extract(PackageInterface $package, $file, $path): ?PromiseInterface
     {
         $processError = null;
 
diff --git a/src/Composer/Downloader/SvnDownloader.php b/src/Composer/Downloader/SvnDownloader.php
index 0d269121c..7dd58493e 100644
--- a/src/Composer/Downloader/SvnDownloader.php
+++ b/src/Composer/Downloader/SvnDownloader.php
@@ -144,7 +144,7 @@ class SvnDownloader extends VcsDownloader
             return parent::cleanChanges($package, $path, $update);
         }
 
-        $changes = array_map(function ($elem) {
+        $changes = array_map(function ($elem): string {
             return '    '.$elem;
         }, Preg::split('{\s*\r?\n\s*}', $changes));
         $countChanges = count($changes);
diff --git a/src/Composer/Downloader/TransportException.php b/src/Composer/Downloader/TransportException.php
index 92be261a6..e36028670 100644
--- a/src/Composer/Downloader/TransportException.php
+++ b/src/Composer/Downloader/TransportException.php
@@ -31,7 +31,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setHeaders($headers)
+    public function setHeaders($headers): void
     {
         $this->headers = $headers;
     }
@@ -39,7 +39,7 @@ class TransportException extends \RuntimeException
     /**
      * @return ?array<string>
      */
-    public function getHeaders()
+    public function getHeaders(): ?array
     {
         return $this->headers;
     }
@@ -49,7 +49,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setResponse($response)
+    public function setResponse($response): void
     {
         $this->response = $response;
     }
@@ -57,7 +57,7 @@ class TransportException extends \RuntimeException
     /**
      * @return ?string
      */
-    public function getResponse()
+    public function getResponse(): ?string
     {
         return $this->response;
     }
@@ -67,7 +67,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setStatusCode($statusCode)
+    public function setStatusCode($statusCode): void
     {
         $this->statusCode = $statusCode;
     }
@@ -75,7 +75,7 @@ class TransportException extends \RuntimeException
     /**
      * @return ?int
      */
-    public function getStatusCode()
+    public function getStatusCode(): ?int
     {
         return $this->statusCode;
     }
@@ -83,7 +83,7 @@ class TransportException extends \RuntimeException
     /**
      * @return array<mixed>
      */
-    public function getResponseInfo()
+    public function getResponseInfo(): array
     {
         return $this->responseInfo;
     }
@@ -93,7 +93,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setResponseInfo(array $responseInfo)
+    public function setResponseInfo(array $responseInfo): void
     {
         $this->responseInfo = $responseInfo;
     }
diff --git a/src/Composer/Downloader/VcsDownloader.php b/src/Composer/Downloader/VcsDownloader.php
index eaea3d9da..836925b66 100644
--- a/src/Composer/Downloader/VcsDownloader.php
+++ b/src/Composer/Downloader/VcsDownloader.php
@@ -200,7 +200,7 @@ abstract class VcsDownloader implements DownloaderInterface, ChangeReportInterfa
             }
 
             if (trim($logs)) {
-                $logs = implode("\n", array_map(function ($line) {
+                $logs = implode("\n", array_map(function ($line): string {
                     return '      ' . $line;
                 }, explode("\n", $logs)));
 
@@ -346,7 +346,7 @@ abstract class VcsDownloader implements DownloaderInterface, ChangeReportInterfa
      *
      * @return string[]
      */
-    private function prepareUrls(array $urls)
+    private function prepareUrls(array $urls): array
     {
         foreach ($urls as $index => $url) {
             if (Filesystem::isLocalPath($url)) {
diff --git a/src/Composer/Downloader/XzDownloader.php b/src/Composer/Downloader/XzDownloader.php
index 7e69a52bc..2072b05e3 100644
--- a/src/Composer/Downloader/XzDownloader.php
+++ b/src/Composer/Downloader/XzDownloader.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 use Composer\Util\ProcessExecutor;
 
@@ -23,7 +24,7 @@ use Composer\Util\ProcessExecutor;
  */
 class XzDownloader extends ArchiveDownloader
 {
-    protected function extract(PackageInterface $package, $file, $path)
+    protected function extract(PackageInterface $package, $file, $path): ?PromiseInterface
     {
         $command = 'tar -xJf ' . ProcessExecutor::escape($file) . ' -C ' . ProcessExecutor::escape($path);
 
diff --git a/src/Composer/Downloader/ZipDownloader.php b/src/Composer/Downloader/ZipDownloader.php
index 23ad6db76..a1f297a9d 100644
--- a/src/Composer/Downloader/ZipDownloader.php
+++ b/src/Composer/Downloader/ZipDownloader.php
@@ -105,7 +105,7 @@ class ZipDownloader extends ArchiveDownloader
      * @param  string           $path Path where to extract file
      * @return PromiseInterface
      */
-    private function extractWithSystemUnzip(PackageInterface $package, $file, $path)
+    private function extractWithSystemUnzip(PackageInterface $package, $file, $path): PromiseInterface
     {
         // Force Exception throwing if the other alternative extraction method is not available
         $isLastChance = !self::$hasZipArchive;
@@ -127,7 +127,7 @@ class ZipDownloader extends ArchiveDownloader
         $executable = $commandSpec[0];
 
         $io = $this->io;
-        $tryFallback = function ($processError) use ($isLastChance, $io, $file, $path, $package, $executable) {
+        $tryFallback = function ($processError) use ($isLastChance, $io, $file, $path, $package, $executable): \React\Promise\PromiseInterface {
             if ($isLastChance) {
                 throw $processError;
             }
@@ -172,7 +172,7 @@ class ZipDownloader extends ArchiveDownloader
      * @param  string           $path Path where to extract file
      * @return PromiseInterface
      */
-    private function extractWithZipArchive(PackageInterface $package, $file, $path)
+    private function extractWithZipArchive(PackageInterface $package, $file, $path): PromiseInterface
     {
         $processError = null;
         $zipArchive = $this->zipArchiveObject ?: new ZipArchive();
diff --git a/src/Composer/EventDispatcher/EventDispatcher.php b/src/Composer/EventDispatcher/EventDispatcher.php
index 29b1f5de7..ea6d51792 100644
--- a/src/Composer/EventDispatcher/EventDispatcher.php
+++ b/src/Composer/EventDispatcher/EventDispatcher.php
@@ -12,10 +12,12 @@
 
 namespace Composer\EventDispatcher;
 
+use Composer\Autoload\AutoloadGenerator;
 use Composer\DependencyResolver\Transaction;
 use Composer\Installer\InstallerEvent;
 use Composer\IO\IOInterface;
 use Composer\Composer;
+use Composer\PartialComposer;
 use Composer\Pcre\Preg;
 use Composer\Util\Platform;
 use Composer\DependencyResolver\Operation\OperationInterface;
@@ -44,7 +46,7 @@ use Symfony\Component\Process\ExecutableFinder;
  */
 class EventDispatcher
 {
-    /** @var Composer */
+    /** @var PartialComposer */
     protected $composer;
     /** @var IOInterface */
     protected $io;
@@ -62,11 +64,11 @@ class EventDispatcher
     /**
      * Constructor.
      *
-     * @param Composer        $composer The composer instance
+     * @param PartialComposer $composer The composer instance
      * @param IOInterface     $io       The IOInterface instance
      * @param ProcessExecutor $process
      */
-    public function __construct(Composer $composer, IOInterface $io, ProcessExecutor $process = null)
+    public function __construct(PartialComposer $composer, IOInterface $io, ProcessExecutor $process = null)
     {
         $this->composer = $composer;
         $this->io = $io;
@@ -116,6 +118,8 @@ class EventDispatcher
      */
     public function dispatchScript($eventName, $devMode = false, $additionalArgs = array(), $flags = array())
     {
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         return $this->doDispatch(new Script\Event($eventName, $this->composer, $this->io, $devMode, $additionalArgs, $flags));
     }
 
@@ -133,6 +137,8 @@ class EventDispatcher
      */
     public function dispatchPackageEvent($eventName, $devMode, RepositoryInterface $localRepo, array $operations, OperationInterface $operation)
     {
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         return $this->doDispatch(new PackageEvent($eventName, $this->composer, $this->io, $devMode, $localRepo, $operations, $operation));
     }
 
@@ -149,6 +155,8 @@ class EventDispatcher
      */
     public function dispatchInstallerEvent($eventName, $devMode, $executeOperations, Transaction $transaction)
     {
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         return $this->doDispatch(new InstallerEvent($eventName, $this->composer, $this->io, $devMode, $executeOperations, $transaction));
     }
 
@@ -486,6 +494,8 @@ class EventDispatcher
             return array();
         }
 
+        assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
+
         if ($this->loader) {
             $this->loader->unregister();
         }
@@ -556,7 +566,7 @@ class EventDispatcher
     /**
      * @return void
      */
-    private function ensureBinDirIsInPath()
+    private function ensureBinDirIsInPath(): void
     {
         $pathEnv = 'PATH';
         if (false === Platform::getEnv('PATH') && false !== Platform::getEnv('Path')) {
diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index 5911b6633..e6ef47563 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -28,6 +28,7 @@ use Composer\Util\Loop;
 use Composer\Util\Silencer;
 use Composer\Plugin\PluginEvents;
 use Composer\EventDispatcher\Event;
+use Phar;
 use Seld\JsonLint\DuplicateKeyException;
 use Symfony\Component\Console\Formatter\OutputFormatter;
 use Symfony\Component\Console\Formatter\OutputFormatterStyle;
@@ -39,6 +40,7 @@ use Composer\Downloader\TransportException;
 use Composer\Json\JsonValidationException;
 use Composer\Repository\InstalledRepositoryInterface;
 use Seld\JsonLint\JsonParser;
+use ZipArchive;
 
 /**
  * Creates a configured instance of composer.
@@ -52,9 +54,8 @@ class Factory
 {
     /**
      * @throws \RuntimeException
-     * @return string
      */
-    protected static function getHomeDir()
+    protected static function getHomeDir(): string
     {
         $home = Platform::getEnv('COMPOSER_HOME');
         if ($home) {
@@ -95,11 +96,7 @@ class Factory
         return $dirs[0];
     }
 
-    /**
-     * @param  string $home
-     * @return string
-     */
-    protected static function getCacheDir($home)
+    protected static function getCacheDir(string $home): string
     {
         $cacheDir = Platform::getEnv('COMPOSER_CACHE_DIR');
         if ($cacheDir) {
@@ -144,11 +141,7 @@ class Factory
         return $home . '/cache';
     }
 
-    /**
-     * @param  string $home
-     * @return string
-     */
-    protected static function getDataDir($home)
+    protected static function getDataDir(string $home): string
     {
         $homeEnv = Platform::getEnv('COMPOSER_HOME');
         if ($homeEnv) {
@@ -169,12 +162,7 @@ class Factory
         return $home;
     }
 
-    /**
-     * @param string|null $cwd
-     *
-     * @return Config
-     */
-    public static function createConfig(IOInterface $io = null, $cwd = null)
+    public static function createConfig(IOInterface $io = null, ?string $cwd = null): Config
     {
         $cwd = $cwd ?: (string) getcwd();
 
@@ -243,20 +231,12 @@ class Factory
         return $config;
     }
 
-    /**
-     * @return string
-     */
-    public static function getComposerFile()
+    public static function getComposerFile(): string
     {
         return trim((string) Platform::getEnv('COMPOSER')) ?: './composer.json';
     }
 
-    /**
-     * @param string $composerFile
-     *
-     * @return string
-     */
-    public static function getLockFile($composerFile)
+    public static function getLockFile(string $composerFile): string
     {
         return "json" === pathinfo($composerFile, PATHINFO_EXTENSION)
                 ? substr($composerFile, 0, -4).'lock'
@@ -266,7 +246,7 @@ class Factory
     /**
      * @return array{highlight: OutputFormatterStyle, warning: OutputFormatterStyle}
      */
-    public static function createAdditionalStyles()
+    public static function createAdditionalStyles(): array
     {
         return array(
             'highlight' => new OutputFormatterStyle('red'),
@@ -274,12 +254,7 @@ class Factory
         );
     }
 
-    /**
-     * Creates a ConsoleOutput instance
-     *
-     * @return ConsoleOutput
-     */
-    public static function createOutput()
+    public static function createOutput(): ConsoleOutput
     {
         $styles = self::createAdditionalStyles();
         $formatter = new OutputFormatter(false, $styles);
@@ -299,9 +274,9 @@ class Factory
      * @param  bool                              $fullLoad       Whether to initialize everything or only main project stuff (used when loading the global composer)
      * @throws \InvalidArgumentException
      * @throws \UnexpectedValueException
-     * @return Composer
+     * @return Composer|PartialComposer Composer if $fullLoad is true, otherwise PartialComposer
      */
-    public function createComposer(IOInterface $io, $localConfig = null, $disablePlugins = false, $cwd = null, $fullLoad = true, $disableScripts = false)
+    public function createComposer(IOInterface $io, $localConfig = null, bool $disablePlugins = false, ?string $cwd = null, bool $fullLoad = true, bool $disableScripts = false)
     {
         $cwd = $cwd ?: (string) getcwd();
 
@@ -363,7 +338,7 @@ class Factory
         $vendorDir = $config->get('vendor-dir');
 
         // initialize composer
-        $composer = new Composer();
+        $composer = $fullLoad ? new Composer() : new PartialComposer();
         $composer->setConfig($config);
 
         if ($fullLoad) {
@@ -410,7 +385,7 @@ class Factory
         $im = $this->createInstallationManager($loop, $io, $dispatcher);
         $composer->setInstallationManager($im);
 
-        if ($fullLoad) {
+        if ($composer instanceof Composer) {
             // initialize download manager
             $dm = $this->createDownloadManager($io, $config, $httpDownloader, $process, $dispatcher);
             $composer->setDownloadManager($dm);
@@ -427,7 +402,7 @@ class Factory
         // add installers to the manager (must happen after download manager is created since they read it out of $composer)
         $this->createDefaultInstallers($im, $composer, $io, $process);
 
-        if ($fullLoad) {
+        if ($composer instanceof Composer) {
             $globalComposer = null;
             if (realpath($config->get('home')) !== $cwd) {
                 $globalComposer = $this->createGlobalComposer($io, $config, $disablePlugins, $disableScripts);
@@ -440,7 +415,7 @@ class Factory
         }
 
         // init locker if possible
-        if ($fullLoad && isset($composerFile)) {
+        if ($composer instanceof Composer && isset($composerFile)) {
             $lockFile = self::getLockFile($composerFile);
 
             $locker = new Package\Locker($io, new JsonFile($lockFile, null, $io), $im, file_get_contents($composerFile), $process);
@@ -460,16 +435,17 @@ class Factory
     }
 
     /**
-     * @param  IOInterface   $io             IO instance
      * @param  bool          $disablePlugins Whether plugins should not be loaded
      * @param  bool          $disableScripts Whether scripts should not be executed
-     * @return Composer|null
      */
-    public static function createGlobal(IOInterface $io, $disablePlugins = false, $disableScripts = false)
+    public static function createGlobal(IOInterface $io, bool $disablePlugins = false, bool $disableScripts = false): ?Composer
     {
         $factory = new static();
 
-        return $factory->createGlobalComposer($io, static::createConfig($io), $disablePlugins, $disableScripts, true);
+        $composer = $factory->createGlobalComposer($io, static::createConfig($io), $disablePlugins, $disableScripts, true);
+        assert(null === $composer || $composer instanceof Composer);
+
+        return $composer;
     }
 
     /**
@@ -478,7 +454,7 @@ class Factory
      *
      * @return void
      */
-    protected function addLocalRepository(IOInterface $io, RepositoryManager $rm, $vendorDir, RootPackageInterface $rootPackage, ProcessExecutor $process = null)
+    protected function addLocalRepository(IOInterface $io, RepositoryManager $rm, $vendorDir, RootPackageInterface $rootPackage, ProcessExecutor $process = null): void
     {
         $fs = null;
         if ($process) {
@@ -489,13 +465,9 @@ class Factory
     }
 
     /**
-     * @param bool $disablePlugins
-     * @param bool $disableScripts
-     * @param bool $fullLoad
-     *
-     * @return Composer|null
+     * @return PartialComposer|Composer|null By default PartialComposer, but Composer if $fullLoad is set to true
      */
-    protected function createGlobalComposer(IOInterface $io, Config $config, $disablePlugins, $disableScripts, $fullLoad = false)
+    protected function createGlobalComposer(IOInterface $io, Config $config, bool $disablePlugins, bool $disableScripts, bool $fullLoad = false): ?PartialComposer
     {
         $composer = null;
         try {
@@ -513,7 +485,7 @@ class Factory
      * @param  EventDispatcher            $eventDispatcher
      * @return Downloader\DownloadManager
      */
-    public function createDownloadManager(IOInterface $io, Config $config, HttpDownloader $httpDownloader, ProcessExecutor $process, EventDispatcher $eventDispatcher = null)
+    public function createDownloadManager(IOInterface $io, Config $config, HttpDownloader $httpDownloader, ProcessExecutor $process, EventDispatcher $eventDispatcher = null): Downloader\DownloadManager
     {
         $cache = null;
         if ($config->get('cache-files-ttl') > 0) {
@@ -566,20 +538,20 @@ class Factory
     public function createArchiveManager(Config $config, Downloader\DownloadManager $dm, Loop $loop)
     {
         $am = new Archiver\ArchiveManager($dm, $loop);
-        $am->addArchiver(new Archiver\ZipArchiver);
-        $am->addArchiver(new Archiver\PharArchiver);
+        if (class_exists(ZipArchive::class)) {
+            $am->addArchiver(new Archiver\ZipArchiver);
+        }
+        if (class_exists(Phar::class)) {
+            $am->addArchiver(new Archiver\PharArchiver);
+        }
 
         return $am;
     }
 
     /**
-     * @param  IOInterface          $io
-     * @param  Composer             $composer
-     * @param  Composer             $globalComposer
-     * @param  bool                 $disablePlugins
      * @return Plugin\PluginManager
      */
-    protected function createPluginManager(IOInterface $io, Composer $composer, Composer $globalComposer = null, $disablePlugins = false)
+    protected function createPluginManager(IOInterface $io, Composer $composer, PartialComposer $globalComposer = null, bool $disablePlugins = false): Plugin\PluginManager
     {
         return new Plugin\PluginManager($io, $composer, $globalComposer, $disablePlugins);
     }
@@ -587,7 +559,7 @@ class Factory
     /**
      * @return Installer\InstallationManager
      */
-    public function createInstallationManager(Loop $loop, IOInterface $io, EventDispatcher $eventDispatcher = null)
+    public function createInstallationManager(Loop $loop, IOInterface $io, EventDispatcher $eventDispatcher = null): Installer\InstallationManager
     {
         return new Installer\InstallationManager($loop, $io, $eventDispatcher);
     }
@@ -595,7 +567,7 @@ class Factory
     /**
      * @return void
      */
-    protected function createDefaultInstallers(Installer\InstallationManager $im, Composer $composer, IOInterface $io, ProcessExecutor $process = null)
+    protected function createDefaultInstallers(Installer\InstallationManager $im, PartialComposer $composer, IOInterface $io, ProcessExecutor $process = null): void
     {
         $fs = new Filesystem($process);
         $binaryInstaller = new Installer\BinaryInstaller($io, rtrim($composer->getConfig()->get('bin-dir'), '/'), $composer->getConfig()->get('bin-compat'), $fs, rtrim($composer->getConfig()->get('vendor-dir'), '/'));
@@ -608,10 +580,8 @@ class Factory
     /**
      * @param InstalledRepositoryInterface   $repo repository to purge packages from
      * @param Installer\InstallationManager  $im   manager to check whether packages are still installed
-     *
-     * @return void
      */
-    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im)
+    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im): void
     {
         foreach ($repo->getPackages() as $package) {
             if (!$im->isPackageInstalled($repo, $package)) {
@@ -620,10 +590,7 @@ class Factory
         }
     }
 
-    /**
-     * @return Package\Loader\RootPackageLoader
-     */
-    protected function loadRootPackage(RepositoryManager $rm, Config $config, VersionParser $parser, VersionGuesser $guesser, IOInterface $io)
+    protected function loadRootPackage(RepositoryManager $rm, Config $config, VersionParser $parser, VersionGuesser $guesser, IOInterface $io): Package\Loader\RootPackageLoader
     {
         return new Package\Loader\RootPackageLoader($rm, $config, $parser, $guesser, $io);
     }
@@ -636,11 +603,14 @@ class Factory
      * @param  bool        $disableScripts Whether scripts should not be run
      * @return Composer
      */
-    public static function create(IOInterface $io, $config = null, $disablePlugins = false, $disableScripts = false)
+    public static function create(IOInterface $io, $config = null, bool $disablePlugins = false, bool $disableScripts = false): Composer
     {
         $factory = new static();
 
-        return $factory->createComposer($io, $config, $disablePlugins, null, true, $disableScripts);
+        $composer = $factory->createComposer($io, $config, $disablePlugins, null, true, $disableScripts);
+        assert($composer instanceof Composer);
+
+        return $composer;
     }
 
     /**
@@ -651,7 +621,7 @@ class Factory
      * @param  mixed[]        $options Array of options passed directly to HttpDownloader constructor
      * @return HttpDownloader
      */
-    public static function createHttpDownloader(IOInterface $io, Config $config, $options = array())
+    public static function createHttpDownloader(IOInterface $io, Config $config, $options = array()): HttpDownloader
     {
         static $warned = false;
         $disableTls = false;
@@ -693,10 +663,7 @@ class Factory
         return $httpDownloader;
     }
 
-    /**
-     * @return bool
-     */
-    private static function useXdg()
+    private static function useXdg(): bool
     {
         foreach (array_keys($_SERVER) as $key) {
             if (strpos($key, 'XDG_') === 0) {
@@ -713,9 +680,8 @@ class Factory
 
     /**
      * @throws \RuntimeException
-     * @return string
      */
-    private static function getUserDir()
+    private static function getUserDir(): string
     {
         $home = Platform::getEnv('HOME');
         if (!$home) {
diff --git a/src/Composer/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilter.php b/src/Composer/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilter.php
index d1fb51b8f..ca37baea4 100644
--- a/src/Composer/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilter.php
+++ b/src/Composer/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilter.php
@@ -20,7 +20,7 @@ final class IgnoreAllPlatformRequirementFilter implements PlatformRequirementFil
      * @param string $req
      * @return bool
      */
-    public function isIgnored($req)
+    public function isIgnored($req): bool
     {
         return PlatformRepository::isPlatformPackage($req);
     }
diff --git a/src/Composer/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilter.php b/src/Composer/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilter.php
index 6c3adf14b..e51d59b39 100644
--- a/src/Composer/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilter.php
+++ b/src/Composer/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilter.php
@@ -55,7 +55,7 @@ final class IgnoreListPlatformRequirementFilter implements PlatformRequirementFi
      * @param string $req
      * @return bool
      */
-    public function isIgnored($req)
+    public function isIgnored($req): bool
     {
         if (!PlatformRepository::isPlatformPackage($req)) {
             return false;
@@ -68,7 +68,7 @@ final class IgnoreListPlatformRequirementFilter implements PlatformRequirementFi
      * @param string $req
      * @return ConstraintInterface
      */
-    public function filterConstraint($req, ConstraintInterface $constraint)
+    public function filterConstraint($req, ConstraintInterface $constraint): ConstraintInterface
     {
         if (!PlatformRepository::isPlatformPackage($req)) {
             return $constraint;
diff --git a/src/Composer/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilter.php b/src/Composer/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilter.php
index 7c4bddbab..2262326c3 100644
--- a/src/Composer/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilter.php
+++ b/src/Composer/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilter.php
@@ -18,7 +18,7 @@ final class IgnoreNothingPlatformRequirementFilter implements PlatformRequiremen
      * @param string $req
      * @return false
      */
-    public function isIgnored($req)
+    public function isIgnored($req): bool
     {
         return false;
     }
diff --git a/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactory.php b/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactory.php
index ac8a83cd7..302bdd915 100644
--- a/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactory.php
+++ b/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactory.php
@@ -19,7 +19,7 @@ final class PlatformRequirementFilterFactory
      *
      * @return PlatformRequirementFilterInterface
      */
-    public static function fromBoolOrList($boolOrList)
+    public static function fromBoolOrList($boolOrList): PlatformRequirementFilterInterface
     {
         if (is_bool($boolOrList)) {
             return $boolOrList ? self::ignoreAll() : self::ignoreNothing();
@@ -40,7 +40,7 @@ final class PlatformRequirementFilterFactory
     /**
      * @return PlatformRequirementFilterInterface
      */
-    public static function ignoreAll()
+    public static function ignoreAll(): PlatformRequirementFilterInterface
     {
         return new IgnoreAllPlatformRequirementFilter();
     }
@@ -48,7 +48,7 @@ final class PlatformRequirementFilterFactory
     /**
      * @return PlatformRequirementFilterInterface
      */
-    public static function ignoreNothing()
+    public static function ignoreNothing(): PlatformRequirementFilterInterface
     {
         return new IgnoreNothingPlatformRequirementFilter();
     }
diff --git a/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterInterface.php b/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterInterface.php
index 89e858152..52cf83e16 100644
--- a/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterInterface.php
+++ b/src/Composer/Filter/PlatformRequirementFilter/PlatformRequirementFilterInterface.php
@@ -18,5 +18,5 @@ interface PlatformRequirementFilterInterface
      * @param string $req
      * @return bool
      */
-    public function isIgnored($req);
+    public function isIgnored($req): bool;
 }
diff --git a/src/Composer/IO/BufferIO.php b/src/Composer/IO/BufferIO.php
index 627abec9d..4d7242120 100644
--- a/src/Composer/IO/BufferIO.php
+++ b/src/Composer/IO/BufferIO.php
@@ -50,13 +50,13 @@ class BufferIO extends ConsoleIO
     /**
      * @return string output
      */
-    public function getOutput()
+    public function getOutput(): string
     {
         fseek($this->output->getStream(), 0);
 
         $output = stream_get_contents($this->output->getStream());
 
-        $output = Preg::replaceCallback("{(?<=^|\n|\x08)(.+?)(\x08+)}", function ($matches) {
+        $output = Preg::replaceCallback("{(?<=^|\n|\x08)(.+?)(\x08+)}", function ($matches): string {
             $pre = strip_tags($matches[1]);
 
             if (strlen($pre) === strlen($matches[2])) {
@@ -77,7 +77,7 @@ class BufferIO extends ConsoleIO
      *
      * @return void
      */
-    public function setUserInputs(array $inputs)
+    public function setUserInputs(array $inputs): void
     {
         if (!$this->input instanceof StreamableInputInterface) {
             throw new \RuntimeException('Setting the user inputs requires at least the version 3.2 of the symfony/console component.');
diff --git a/src/Composer/IO/ConsoleIO.php b/src/Composer/IO/ConsoleIO.php
index 63337ec93..2fd2cb178 100644
--- a/src/Composer/IO/ConsoleIO.php
+++ b/src/Composer/IO/ConsoleIO.php
@@ -157,7 +157,7 @@ class ConsoleIO extends BaseIO
      *
      * @return void
      */
-    private function doWrite($messages, $newline, $stderr, $verbosity, $raw = false)
+    private function doWrite($messages, $newline, $stderr, $verbosity, $raw = false): void
     {
         $sfVerbosity = $this->verbosityMap[$verbosity];
         if ($sfVerbosity > $this->output->getVerbosity()) {
@@ -175,7 +175,7 @@ class ConsoleIO extends BaseIO
         if (null !== $this->startTime) {
             $memoryUsage = memory_get_usage() / 1024 / 1024;
             $timeSpent = microtime(true) - $this->startTime;
-            $messages = array_map(function ($message) use ($memoryUsage, $timeSpent) {
+            $messages = array_map(function ($message) use ($memoryUsage, $timeSpent): string {
                 return sprintf('[%.1fMiB/%.2fs] %s', $memoryUsage, $timeSpent, $message);
             }, (array) $messages);
         }
@@ -216,7 +216,7 @@ class ConsoleIO extends BaseIO
      *
      * @return void
      */
-    private function doOverwrite($messages, $newline, $size, $stderr, $verbosity)
+    private function doOverwrite($messages, $newline, $size, $stderr, $verbosity): void
     {
         // messages can be an array, let's convert it to string anyway
         $messages = implode($newline ? "\n" : '', (array) $messages);
@@ -345,7 +345,7 @@ class ConsoleIO extends BaseIO
     /**
      * @return OutputInterface
      */
-    private function getErrorOutput()
+    private function getErrorOutput(): OutputInterface
     {
         if ($this->output instanceof ConsoleOutputInterface) {
             return $this->output->getErrorOutput();
diff --git a/src/Composer/IO/NullIO.php b/src/Composer/IO/NullIO.php
index 89c574e43..278b945b3 100644
--- a/src/Composer/IO/NullIO.php
+++ b/src/Composer/IO/NullIO.php
@@ -22,7 +22,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function isInteractive()
+    public function isInteractive(): bool
     {
         return false;
     }
@@ -30,7 +30,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function isVerbose()
+    public function isVerbose(): bool
     {
         return false;
     }
@@ -38,7 +38,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function isVeryVerbose()
+    public function isVeryVerbose(): bool
     {
         return false;
     }
@@ -46,7 +46,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function isDebug()
+    public function isDebug(): bool
     {
         return false;
     }
@@ -54,7 +54,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function isDecorated()
+    public function isDecorated(): bool
     {
         return false;
     }
@@ -62,28 +62,28 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function write($messages, $newline = true, $verbosity = self::NORMAL)
+    public function write($messages, $newline = true, $verbosity = self::NORMAL): void
     {
     }
 
     /**
      * @inheritDoc
      */
-    public function writeError($messages, $newline = true, $verbosity = self::NORMAL)
+    public function writeError($messages, $newline = true, $verbosity = self::NORMAL): void
     {
     }
 
     /**
      * @inheritDoc
      */
-    public function overwrite($messages, $newline = true, $size = 80, $verbosity = self::NORMAL)
+    public function overwrite($messages, $newline = true, $size = 80, $verbosity = self::NORMAL): void
     {
     }
 
     /**
      * @inheritDoc
      */
-    public function overwriteError($messages, $newline = true, $size = 80, $verbosity = self::NORMAL)
+    public function overwriteError($messages, $newline = true, $size = 80, $verbosity = self::NORMAL): void
     {
     }
 
@@ -98,7 +98,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function askConfirmation($question, $default = true)
+    public function askConfirmation($question, $default = true): bool
     {
         return $default;
     }
@@ -114,7 +114,7 @@ class NullIO extends BaseIO
     /**
      * @inheritDoc
      */
-    public function askAndHideAnswer($question)
+    public function askAndHideAnswer($question): ?string
     {
         return null;
     }
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index 8bd88e5b9..18b798d7b 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -226,7 +226,7 @@ class Installer
      * @return int        0 on success or a positive error code on failure
      * @phpstan-return self::ERROR_*
      */
-    public function run()
+    public function run(): int
     {
         // Disable GC to save CPU cycles, as the dependency solver can create hundreds of thousands
         // of PHP objects, the GC can spend quite some time walking the tree of references looking
@@ -386,7 +386,7 @@ class Installer
      * @return int
      * @phpstan-return self::ERROR_*
      */
-    protected function doUpdate(InstalledRepositoryInterface $localRepo, $doInstall)
+    protected function doUpdate(InstalledRepositoryInterface $localRepo, $doInstall): int
     {
         $platformRepo = $this->createPlatformRepo(true);
         $aliases = $this->getRootAliases(true);
@@ -522,7 +522,7 @@ class Installer
             }
         }
 
-        $sortByName = function ($a, $b) {
+        $sortByName = function ($a, $b): int {
             if ($a instanceof UpdateOperation) {
                 $a = $a->getTargetPackage()->getName();
             } else {
@@ -597,7 +597,7 @@ class Installer
      * @phpstan-param list<array{package: string, version: string, alias: string, alias_normalized: string}> $aliases
      * @phpstan-return self::ERROR_*
      */
-    protected function extractDevPackages(LockTransaction $lockTransaction, PlatformRepository $platformRepo, array $aliases, PolicyInterface $policy, LockArrayRepository $lockedRepository = null)
+    protected function extractDevPackages(LockTransaction $lockTransaction, PlatformRepository $platformRepo, array $aliases, PolicyInterface $policy, LockArrayRepository $lockedRepository = null): int
     {
         if (!$this->package->getDevRequires()) {
             return 0;
@@ -648,7 +648,7 @@ class Installer
      * @return int                          exit code
      * @phpstan-return self::ERROR_*
      */
-    protected function doInstall(InstalledRepositoryInterface $localRepo, $alreadySolved = false)
+    protected function doInstall(InstalledRepositoryInterface $localRepo, $alreadySolved = false): int
     {
         $this->io->writeError('<info>Installing dependencies from lock file'.($this->devMode ? ' (including require-dev)' : '').'</info>');
 
@@ -769,7 +769,7 @@ class Installer
      *
      * @return PlatformRepository
      */
-    protected function createPlatformRepo($forUpdate)
+    protected function createPlatformRepo($forUpdate): PlatformRepository
     {
         if ($forUpdate) {
             $platformOverrides = $this->config->get('platform') ?: array();
@@ -789,7 +789,7 @@ class Installer
      *
      * @phpstan-param list<array{package: string, version: string, alias: string, alias_normalized: string}> $rootAliases
      */
-    private function createRepositorySet($forUpdate, PlatformRepository $platformRepo, array $rootAliases = array(), $lockedRepository = null)
+    private function createRepositorySet($forUpdate, PlatformRepository $platformRepo, array $rootAliases = array(), $lockedRepository = null): RepositorySet
     {
         if ($forUpdate) {
             $minimumStability = $this->package->getMinimumStability();
@@ -858,7 +858,7 @@ class Installer
      *
      * @return DefaultPolicy
      */
-    private function createPolicy($forUpdate)
+    private function createPolicy($forUpdate): DefaultPolicy
     {
         $preferStable = null;
         $preferLowest = null;
@@ -882,7 +882,7 @@ class Installer
      * @param RootPackageInterface&BasePackage $rootPackage
      * @return Request
      */
-    private function createRequest(RootPackageInterface $rootPackage, PlatformRepository $platformRepo, LockArrayRepository $lockedRepository = null)
+    private function createRequest(RootPackageInterface $rootPackage, PlatformRepository $platformRepo, LockArrayRepository $lockedRepository = null): Request
     {
         $request = new Request($lockedRepository);
 
@@ -919,7 +919,7 @@ class Installer
      *
      * @return void
      */
-    private function requirePackagesForUpdate(Request $request, LockArrayRepository $lockedRepository = null, $includeDevRequires = true)
+    private function requirePackagesForUpdate(Request $request, LockArrayRepository $lockedRepository = null, $includeDevRequires = true): void
     {
         // if we're updating mirrors we want to keep exactly the same versions installed which are in the lock file, but we want current remote metadata
         if ($this->updateMirrors) {
@@ -953,7 +953,7 @@ class Installer
      *
      * @phpstan-return list<array{package: string, version: string, alias: string, alias_normalized: string}>
      */
-    private function getRootAliases($forUpdate)
+    private function getRootAliases($forUpdate): array
     {
         if ($forUpdate) {
             $aliases = $this->package->getAliases();
@@ -969,7 +969,7 @@ class Installer
      *
      * @return array<string, string>
      */
-    private function extractPlatformRequirements(array $links)
+    private function extractPlatformRequirements(array $links): array
     {
         $platformReqs = array();
         foreach ($links as $link) {
@@ -988,7 +988,7 @@ class Installer
      *
      * @return void
      */
-    private function mockLocalRepositories(RepositoryManager $rm)
+    private function mockLocalRepositories(RepositoryManager $rm): void
     {
         $packages = array();
         foreach ($rm->getLocalRepository()->getPackages() as $package) {
@@ -1009,7 +1009,7 @@ class Installer
     /**
      * @return PoolOptimizer|null
      */
-    private function createPoolOptimizer(PolicyInterface $policy)
+    private function createPoolOptimizer(PolicyInterface $policy): ?PoolOptimizer
     {
         // Not the best architectural decision here, would need to be able
         // to configure from the outside of Installer but this is only
@@ -1030,7 +1030,7 @@ class Installer
      * @param  Composer    $composer
      * @return Installer
      */
-    public static function create(IOInterface $io, Composer $composer)
+    public static function create(IOInterface $io, Composer $composer): Installer
     {
         return new static(
             $io,
@@ -1062,7 +1062,7 @@ class Installer
      * @param  bool      $dryRun
      * @return Installer
      */
-    public function setDryRun($dryRun = true)
+    public function setDryRun($dryRun = true): Installer
     {
         $this->dryRun = (bool) $dryRun;
 
@@ -1074,7 +1074,7 @@ class Installer
      *
      * @return bool
      */
-    public function isDryRun()
+    public function isDryRun(): bool
     {
         return $this->dryRun;
     }
@@ -1085,7 +1085,7 @@ class Installer
      * @param  bool      $preferSource
      * @return Installer
      */
-    public function setPreferSource($preferSource = true)
+    public function setPreferSource($preferSource = true): Installer
     {
         $this->preferSource = (bool) $preferSource;
 
@@ -1098,7 +1098,7 @@ class Installer
      * @param  bool      $preferDist
      * @return Installer
      */
-    public function setPreferDist($preferDist = true)
+    public function setPreferDist($preferDist = true): Installer
     {
         $this->preferDist = (bool) $preferDist;
 
@@ -1111,7 +1111,7 @@ class Installer
      * @param  bool      $optimizeAutoloader
      * @return Installer
      */
-    public function setOptimizeAutoloader($optimizeAutoloader)
+    public function setOptimizeAutoloader($optimizeAutoloader): Installer
     {
         $this->optimizeAutoloader = (bool) $optimizeAutoloader;
         if (!$this->optimizeAutoloader) {
@@ -1130,7 +1130,7 @@ class Installer
      * @param  bool      $classMapAuthoritative
      * @return Installer
      */
-    public function setClassMapAuthoritative($classMapAuthoritative)
+    public function setClassMapAuthoritative($classMapAuthoritative): Installer
     {
         $this->classMapAuthoritative = (bool) $classMapAuthoritative;
         if ($this->classMapAuthoritative) {
@@ -1148,7 +1148,7 @@ class Installer
      * @param  string|null $apcuAutoloaderPrefix
      * @return Installer
      */
-    public function setApcuAutoloader($apcuAutoloader, $apcuAutoloaderPrefix = null)
+    public function setApcuAutoloader($apcuAutoloader, $apcuAutoloaderPrefix = null): Installer
     {
         $this->apcuAutoloader = $apcuAutoloader;
         $this->apcuAutoloaderPrefix = $apcuAutoloaderPrefix;
@@ -1162,7 +1162,7 @@ class Installer
      * @param  bool      $update
      * @return Installer
      */
-    public function setUpdate($update)
+    public function setUpdate($update): Installer
     {
         $this->update = (bool) $update;
 
@@ -1175,7 +1175,7 @@ class Installer
      * @param  bool      $install
      * @return Installer
      */
-    public function setInstall($install)
+    public function setInstall($install): Installer
     {
         $this->install = (bool) $install;
 
@@ -1188,7 +1188,7 @@ class Installer
      * @param  bool      $devMode
      * @return Installer
      */
-    public function setDevMode($devMode = true)
+    public function setDevMode($devMode = true): Installer
     {
         $this->devMode = (bool) $devMode;
 
@@ -1203,7 +1203,7 @@ class Installer
      * @param  bool      $dumpAutoloader
      * @return Installer
      */
-    public function setDumpAutoloader($dumpAutoloader = true)
+    public function setDumpAutoloader($dumpAutoloader = true): Installer
     {
         $this->dumpAutoloader = (bool) $dumpAutoloader;
 
@@ -1219,7 +1219,7 @@ class Installer
      * @return Installer
      * @deprecated Use setRunScripts(false) on the EventDispatcher instance being injected instead
      */
-    public function setRunScripts($runScripts = true)
+    public function setRunScripts($runScripts = true): Installer
     {
         $this->runScripts = (bool) $runScripts;
 
@@ -1232,7 +1232,7 @@ class Installer
      * @param  Config    $config
      * @return Installer
      */
-    public function setConfig(Config $config)
+    public function setConfig(Config $config): Installer
     {
         $this->config = $config;
 
@@ -1245,7 +1245,7 @@ class Installer
      * @param  bool      $verbose
      * @return Installer
      */
-    public function setVerbose($verbose = true)
+    public function setVerbose($verbose = true): Installer
     {
         $this->verbose = (bool) $verbose;
 
@@ -1257,7 +1257,7 @@ class Installer
      *
      * @return bool
      */
-    public function isVerbose()
+    public function isVerbose(): bool
     {
         return $this->verbose;
     }
@@ -1275,7 +1275,7 @@ class Installer
      *
      * @deprecated use setPlatformRequirementFilter instead
      */
-    public function setIgnorePlatformRequirements($ignorePlatformReqs)
+    public function setIgnorePlatformRequirements($ignorePlatformReqs): Installer
     {
         trigger_error('Installer::setIgnorePlatformRequirements is deprecated since Composer 2.2, use setPlatformRequirementFilter instead.', E_USER_DEPRECATED);
 
@@ -1286,7 +1286,7 @@ class Installer
      * @param PlatformRequirementFilterInterface $platformRequirementFilter
      * @return Installer
      */
-    public function setPlatformRequirementFilter(PlatformRequirementFilterInterface $platformRequirementFilter)
+    public function setPlatformRequirementFilter(PlatformRequirementFilterInterface $platformRequirementFilter): Installer
     {
         $this->platformRequirementFilter = $platformRequirementFilter;
 
@@ -1299,7 +1299,7 @@ class Installer
      * @param  bool      $updateMirrors
      * @return Installer
      */
-    public function setUpdateMirrors($updateMirrors)
+    public function setUpdateMirrors($updateMirrors): Installer
     {
         $this->updateMirrors = $updateMirrors;
 
@@ -1314,7 +1314,7 @@ class Installer
      *
      * @return Installer
      */
-    public function setUpdateAllowList(array $packages)
+    public function setUpdateAllowList(array $packages): Installer
     {
         $this->updateAllowList = array_flip(array_map('strtolower', $packages));
 
@@ -1330,7 +1330,7 @@ class Installer
      * @param  int       $updateAllowTransitiveDependencies One of the UPDATE_ constants on the Request class
      * @return Installer
      */
-    public function setUpdateAllowTransitiveDependencies($updateAllowTransitiveDependencies)
+    public function setUpdateAllowTransitiveDependencies($updateAllowTransitiveDependencies): Installer
     {
         if (!in_array($updateAllowTransitiveDependencies, array(Request::UPDATE_ONLY_LISTED, Request::UPDATE_LISTED_WITH_TRANSITIVE_DEPS_NO_ROOT_REQUIRE, Request::UPDATE_LISTED_WITH_TRANSITIVE_DEPS), true)) {
             throw new \RuntimeException("Invalid value for updateAllowTransitiveDependencies supplied");
@@ -1347,7 +1347,7 @@ class Installer
      * @param  bool      $preferStable
      * @return Installer
      */
-    public function setPreferStable($preferStable = true)
+    public function setPreferStable($preferStable = true): Installer
     {
         $this->preferStable = (bool) $preferStable;
 
@@ -1360,7 +1360,7 @@ class Installer
      * @param  bool      $preferLowest
      * @return Installer
      */
-    public function setPreferLowest($preferLowest = true)
+    public function setPreferLowest($preferLowest = true): Installer
     {
         $this->preferLowest = (bool) $preferLowest;
 
@@ -1375,7 +1375,7 @@ class Installer
      * @param  bool      $writeLock
      * @return Installer
      */
-    public function setWriteLock($writeLock = true)
+    public function setWriteLock($writeLock = true): Installer
     {
         $this->writeLock = (bool) $writeLock;
 
@@ -1390,7 +1390,7 @@ class Installer
      * @param  bool      $executeOperations
      * @return Installer
      */
-    public function setExecuteOperations($executeOperations = true)
+    public function setExecuteOperations($executeOperations = true): Installer
     {
         $this->executeOperations = (bool) $executeOperations;
 
@@ -1406,7 +1406,7 @@ class Installer
      *
      * @return Installer
      */
-    public function disablePlugins()
+    public function disablePlugins(): Installer
     {
         $this->installationManager->disablePlugins();
 
@@ -1417,7 +1417,7 @@ class Installer
      * @param  SuggestedPackagesReporter $suggestedPackagesReporter
      * @return Installer
      */
-    public function setSuggestedPackagesReporter(SuggestedPackagesReporter $suggestedPackagesReporter)
+    public function setSuggestedPackagesReporter(SuggestedPackagesReporter $suggestedPackagesReporter): Installer
     {
         $this->suggestedPackagesReporter = $suggestedPackagesReporter;
 
diff --git a/src/Composer/Installer/BinaryInstaller.php b/src/Composer/Installer/BinaryInstaller.php
index 417e9be2b..4af59ad23 100644
--- a/src/Composer/Installer/BinaryInstaller.php
+++ b/src/Composer/Installer/BinaryInstaller.php
@@ -62,7 +62,7 @@ class BinaryInstaller
      *
      * @return void
      */
-    public function installBinaries(PackageInterface $package, $installPath, $warnOnOverwrite = true)
+    public function installBinaries(PackageInterface $package, $installPath, $warnOnOverwrite = true): void
     {
         $binaries = $this->getBinaries($package);
         if (!$binaries) {
@@ -120,7 +120,7 @@ class BinaryInstaller
     /**
      * @return void
      */
-    public function removeBinaries(PackageInterface $package)
+    public function removeBinaries(PackageInterface $package): void
     {
         $this->initializeBinDir();
 
@@ -149,7 +149,7 @@ class BinaryInstaller
      *
      * @return string
      */
-    public static function determineBinaryCaller($bin)
+    public static function determineBinaryCaller($bin): string
     {
         if ('.bat' === substr($bin, -4) || '.exe' === substr($bin, -4)) {
             return 'call';
@@ -168,7 +168,7 @@ class BinaryInstaller
     /**
      * @return string[]
      */
-    protected function getBinaries(PackageInterface $package)
+    protected function getBinaries(PackageInterface $package): array
     {
         return $package->getBinaries();
     }
@@ -180,7 +180,7 @@ class BinaryInstaller
      *
      * @return void
      */
-    protected function installFullBinaries($binPath, $link, $bin, PackageInterface $package)
+    protected function installFullBinaries($binPath, $link, $bin, PackageInterface $package): void
     {
         // add unixy support for cygwin and similar environments
         if ('.bat' !== substr($binPath, -4)) {
@@ -202,7 +202,7 @@ class BinaryInstaller
      *
      * @return void
      */
-    protected function installUnixyProxyBinaries($binPath, $link)
+    protected function installUnixyProxyBinaries($binPath, $link): void
     {
         file_put_contents($link, $this->generateUnixyProxyCode($binPath, $link));
         Silencer::call('chmod', $link, 0777 & ~umask());
@@ -211,7 +211,7 @@ class BinaryInstaller
     /**
      * @return void
      */
-    protected function initializeBinDir()
+    protected function initializeBinDir(): void
     {
         $this->filesystem->ensureDirectoryExists($this->binDir);
         $this->binDir = realpath($this->binDir);
@@ -223,7 +223,7 @@ class BinaryInstaller
      *
      * @return string
      */
-    protected function generateWindowsProxyCode($bin, $link)
+    protected function generateWindowsProxyCode($bin, $link): string
     {
         $binPath = $this->filesystem->findShortestPath($link, $bin);
         $caller = self::determineBinaryCaller($bin);
@@ -252,7 +252,7 @@ class BinaryInstaller
      *
      * @return string
      */
-    protected function generateUnixyProxyCode($bin, $link)
+    protected function generateUnixyProxyCode($bin, $link): string
     {
         $binPath = $this->filesystem->findShortestPath($link, $bin);
 
diff --git a/src/Composer/Installer/InstallationManager.php b/src/Composer/Installer/InstallationManager.php
index 71aa3d986..788d367e7 100644
--- a/src/Composer/Installer/InstallationManager.php
+++ b/src/Composer/Installer/InstallationManager.php
@@ -199,18 +199,18 @@ class InstallationManager
 
         $loop = $this->loop;
         $io = $this->io;
-        $runCleanup = function () use (&$cleanupPromises, $loop) {
+        $runCleanup = function () use (&$cleanupPromises, $loop): void {
             $promises = array();
 
             $loop->abortJobs();
 
             foreach ($cleanupPromises as $cleanup) {
-                $promises[] = new \React\Promise\Promise(function ($resolve, $reject) use ($cleanup) {
+                $promises[] = new \React\Promise\Promise(function ($resolve, $reject) use ($cleanup): void {
                     $promise = $cleanup();
                     if (!$promise instanceof PromiseInterface) {
                         $resolve();
                     } else {
-                        $promise->then(function () use ($resolve) {
+                        $promise->then(function () use ($resolve): void {
                             $resolve();
                         });
                     }
@@ -229,7 +229,7 @@ class InstallationManager
         if ($handleInterruptsUnix) {
             pcntl_async_signals(true);
             $prevHandler = pcntl_signal_get_handler(SIGINT);
-            pcntl_signal(SIGINT, function ($sig) use ($runCleanup, $prevHandler, $io) {
+            pcntl_signal(SIGINT, function ($sig) use ($runCleanup, $prevHandler, $io): void {
                 $io->writeError('Received SIGINT, aborting', true, IOInterface::DEBUG);
                 $runCleanup();
 
@@ -241,7 +241,7 @@ class InstallationManager
             });
         }
         if ($handleInterruptsWindows) {
-            $windowsHandler = function ($event) use ($runCleanup, $io) {
+            $windowsHandler = function ($event) use ($runCleanup, $io): void {
                 if ($event !== PHP_WINDOWS_EVENT_CTRL_C) {
                     return;
                 }
@@ -316,7 +316,7 @@ class InstallationManager
      *
      * @return void
      */
-    private function downloadAndExecuteBatch(InstalledRepositoryInterface $repo, array $operations, array &$cleanupPromises, $devMode, $runScripts, array $allOperations)
+    private function downloadAndExecuteBatch(InstalledRepositoryInterface $repo, array $operations, array &$cleanupPromises, $devMode, $runScripts, array $allOperations): void
     {
         $promises = array();
 
@@ -400,7 +400,7 @@ class InstallationManager
      *
      * @return void
      */
-    private function executeBatch(InstalledRepositoryInterface $repo, array $operations, array $cleanupPromises, $devMode, $runScripts, array $allOperations)
+    private function executeBatch(InstalledRepositoryInterface $repo, array $operations, array $cleanupPromises, $devMode, $runScripts, array $allOperations): void
     {
         $promises = array();
         $postExecCallbacks = array();
@@ -446,15 +446,15 @@ class InstallationManager
             $promise = $promise->then(function () use ($opType, $repo, $operation) {
                 return $this->$opType($repo, $operation);
             })->then($cleanupPromises[$index])
-            ->then(function () use ($devMode, $repo) {
+            ->then(function () use ($devMode, $repo): void {
                 $repo->write($devMode, $this);
-            }, function ($e) use ($opType, $package, $io) {
+            }, function ($e) use ($opType, $package, $io): void {
                 $io->writeError('    <error>' . ucfirst($opType) .' of '.$package->getPrettyName().' failed</error>');
 
                 throw $e;
             });
 
-            $postExecCallbacks[] = function () use ($opType, $runScripts, $dispatcher, $devMode, $repo, $allOperations, $operation) {
+            $postExecCallbacks[] = function () use ($opType, $runScripts, $dispatcher, $devMode, $repo, $allOperations, $operation): void {
                 $event = 'Composer\Installer\PackageEvents::POST_PACKAGE_'.strtoupper($opType);
                 if (defined($event) && $runScripts && $dispatcher) {
                     $dispatcher->dispatchPackageEvent(constant($event), $devMode, $repo, $allOperations, $operation);
@@ -481,7 +481,7 @@ class InstallationManager
      *
      * @return void
      */
-    private function waitOnPromises(array $promises)
+    private function waitOnPromises(array $promises): void
     {
         $progress = null;
         if (
@@ -548,8 +548,13 @@ class InstallationManager
             }
 
             $installer = $this->getInstaller($targetType);
-            $promise = $promise->then(function () use ($installer, $repo, $target) {
-                return $installer->install($repo, $target);
+            $promise = $promise->then(function () use ($installer, $repo, $target): PromiseInterface {
+                $promise = $installer->install($repo, $target);
+                if ($promise instanceof PromiseInterface) {
+                    return $promise;
+                }
+                
+                return \React\Promise\resolve();
             });
         }
 
@@ -700,7 +705,7 @@ class InstallationManager
     /**
      * @return void
      */
-    private function markForNotification(PackageInterface $package)
+    private function markForNotification(PackageInterface $package): void
     {
         if ($package->getNotificationUrl()) {
             $this->notifiablePackages[$package->getNotificationUrl()][$package->getName()] = $package;
diff --git a/src/Composer/Installer/LibraryInstaller.php b/src/Composer/Installer/LibraryInstaller.php
index 7490e11a5..f5525341d 100644
--- a/src/Composer/Installer/LibraryInstaller.php
+++ b/src/Composer/Installer/LibraryInstaller.php
@@ -14,6 +14,7 @@ namespace Composer\Installer;
 
 use Composer\Composer;
 use Composer\IO\IOInterface;
+use Composer\PartialComposer;
 use Composer\Pcre\Preg;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
@@ -31,11 +32,11 @@ use Composer\Downloader\DownloadManager;
  */
 class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
 {
-    /** @var Composer */
+    /** @var PartialComposer */
     protected $composer;
     /** @var string */
     protected $vendorDir;
-    /** @var DownloadManager */
+    /** @var DownloadManager|null */
     protected $downloadManager;
     /** @var IOInterface */
     protected $io;
@@ -50,15 +51,15 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
      * Initializes library installer.
      *
      * @param IOInterface     $io
-     * @param Composer        $composer
+     * @param PartialComposer $composer
      * @param string|null     $type
      * @param Filesystem      $filesystem
      * @param BinaryInstaller $binaryInstaller
      */
-    public function __construct(IOInterface $io, Composer $composer, $type = 'library', Filesystem $filesystem = null, BinaryInstaller $binaryInstaller = null)
+    public function __construct(IOInterface $io, PartialComposer $composer, $type = 'library', Filesystem $filesystem = null, BinaryInstaller $binaryInstaller = null)
     {
         $this->composer = $composer;
-        $this->downloadManager = $composer->getDownloadManager();
+        $this->downloadManager = $composer instanceof Composer ? $composer->getDownloadManager() : null;
         $this->io = $io;
         $this->type = $type;
 
@@ -101,7 +102,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->initializeVendorDir();
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->download($package, $downloadPath, $prevPackage);
+        return $this->getDownloadManager()->download($package, $downloadPath, $prevPackage);
     }
 
     /**
@@ -112,7 +113,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->initializeVendorDir();
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->prepare($type, $package, $downloadPath, $prevPackage);
+        return $this->getDownloadManager()->prepare($type, $package, $downloadPath, $prevPackage);
     }
 
     /**
@@ -123,7 +124,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->initializeVendorDir();
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->cleanup($type, $package, $downloadPath, $prevPackage);
+        return $this->getDownloadManager()->cleanup($type, $package, $downloadPath, $prevPackage);
     }
 
     /**
@@ -147,7 +148,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $binaryInstaller = $this->binaryInstaller;
         $installPath = $this->getInstallPath($package);
 
-        return $promise->then(function () use ($binaryInstaller, $installPath, $package, $repo) {
+        return $promise->then(function () use ($binaryInstaller, $installPath, $package, $repo): void {
             $binaryInstaller->installBinaries($package, $installPath);
             if (!$repo->hasPackage($package)) {
                 $repo->addPackage(clone $package);
@@ -175,7 +176,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $binaryInstaller = $this->binaryInstaller;
         $installPath = $this->getInstallPath($target);
 
-        return $promise->then(function () use ($binaryInstaller, $installPath, $target, $initial, $repo) {
+        return $promise->then(function () use ($binaryInstaller, $installPath, $target, $initial, $repo): void {
             $binaryInstaller->installBinaries($target, $installPath);
             $repo->removePackage($initial);
             if (!$repo->hasPackage($target)) {
@@ -202,7 +203,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $downloadPath = $this->getPackageBasePath($package);
         $filesystem = $this->filesystem;
 
-        return $promise->then(function () use ($binaryInstaller, $filesystem, $downloadPath, $package, $repo) {
+        return $promise->then(function () use ($binaryInstaller, $filesystem, $downloadPath, $package, $repo): void {
             $binaryInstaller->removeBinaries($package);
             $repo->removePackage($package);
 
@@ -266,7 +267,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
     {
         $downloadPath = $this->getInstallPath($package);
 
-        return $this->downloadManager->install($package, $downloadPath);
+        return $this->getDownloadManager()->install($package, $downloadPath);
     }
 
     /**
@@ -287,15 +288,20 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
                     $promise = \React\Promise\resolve();
                 }
 
-                return $promise->then(function () use ($target) {
-                    return $this->installCode($target);
+                return $promise->then(function () use ($target): PromiseInterface {
+                    $promise = $this->installCode($target);
+                    if ($promise instanceof PromiseInterface) {
+                        return $promise;
+                    }
+
+                    return \React\Promise\resolve();
                 });
             }
 
             $this->filesystem->rename($initialDownloadPath, $targetDownloadPath);
         }
 
-        return $this->downloadManager->update($initial, $target, $targetDownloadPath);
+        return $this->getDownloadManager()->update($initial, $target, $targetDownloadPath);
     }
 
     /**
@@ -305,7 +311,7 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
     {
         $downloadPath = $this->getPackageBasePath($package);
 
-        return $this->downloadManager->remove($package, $downloadPath);
+        return $this->getDownloadManager()->remove($package, $downloadPath);
     }
 
     /**
@@ -316,4 +322,11 @@ class LibraryInstaller implements InstallerInterface, BinaryPresenceInterface
         $this->filesystem->ensureDirectoryExists($this->vendorDir);
         $this->vendorDir = realpath($this->vendorDir);
     }
+
+    protected function getDownloadManager(): DownloadManager
+    {
+        assert($this->downloadManager instanceof DownloadManager, new \LogicException(self::class.' should be initialized with a fully loaded Composer instance to be able to install/... packages'));
+
+        return $this->downloadManager;
+    }
 }
diff --git a/src/Composer/Installer/PluginInstaller.php b/src/Composer/Installer/PluginInstaller.php
index e4d0f0404..2dded4eac 100644
--- a/src/Composer/Installer/PluginInstaller.php
+++ b/src/Composer/Installer/PluginInstaller.php
@@ -14,8 +14,10 @@ namespace Composer\Installer;
 
 use Composer\Composer;
 use Composer\IO\IOInterface;
+use Composer\PartialComposer;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
+use Composer\Plugin\PluginManager;
 use Composer\Util\Filesystem;
 use Composer\Util\Platform;
 use React\Promise\PromiseInterface;
@@ -28,13 +30,7 @@ use React\Promise\PromiseInterface;
  */
 class PluginInstaller extends LibraryInstaller
 {
-    /**
-     * Initializes Plugin installer.
-     *
-     * @param IOInterface $io
-     * @param Composer    $composer
-     */
-    public function __construct(IOInterface $io, Composer $composer, Filesystem $fs = null, BinaryInstaller $binaryInstaller = null)
+    public function __construct(IOInterface $io, PartialComposer $composer, Filesystem $fs = null, BinaryInstaller $binaryInstaller = null)
     {
         parent::__construct($io, $composer, 'composer-plugin', $fs, $binaryInstaller);
     }
@@ -70,10 +66,10 @@ class PluginInstaller extends LibraryInstaller
             $promise = \React\Promise\resolve();
         }
 
-        return $promise->then(function () use ($package, $repo) {
+        return $promise->then(function () use ($package, $repo): void {
             try {
                 Platform::workaroundFilesystemIssues();
-                $this->composer->getPluginManager()->registerPackage($package, true);
+                $this->getPluginManager()->registerPackage($package, true);
             } catch (\Exception $e) {
                 $this->rollbackInstall($e, $repo, $package);
             }
@@ -90,11 +86,11 @@ class PluginInstaller extends LibraryInstaller
             $promise = \React\Promise\resolve();
         }
 
-        return $promise->then(function () use ($initial, $target, $repo) {
+        return $promise->then(function () use ($initial, $target, $repo): void {
             try {
                 Platform::workaroundFilesystemIssues();
-                $this->composer->getPluginManager()->deactivatePackage($initial);
-                $this->composer->getPluginManager()->registerPackage($target, true);
+                $this->getPluginManager()->deactivatePackage($initial);
+                $this->getPluginManager()->registerPackage($target, true);
             } catch (\Exception $e) {
                 $this->rollbackInstall($e, $repo, $target);
             }
@@ -103,7 +99,7 @@ class PluginInstaller extends LibraryInstaller
 
     public function uninstall(InstalledRepositoryInterface $repo, PackageInterface $package)
     {
-        $this->composer->getPluginManager()->uninstallPackage($package);
+        $this->getPluginManager()->uninstallPackage($package);
 
         return parent::uninstall($repo, $package);
     }
@@ -114,4 +110,12 @@ class PluginInstaller extends LibraryInstaller
         parent::uninstall($repo, $package);
         throw $e;
     }
+
+    protected function getPluginManager(): PluginManager
+    {
+        assert($this->composer instanceof Composer, new \LogicException(self::class.' should be initialized with a fully loaded Composer instance.'));
+        $pluginManager = $this->composer->getPluginManager();
+
+        return $pluginManager;
+    }
 }
diff --git a/src/Composer/Installer/ProjectInstaller.php b/src/Composer/Installer/ProjectInstaller.php
index 42bb77aca..7e31fdaf9 100644
--- a/src/Composer/Installer/ProjectInstaller.php
+++ b/src/Composer/Installer/ProjectInstaller.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Installer;
 
+use React\Promise\PromiseInterface;
 use Composer\Package\PackageInterface;
 use Composer\Downloader\DownloadManager;
 use Composer\Repository\InstalledRepositoryInterface;
@@ -48,7 +49,7 @@ class ProjectInstaller implements InstallerInterface
      * @param  string $packageType
      * @return bool
      */
-    public function supports($packageType)
+    public function supports($packageType): bool
     {
         return true;
     }
@@ -56,7 +57,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function isInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
+    public function isInstalled(InstalledRepositoryInterface $repo, PackageInterface $package): bool
     {
         return false;
     }
@@ -64,7 +65,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function download(PackageInterface $package, PackageInterface $prevPackage = null)
+    public function download(PackageInterface $package, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         $installPath = $this->installPath;
         if (file_exists($installPath) && !$this->filesystem->isDirEmpty($installPath)) {
@@ -80,7 +81,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function prepare($type, PackageInterface $package, PackageInterface $prevPackage = null)
+    public function prepare($type, PackageInterface $package, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         return $this->downloadManager->prepare($type, $package, $this->installPath, $prevPackage);
     }
@@ -88,7 +89,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function cleanup($type, PackageInterface $package, PackageInterface $prevPackage = null)
+    public function cleanup($type, PackageInterface $package, PackageInterface $prevPackage = null): ?PromiseInterface
     {
         return $this->downloadManager->cleanup($type, $package, $this->installPath, $prevPackage);
     }
@@ -96,7 +97,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function install(InstalledRepositoryInterface $repo, PackageInterface $package)
+    public function install(InstalledRepositoryInterface $repo, PackageInterface $package): ?PromiseInterface
     {
         return $this->downloadManager->install($package, $this->installPath);
     }
@@ -104,7 +105,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function update(InstalledRepositoryInterface $repo, PackageInterface $initial, PackageInterface $target)
+    public function update(InstalledRepositoryInterface $repo, PackageInterface $initial, PackageInterface $target): ?PromiseInterface
     {
         throw new \InvalidArgumentException("not supported");
     }
@@ -112,7 +113,7 @@ class ProjectInstaller implements InstallerInterface
     /**
      * @inheritDoc
      */
-    public function uninstall(InstalledRepositoryInterface $repo, PackageInterface $package)
+    public function uninstall(InstalledRepositoryInterface $repo, PackageInterface $package): ?PromiseInterface
     {
         throw new \InvalidArgumentException("not supported");
     }
@@ -123,7 +124,7 @@ class ProjectInstaller implements InstallerInterface
      * @param  PackageInterface $package
      * @return string           path
      */
-    public function getInstallPath(PackageInterface $package)
+    public function getInstallPath(PackageInterface $package): string
     {
         return $this->installPath;
     }
diff --git a/src/Composer/Installer/SuggestedPackagesReporter.php b/src/Composer/Installer/SuggestedPackagesReporter.php
index f8fb9d887..17e51b5f6 100644
--- a/src/Composer/Installer/SuggestedPackagesReporter.php
+++ b/src/Composer/Installer/SuggestedPackagesReporter.php
@@ -47,7 +47,7 @@ class SuggestedPackagesReporter
     /**
      * @return array<array{source: string, target: string, reason: string}> Suggested packages with source, target and reason keys.
      */
-    public function getPackages()
+    public function getPackages(): array
     {
         return $this->suggestedPackages;
     }
@@ -63,7 +63,7 @@ class SuggestedPackagesReporter
      * @param  string                    $reason Reason the target package to be suggested
      * @return SuggestedPackagesReporter
      */
-    public function addPackage($source, $target, $reason)
+    public function addPackage($source, $target, $reason): SuggestedPackagesReporter
     {
         $this->suggestedPackages[] = array(
             'source' => $source,
@@ -80,7 +80,7 @@ class SuggestedPackagesReporter
      * @param  PackageInterface          $package
      * @return SuggestedPackagesReporter
      */
-    public function addSuggestionsFromPackage(PackageInterface $package)
+    public function addSuggestionsFromPackage(PackageInterface $package): SuggestedPackagesReporter
     {
         $source = $package->getPrettyName();
         foreach ($package->getSuggests() as $target => $reason) {
@@ -104,7 +104,7 @@ class SuggestedPackagesReporter
      * @param  PackageInterface|null    $onlyDependentsOf If passed in, only the suggestions from direct dependents of that package, or from the package itself, will be shown
      * @return void
      */
-    public function output($mode, InstalledRepository $installedRepo = null, PackageInterface $onlyDependentsOf = null)
+    public function output($mode, InstalledRepository $installedRepo = null, PackageInterface $onlyDependentsOf = null): void
     {
         $suggestedPackages = $this->getFilteredSuggestions($installedRepo, $onlyDependentsOf);
 
@@ -170,7 +170,7 @@ class SuggestedPackagesReporter
      * @param  PackageInterface|null    $onlyDependentsOf If passed in, only the suggestions from direct dependents of that package, or from the package itself, will be shown
      * @return void
      */
-    public function outputMinimalistic(InstalledRepository $installedRepo = null, PackageInterface $onlyDependentsOf = null)
+    public function outputMinimalistic(InstalledRepository $installedRepo = null, PackageInterface $onlyDependentsOf = null): void
     {
         $suggestedPackages = $this->getFilteredSuggestions($installedRepo, $onlyDependentsOf);
         if ($suggestedPackages) {
@@ -183,7 +183,7 @@ class SuggestedPackagesReporter
      * @param  PackageInterface|null    $onlyDependentsOf If passed in, only the suggestions from direct dependents of that package, or from the package itself, will be shown
      * @return mixed[]
      */
-    private function getFilteredSuggestions(InstalledRepository $installedRepo = null, PackageInterface $onlyDependentsOf = null)
+    private function getFilteredSuggestions(InstalledRepository $installedRepo = null, PackageInterface $onlyDependentsOf = null): array
     {
         $suggestedPackages = $this->getPackages();
         $installedNames = array();
@@ -198,7 +198,7 @@ class SuggestedPackagesReporter
 
         $sourceFilter = array();
         if ($onlyDependentsOf) {
-            $sourceFilter = array_map(function ($link) {
+            $sourceFilter = array_map(function ($link): string {
                 return $link->getTarget();
             }, array_merge($onlyDependentsOf->getRequires(), $onlyDependentsOf->getDevRequires()));
             $sourceFilter[] = $onlyDependentsOf->getName();
@@ -220,7 +220,7 @@ class SuggestedPackagesReporter
      * @param  string $string
      * @return string
      */
-    private function escapeOutput($string)
+    private function escapeOutput($string): string
     {
         return OutputFormatter::escape(
             $this->removeControlCharacters($string)
@@ -231,7 +231,7 @@ class SuggestedPackagesReporter
      * @param  string $string
      * @return string
      */
-    private function removeControlCharacters($string)
+    private function removeControlCharacters($string): string
     {
         return Preg::replace(
             '/[[:cntrl:]]/',
diff --git a/src/Composer/Json/JsonFile.php b/src/Composer/Json/JsonFile.php
index 35661c925..aa81fb61a 100644
--- a/src/Composer/Json/JsonFile.php
+++ b/src/Composer/Json/JsonFile.php
@@ -256,7 +256,7 @@ class JsonFile
      * @throws \RuntimeException
      * @return void
      */
-    private static function throwEncodeError($code)
+    private static function throwEncodeError($code): void
     {
         switch ($code) {
             case JSON_ERROR_DEPTH:
diff --git a/src/Composer/Json/JsonFormatter.php b/src/Composer/Json/JsonFormatter.php
index a85357f87..dd705db78 100644
--- a/src/Composer/Json/JsonFormatter.php
+++ b/src/Composer/Json/JsonFormatter.php
@@ -38,7 +38,7 @@ class JsonFormatter
      * @param  bool   $unescapeSlashes Un escape slashes
      * @return string
      */
-    public static function format($json, $unescapeUnicode, $unescapeSlashes)
+    public static function format($json, $unescapeUnicode, $unescapeSlashes): string
     {
         $result = '';
         $pos = 0;
diff --git a/src/Composer/Json/JsonManipulator.php b/src/Composer/Json/JsonManipulator.php
index 1e51f4dcc..8a309086d 100644
--- a/src/Composer/Json/JsonManipulator.php
+++ b/src/Composer/Json/JsonManipulator.php
@@ -94,7 +94,7 @@ class JsonManipulator
             // update existing link
             $existingPackage = $packageMatches['package'];
             $packageRegex = str_replace('/', '\\\\?/', preg_quote($existingPackage));
-            $links = Preg::replaceCallback('{'.self::$DEFINES.'"'.$packageRegex.'"(?P<separator>\s*:\s*)(?&string)}ix', function ($m) use ($existingPackage, $constraint) {
+            $links = Preg::replaceCallback('{'.self::$DEFINES.'"'.$packageRegex.'"(?P<separator>\s*:\s*)(?&string)}ix', function ($m) use ($existingPackage, $constraint): string {
                 return JsonFile::encode(str_replace('\\/', '/', $existingPackage)) . $m['separator'] . '"' . $constraint . '"';
             }, $links);
         } else {
@@ -133,9 +133,9 @@ class JsonManipulator
      * @param array<string> $packages
      * @return void
      */
-    private function sortPackages(array &$packages = array())
+    private function sortPackages(array &$packages = array()): void
     {
-        $prefix = function ($requirement) {
+        $prefix = function ($requirement): string {
             if (PlatformRepository::isPlatformPackage($requirement)) {
                 return Preg::replace(
                     array(
@@ -159,7 +159,7 @@ class JsonManipulator
             return '5-'.$requirement;
         };
 
-        uksort($packages, function ($a, $b) use ($prefix) {
+        uksort($packages, function ($a, $b) use ($prefix): int {
             return strnatcmp($prefix($a), $prefix($b));
         });
     }
@@ -297,7 +297,7 @@ class JsonManipulator
         // child exists
         $childRegex = '{'.self::$DEFINES.'(?P<start>"'.preg_quote($name).'"\s*:\s*)(?P<content>(?&json))(?P<end>,?)}x';
         if (Preg::isMatch($childRegex, $children, $matches)) {
-            $children = Preg::replaceCallback($childRegex, function ($matches) use ($subName, $value) {
+            $children = Preg::replaceCallback($childRegex, function ($matches) use ($subName, $value): string {
                 if ($subName !== null) {
                     $curVal = json_decode($matches['content'], true);
                     if (!is_array($curVal)) {
@@ -351,7 +351,7 @@ class JsonManipulator
             }
         }
 
-        $this->contents = Preg::replaceCallback($nodeRegex, function ($m) use ($children) {
+        $this->contents = Preg::replaceCallback($nodeRegex, function ($m) use ($children): string {
             return $m['start'] . $children . $m['end'];
         }, $this->contents);
 
@@ -436,7 +436,7 @@ class JsonManipulator
             $newline = $this->newline;
             $indent = $this->indent;
 
-            $this->contents = Preg::replaceCallback($nodeRegex, function ($matches) use ($indent, $newline) {
+            $this->contents = Preg::replaceCallback($nodeRegex, function ($matches) use ($indent, $newline): string {
                 return $matches['start'] . '{' . $newline . $indent . '}' . $matches['end'];
             }, $this->contents);
 
@@ -450,7 +450,7 @@ class JsonManipulator
             return true;
         }
 
-        $this->contents = Preg::replaceCallback($nodeRegex, function ($matches) use ($name, $subName, $childrenClean) {
+        $this->contents = Preg::replaceCallback($nodeRegex, function ($matches) use ($name, $subName, $childrenClean): string {
             if ($subName !== null) {
                 $curVal = json_decode($matches['content'], true);
                 unset($curVal[$name][$subName]);
diff --git a/src/Composer/Json/JsonValidationException.php b/src/Composer/Json/JsonValidationException.php
index e4533f672..35f61d32d 100644
--- a/src/Composer/Json/JsonValidationException.php
+++ b/src/Composer/Json/JsonValidationException.php
@@ -28,7 +28,7 @@ class JsonValidationException extends Exception
      * @param string   $message
      * @param string[] $errors
      */
-    public function __construct($message, $errors = array(), Exception $previous = null)
+    public function __construct($message, array $errors = array(), Exception $previous = null)
     {
         $this->errors = $errors;
         parent::__construct((string) $message, 0, $previous);
@@ -37,7 +37,7 @@ class JsonValidationException extends Exception
     /**
      * @return string[]
      */
-    public function getErrors()
+    public function getErrors(): array
     {
         return $this->errors;
     }
diff --git a/src/Composer/Package/AliasPackage.php b/src/Composer/Package/AliasPackage.php
index 4dd2bd9aa..5c2645eb1 100644
--- a/src/Composer/Package/AliasPackage.php
+++ b/src/Composer/Package/AliasPackage.php
@@ -83,7 +83,7 @@ class AliasPackage extends BasePackage
     /**
      * @inheritDoc
      */
-    public function getVersion()
+    public function getVersion(): string
     {
         return $this->version;
     }
@@ -91,7 +91,7 @@ class AliasPackage extends BasePackage
     /**
      * @inheritDoc
      */
-    public function getStability()
+    public function getStability(): string
     {
         return $this->stability;
     }
@@ -99,7 +99,7 @@ class AliasPackage extends BasePackage
     /**
      * @inheritDoc
      */
-    public function getPrettyVersion()
+    public function getPrettyVersion(): string
     {
         return $this->prettyVersion;
     }
@@ -107,7 +107,7 @@ class AliasPackage extends BasePackage
     /**
      * @inheritDoc
      */
-    public function isDev()
+    public function isDev(): bool
     {
         return $this->dev;
     }
@@ -115,7 +115,7 @@ class AliasPackage extends BasePackage
     /**
      * @inheritDoc
      */
-    public function getRequires()
+    public function getRequires(): array
     {
         return $this->requires;
     }
@@ -124,7 +124,7 @@ class AliasPackage extends BasePackage
      * @inheritDoc
      * @return array<string|int, Link>
      */
-    public function getConflicts()
+    public function getConflicts(): array
     {
         return $this->conflicts;
     }
@@ -133,7 +133,7 @@ class AliasPackage extends BasePackage
      * @inheritDoc
      * @return array<string|int, Link>
      */
-    public function getProvides()
+    public function getProvides(): array
     {
         return $this->provides;
     }
@@ -142,7 +142,7 @@ class AliasPackage extends BasePackage
      * @inheritDoc
      * @return array<string|int, Link>
      */
-    public function getReplaces()
+    public function getReplaces(): array
     {
         return $this->replaces;
     }
@@ -150,7 +150,7 @@ class AliasPackage extends BasePackage
     /**
      * @inheritDoc
      */
-    public function getDevRequires()
+    public function getDevRequires(): array
     {
         return $this->devRequires;
     }
@@ -173,7 +173,7 @@ class AliasPackage extends BasePackage
      * @see setRootPackageAlias
      * @return bool
      */
-    public function isRootPackageAlias()
+    public function isRootPackageAlias(): bool
     {
         return $this->rootPackageAlias;
     }
@@ -184,7 +184,7 @@ class AliasPackage extends BasePackage
      *
      * @return Link[]
      */
-    protected function replaceSelfVersionDependencies(array $links, $linkType)
+    protected function replaceSelfVersionDependencies(array $links, $linkType): array
     {
         // for self.version requirements, we use the original package's branch name instead, to avoid leaking the magic dev-master-alias to users
         $prettyVersion = $this->prettyVersion;
@@ -220,7 +220,7 @@ class AliasPackage extends BasePackage
     /**
      * @return bool
      */
-    public function hasSelfVersionRequires()
+    public function hasSelfVersionRequires(): bool
     {
         return $this->hasSelfVersionRequires;
     }
@@ -234,167 +234,167 @@ class AliasPackage extends BasePackage
      * Wrappers around the aliased package *
      ***************************************/
 
-    public function getType()
+    public function getType(): string
     {
         return $this->aliasOf->getType();
     }
 
-    public function getTargetDir()
+    public function getTargetDir(): ?string
     {
         return $this->aliasOf->getTargetDir();
     }
 
-    public function getExtra()
+    public function getExtra(): array
     {
         return $this->aliasOf->getExtra();
     }
 
-    public function setInstallationSource($type)
+    public function setInstallationSource($type): void
     {
         $this->aliasOf->setInstallationSource($type);
     }
 
-    public function getInstallationSource()
+    public function getInstallationSource(): ?string
     {
         return $this->aliasOf->getInstallationSource();
     }
 
-    public function getSourceType()
+    public function getSourceType(): ?string
     {
         return $this->aliasOf->getSourceType();
     }
 
-    public function getSourceUrl()
+    public function getSourceUrl(): ?string
     {
         return $this->aliasOf->getSourceUrl();
     }
 
-    public function getSourceUrls()
+    public function getSourceUrls(): array
     {
         return $this->aliasOf->getSourceUrls();
     }
 
-    public function getSourceReference()
+    public function getSourceReference(): ?string
     {
         return $this->aliasOf->getSourceReference();
     }
 
-    public function setSourceReference($reference)
+    public function setSourceReference($reference): void
     {
         $this->aliasOf->setSourceReference($reference);
     }
 
-    public function setSourceMirrors($mirrors)
+    public function setSourceMirrors($mirrors): void
     {
         $this->aliasOf->setSourceMirrors($mirrors);
     }
 
-    public function getSourceMirrors()
+    public function getSourceMirrors(): ?array
     {
         return $this->aliasOf->getSourceMirrors();
     }
 
-    public function getDistType()
+    public function getDistType(): ?string
     {
         return $this->aliasOf->getDistType();
     }
 
-    public function getDistUrl()
+    public function getDistUrl(): ?string
     {
         return $this->aliasOf->getDistUrl();
     }
 
-    public function getDistUrls()
+    public function getDistUrls(): array
     {
         return $this->aliasOf->getDistUrls();
     }
 
-    public function getDistReference()
+    public function getDistReference(): ?string
     {
         return $this->aliasOf->getDistReference();
     }
 
-    public function setDistReference($reference)
+    public function setDistReference($reference): void
     {
         $this->aliasOf->setDistReference($reference);
     }
 
-    public function getDistSha1Checksum()
+    public function getDistSha1Checksum(): ?string
     {
         return $this->aliasOf->getDistSha1Checksum();
     }
 
-    public function setTransportOptions(array $options)
+    public function setTransportOptions(array $options): void
     {
         $this->aliasOf->setTransportOptions($options);
     }
 
-    public function getTransportOptions()
+    public function getTransportOptions(): array
     {
         return $this->aliasOf->getTransportOptions();
     }
 
-    public function setDistMirrors($mirrors)
+    public function setDistMirrors($mirrors): void
     {
         $this->aliasOf->setDistMirrors($mirrors);
     }
 
-    public function getDistMirrors()
+    public function getDistMirrors(): ?array
     {
         return $this->aliasOf->getDistMirrors();
     }
 
-    public function getAutoload()
+    public function getAutoload(): array
     {
         return $this->aliasOf->getAutoload();
     }
 
-    public function getDevAutoload()
+    public function getDevAutoload(): array
     {
         return $this->aliasOf->getDevAutoload();
     }
 
-    public function getIncludePaths()
+    public function getIncludePaths(): array
     {
         return $this->aliasOf->getIncludePaths();
     }
 
-    public function getReleaseDate()
+    public function getReleaseDate(): ?\DateTime
     {
         return $this->aliasOf->getReleaseDate();
     }
 
-    public function getBinaries()
+    public function getBinaries(): array
     {
         return $this->aliasOf->getBinaries();
     }
 
-    public function getSuggests()
+    public function getSuggests(): array
     {
         return $this->aliasOf->getSuggests();
     }
 
-    public function getNotificationUrl()
+    public function getNotificationUrl(): ?string
     {
         return $this->aliasOf->getNotificationUrl();
     }
 
-    public function isDefaultBranch()
+    public function isDefaultBranch(): bool
     {
         return $this->aliasOf->isDefaultBranch();
     }
 
-    public function setDistUrl($url)
+    public function setDistUrl($url): void
     {
         $this->aliasOf->setDistUrl($url);
     }
 
-    public function setDistType($type)
+    public function setDistType($type): void
     {
         $this->aliasOf->setDistType($type);
     }
 
-    public function setSourceDistReferences($reference)
+    public function setSourceDistReferences($reference): void
     {
         $this->aliasOf->setSourceDistReferences($reference);
     }
diff --git a/src/Composer/Package/Archiver/ArchivableFilesFilter.php b/src/Composer/Package/Archiver/ArchivableFilesFilter.php
index 3242df7e8..9a5be1a19 100644
--- a/src/Composer/Package/Archiver/ArchivableFilesFilter.php
+++ b/src/Composer/Package/Archiver/ArchivableFilesFilter.php
@@ -40,7 +40,7 @@ class ArchivableFilesFilter extends FilterIterator
      *
      * @return void
      */
-    public function addEmptyDir(PharData $phar, $sources)
+    public function addEmptyDir(PharData $phar, $sources): void
     {
         foreach ($this->dirs as $filepath) {
             $localname = str_replace($sources . "/", '', $filepath);
diff --git a/src/Composer/Package/Archiver/ArchivableFilesFinder.php b/src/Composer/Package/Archiver/ArchivableFilesFinder.php
index b1d5650ed..31e81e295 100644
--- a/src/Composer/Package/Archiver/ArchivableFilesFinder.php
+++ b/src/Composer/Package/Archiver/ArchivableFilesFinder.php
@@ -57,7 +57,7 @@ class ArchivableFilesFinder extends \FilterIterator
 
         $this->finder = new Finder();
 
-        $filter = function (\SplFileInfo $file) use ($sources, $filters, $fs) {
+        $filter = function (\SplFileInfo $file) use ($sources, $filters, $fs): bool {
             if ($file->isLink() && strpos($file->getRealPath(), $sources) !== 0) {
                 return false;
             }
diff --git a/src/Composer/Package/Archiver/ArchiveManager.php b/src/Composer/Package/Archiver/ArchiveManager.php
index a6682fb3b..de8ff52e8 100644
--- a/src/Composer/Package/Archiver/ArchiveManager.php
+++ b/src/Composer/Package/Archiver/ArchiveManager.php
@@ -56,7 +56,7 @@ class ArchiveManager
      *
      * @return void
      */
-    public function addArchiver(ArchiverInterface $archiver)
+    public function addArchiver(ArchiverInterface $archiver): void
     {
         $this->archivers[] = $archiver;
     }
@@ -82,7 +82,7 @@ class ArchiveManager
      *
      * @return string A filename without an extension
      */
-    public function getPackageFilename(CompletePackageInterface $package)
+    public function getPackageFilename(CompletePackageInterface $package): string
     {
         if ($package->getArchiveName()) {
             $baseName = $package->getArchiveName();
@@ -101,7 +101,7 @@ class ArchiveManager
             $nameParts[] = substr(sha1($package->getSourceReference()), 0, 6);
         }
 
-        $name = implode('-', array_filter($nameParts, function ($p) {
+        $name = implode('-', array_filter($nameParts, function ($p): bool {
             return !empty($p);
         }));
 
@@ -121,7 +121,7 @@ class ArchiveManager
      * @throws \RuntimeException
      * @return string                    The path of the created archive
      */
-    public function archive(CompletePackageInterface $package, $format, $targetDir, $fileName = null, $ignoreFilters = false)
+    public function archive(CompletePackageInterface $package, $format, $targetDir, $fileName = null, $ignoreFilters = false): string
     {
         if (empty($format)) {
             throw new \InvalidArgumentException('Format must be specified');
diff --git a/src/Composer/Package/Archiver/ArchiverInterface.php b/src/Composer/Package/Archiver/ArchiverInterface.php
index 4ea8dc485..a9038ca76 100644
--- a/src/Composer/Package/Archiver/ArchiverInterface.php
+++ b/src/Composer/Package/Archiver/ArchiverInterface.php
@@ -30,7 +30,7 @@ interface ArchiverInterface
      *
      * @return string The path to the written archive file
      */
-    public function archive($sources, $target, $format, array $excludes = array(), $ignoreFilters = false);
+    public function archive($sources, $target, $format, array $excludes = array(), $ignoreFilters = false): string;
 
     /**
      * Format supported by the archiver.
@@ -40,5 +40,5 @@ interface ArchiverInterface
      *
      * @return bool true if the format is supported by the archiver
      */
-    public function supports($format, $sourceType);
+    public function supports($format, $sourceType): bool;
 }
diff --git a/src/Composer/Package/Archiver/BaseExcludeFilter.php b/src/Composer/Package/Archiver/BaseExcludeFilter.php
index 9a9c3b8c2..2fbd22796 100644
--- a/src/Composer/Package/Archiver/BaseExcludeFilter.php
+++ b/src/Composer/Package/Archiver/BaseExcludeFilter.php
@@ -49,7 +49,7 @@ abstract class BaseExcludeFilter
      *
      * @return bool Whether the file should be excluded
      */
-    public function filter($relativePath, $exclude)
+    public function filter($relativePath, $exclude): bool
     {
         foreach ($this->excludePatterns as $patternData) {
             list($pattern, $negate, $stripLeadingSlash) = $patternData;
@@ -80,7 +80,7 @@ abstract class BaseExcludeFilter
      *
      * @return array<array{0: non-empty-string, 1: bool, 2: bool}> Exclude patterns to be used in filter()
      */
-    protected function parseLines(array $lines, $lineParser)
+    protected function parseLines(array $lines, $lineParser): array
     {
         return array_filter(
             array_map(
@@ -95,7 +95,7 @@ abstract class BaseExcludeFilter
                 },
                 $lines
             ),
-            function ($pattern) {
+            function ($pattern): bool {
                 return $pattern !== null;
             }
         );
@@ -108,7 +108,7 @@ abstract class BaseExcludeFilter
      *
      * @return array<int, array{0: non-empty-string, 1: bool, 2: bool}> Exclude patterns
      */
-    protected function generatePatterns($rules)
+    protected function generatePatterns($rules): array
     {
         $patterns = array();
         foreach ($rules as $rule) {
@@ -125,7 +125,7 @@ abstract class BaseExcludeFilter
      *
      * @return array{0: non-empty-string, 1: bool, 2: bool} An exclude pattern
      */
-    protected function generatePattern($rule)
+    protected function generatePattern($rule): array
     {
         $negate = false;
         $pattern = '';
diff --git a/src/Composer/Package/Archiver/GitExcludeFilter.php b/src/Composer/Package/Archiver/GitExcludeFilter.php
index 6a9294529..351ca4e7d 100644
--- a/src/Composer/Package/Archiver/GitExcludeFilter.php
+++ b/src/Composer/Package/Archiver/GitExcludeFilter.php
@@ -50,7 +50,7 @@ class GitExcludeFilter extends BaseExcludeFilter
      *
      * @return array{0: string, 1: bool, 2: bool}|null An exclude pattern for filter()
      */
-    public function parseGitAttributesLine($line)
+    public function parseGitAttributesLine($line): ?array
     {
         $parts = Preg::split('#\s+#', $line);
 
diff --git a/src/Composer/Package/Archiver/PharArchiver.php b/src/Composer/Package/Archiver/PharArchiver.php
index 582e4537e..f0d3abaf5 100644
--- a/src/Composer/Package/Archiver/PharArchiver.php
+++ b/src/Composer/Package/Archiver/PharArchiver.php
@@ -36,7 +36,7 @@ class PharArchiver implements ArchiverInterface
     /**
      * @inheritDoc
      */
-    public function archive($sources, $target, $format, array $excludes = array(), $ignoreFilters = false)
+    public function archive($sources, $target, $format, array $excludes = array(), $ignoreFilters = false): string
     {
         $sources = realpath($sources);
 
@@ -97,7 +97,7 @@ class PharArchiver implements ArchiverInterface
     /**
      * @inheritDoc
      */
-    public function supports($format, $sourceType)
+    public function supports($format, $sourceType): bool
     {
         return isset(static::$formats[$format]);
     }
diff --git a/src/Composer/Package/Archiver/ZipArchiver.php b/src/Composer/Package/Archiver/ZipArchiver.php
index 4e83a2f4d..8c7a4e59c 100644
--- a/src/Composer/Package/Archiver/ZipArchiver.php
+++ b/src/Composer/Package/Archiver/ZipArchiver.php
@@ -28,7 +28,7 @@ class ZipArchiver implements ArchiverInterface
     /**
      * @inheritDoc
      */
-    public function archive($sources, $target, $format, array $excludes = array(), $ignoreFilters = false)
+    public function archive($sources, $target, $format, array $excludes = array(), $ignoreFilters = false): string
     {
         $fs = new Filesystem();
         $sources = $fs->normalizePath($sources);
@@ -78,7 +78,7 @@ class ZipArchiver implements ArchiverInterface
     /**
      * @inheritDoc
      */
-    public function supports($format, $sourceType)
+    public function supports($format, $sourceType): bool
     {
         return isset(static::$formats[$format]) && $this->compressionAvailable();
     }
@@ -86,7 +86,7 @@ class ZipArchiver implements ArchiverInterface
     /**
      * @return bool
      */
-    private function compressionAvailable()
+    private function compressionAvailable(): bool
     {
         return class_exists('ZipArchive');
     }
diff --git a/src/Composer/Package/BasePackage.php b/src/Composer/Package/BasePackage.php
index a3ec9c570..f46cec9fd 100644
--- a/src/Composer/Package/BasePackage.php
+++ b/src/Composer/Package/BasePackage.php
@@ -274,7 +274,7 @@ abstract class BasePackage implements PackageInterface
     public static function packageNamesToRegexp(array $packageNames, $wrap = '{^(?:%s)$}iD')
     {
         $packageNames = array_map(
-            function ($packageName) {
+            function ($packageName): string {
                 return BasePackage::packageNameToRegexp($packageName, '%s');
             },
             $packageNames
diff --git a/src/Composer/Package/Comparer/Comparer.php b/src/Composer/Package/Comparer/Comparer.php
index b24661ad3..d7c2a5fc7 100644
--- a/src/Composer/Package/Comparer/Comparer.php
+++ b/src/Composer/Package/Comparer/Comparer.php
@@ -31,7 +31,7 @@ class Comparer
      *
      * @return void
      */
-    public function setSource($source)
+    public function setSource($source): void
     {
         $this->source = $source;
     }
@@ -41,7 +41,7 @@ class Comparer
      *
      * @return void
      */
-    public function setUpdate($update)
+    public function setUpdate($update): void
     {
         $this->update = $update;
     }
@@ -82,7 +82,7 @@ class Comparer
     /**
      * @return void
      */
-    public function doCompare()
+    public function doCompare(): void
     {
         $source = array();
         $destination = array();
diff --git a/src/Composer/Package/CompleteAliasPackage.php b/src/Composer/Package/CompleteAliasPackage.php
index 5c9e340f3..ea2dfad45 100644
--- a/src/Composer/Package/CompleteAliasPackage.php
+++ b/src/Composer/Package/CompleteAliasPackage.php
@@ -40,127 +40,127 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf;
     }
 
-    public function getScripts()
+    public function getScripts(): array
     {
         return $this->aliasOf->getScripts();
     }
 
-    public function setScripts(array $scripts)
+    public function setScripts(array $scripts): void
     {
         $this->aliasOf->setScripts($scripts);
     }
 
-    public function getRepositories()
+    public function getRepositories(): array
     {
         return $this->aliasOf->getRepositories();
     }
 
-    public function setRepositories(array $repositories)
+    public function setRepositories(array $repositories): void
     {
         $this->aliasOf->setRepositories($repositories);
     }
 
-    public function getLicense()
+    public function getLicense(): array
     {
         return $this->aliasOf->getLicense();
     }
 
-    public function setLicense(array $license)
+    public function setLicense(array $license): void
     {
         $this->aliasOf->setLicense($license);
     }
 
-    public function getKeywords()
+    public function getKeywords(): array
     {
         return $this->aliasOf->getKeywords();
     }
 
-    public function setKeywords(array $keywords)
+    public function setKeywords(array $keywords): void
     {
         $this->aliasOf->setKeywords($keywords);
     }
 
-    public function getDescription()
+    public function getDescription(): ?string
     {
         return $this->aliasOf->getDescription();
     }
 
-    public function setDescription($description)
+    public function setDescription($description): void
     {
         $this->aliasOf->setDescription($description);
     }
 
-    public function getHomepage()
+    public function getHomepage(): ?string
     {
         return $this->aliasOf->getHomepage();
     }
 
-    public function setHomepage($homepage)
+    public function setHomepage($homepage): void
     {
         $this->aliasOf->setHomepage($homepage);
     }
 
-    public function getAuthors()
+    public function getAuthors(): array
     {
         return $this->aliasOf->getAuthors();
     }
 
-    public function setAuthors(array $authors)
+    public function setAuthors(array $authors): void
     {
         $this->aliasOf->setAuthors($authors);
     }
 
-    public function getSupport()
+    public function getSupport(): array
     {
         return $this->aliasOf->getSupport();
     }
 
-    public function setSupport(array $support)
+    public function setSupport(array $support): void
     {
         $this->aliasOf->setSupport($support);
     }
 
-    public function getFunding()
+    public function getFunding(): array
     {
         return $this->aliasOf->getFunding();
     }
 
-    public function setFunding(array $funding)
+    public function setFunding(array $funding): void
     {
         $this->aliasOf->setFunding($funding);
     }
 
-    public function isAbandoned()
+    public function isAbandoned(): bool
     {
         return $this->aliasOf->isAbandoned();
     }
 
-    public function getReplacementPackage()
+    public function getReplacementPackage(): ?string
     {
         return $this->aliasOf->getReplacementPackage();
     }
 
-    public function setAbandoned($abandoned)
+    public function setAbandoned($abandoned): void
     {
         $this->aliasOf->setAbandoned($abandoned);
     }
 
-    public function getArchiveName()
+    public function getArchiveName(): ?string
     {
         return $this->aliasOf->getArchiveName();
     }
 
-    public function setArchiveName($name)
+    public function setArchiveName($name): void
     {
         $this->aliasOf->setArchiveName($name);
     }
 
-    public function getArchiveExcludes()
+    public function getArchiveExcludes(): array
     {
         return $this->aliasOf->getArchiveExcludes();
     }
 
-    public function setArchiveExcludes(array $excludes)
+    public function setArchiveExcludes(array $excludes): void
     {
         $this->aliasOf->setArchiveExcludes($excludes);
     }
diff --git a/src/Composer/Package/Dumper/ArrayDumper.php b/src/Composer/Package/Dumper/ArrayDumper.php
index 9b6e40946..fd54a516f 100644
--- a/src/Composer/Package/Dumper/ArrayDumper.php
+++ b/src/Composer/Package/Dumper/ArrayDumper.php
@@ -26,7 +26,7 @@ class ArrayDumper
     /**
      * @return array<string, mixed>
      */
-    public function dump(PackageInterface $package)
+    public function dump(PackageInterface $package): array
     {
         $keys = array(
             'binaries' => 'bin',
@@ -148,7 +148,7 @@ class ArrayDumper
      *
      * @return array<string, mixed>
      */
-    private function dumpValues(PackageInterface $package, array $keys, array $data)
+    private function dumpValues(PackageInterface $package, array $keys, array $data): array
     {
         foreach ($keys as $method => $key) {
             if (is_numeric($method)) {
diff --git a/src/Composer/Package/Link.php b/src/Composer/Package/Link.php
index af08705df..8e613a565 100644
--- a/src/Composer/Package/Link.php
+++ b/src/Composer/Package/Link.php
@@ -105,7 +105,7 @@ class Link
     /**
      * @return string
      */
-    public function getDescription()
+    public function getDescription(): string
     {
         return $this->description;
     }
@@ -113,7 +113,7 @@ class Link
     /**
      * @return string
      */
-    public function getSource()
+    public function getSource(): string
     {
         return $this->source;
     }
@@ -121,7 +121,7 @@ class Link
     /**
      * @return string
      */
-    public function getTarget()
+    public function getTarget(): string
     {
         return $this->target;
     }
@@ -129,7 +129,7 @@ class Link
     /**
      * @return ConstraintInterface
      */
-    public function getConstraint()
+    public function getConstraint(): ConstraintInterface
     {
         return $this->constraint;
     }
@@ -138,7 +138,7 @@ class Link
      * @throws \UnexpectedValueException If no pretty constraint was provided
      * @return string
      */
-    public function getPrettyConstraint()
+    public function getPrettyConstraint(): string
     {
         if (null === $this->prettyConstraint) {
             throw new \UnexpectedValueException(sprintf('Link %s has been misconfigured and had no prettyConstraint given.', $this));
@@ -159,7 +159,7 @@ class Link
      * @param  PackageInterface $sourcePackage
      * @return string
      */
-    public function getPrettyString(PackageInterface $sourcePackage)
+    public function getPrettyString(PackageInterface $sourcePackage): string
     {
         return $sourcePackage->getPrettyString().' '.$this->description.' '.$this->target.' '.$this->constraint->getPrettyString();
     }
diff --git a/src/Composer/Package/Loader/ArrayLoader.php b/src/Composer/Package/Loader/ArrayLoader.php
index 403da9a45..16cede400 100644
--- a/src/Composer/Package/Loader/ArrayLoader.php
+++ b/src/Composer/Package/Loader/ArrayLoader.php
@@ -321,7 +321,7 @@ class ArrayLoader implements LoaderInterface
      *
      * @return void
      */
-    private function configureCachedLinks(&$linkCache, $package, array $config)
+    private function configureCachedLinks(&$linkCache, $package, array $config): void
     {
         $name = $package->getName();
         $prettyVersion = $package->getPrettyVersion();
@@ -385,7 +385,7 @@ class ArrayLoader implements LoaderInterface
      * @param  string       $prettyConstraint constraint string
      * @return Link
      */
-    private function createLink($source, $sourceVersion, $description, $target, $prettyConstraint)
+    private function createLink($source, $sourceVersion, $description, $target, $prettyConstraint): Link
     {
         if (!\is_string($prettyConstraint)) {
             throw new \UnexpectedValueException('Link constraint in '.$source.' '.$description.' > '.$target.' should be a string, got '.\gettype($prettyConstraint) . ' (' . var_export($prettyConstraint, true) . ')');
diff --git a/src/Composer/Package/Loader/InvalidPackageException.php b/src/Composer/Package/Loader/InvalidPackageException.php
index 25141de25..80c713544 100644
--- a/src/Composer/Package/Loader/InvalidPackageException.php
+++ b/src/Composer/Package/Loader/InvalidPackageException.php
@@ -40,7 +40,7 @@ class InvalidPackageException extends \Exception
     /**
      * @return mixed[]
      */
-    public function getData()
+    public function getData(): array
     {
         return $this->data;
     }
@@ -48,7 +48,7 @@ class InvalidPackageException extends \Exception
     /**
      * @return string[]
      */
-    public function getErrors()
+    public function getErrors(): array
     {
         return $this->errors;
     }
@@ -56,7 +56,7 @@ class InvalidPackageException extends \Exception
     /**
      * @return string[]
      */
-    public function getWarnings()
+    public function getWarnings(): array
     {
         return $this->warnings;
     }
diff --git a/src/Composer/Package/Loader/RootPackageLoader.php b/src/Composer/Package/Loader/RootPackageLoader.php
index cd061941c..9c0c47c8f 100644
--- a/src/Composer/Package/Loader/RootPackageLoader.php
+++ b/src/Composer/Package/Loader/RootPackageLoader.php
@@ -194,7 +194,7 @@ class RootPackageLoader extends ArrayLoader
      *
      * @return list<array{package: string, version: string, alias: string, alias_normalized: string}>
      */
-    private function extractAliases(array $requires, array $aliases)
+    private function extractAliases(array $requires, array $aliases): array
     {
         foreach ($requires as $reqName => $reqVersion) {
             if (Preg::isMatch('{^([^,\s#]+)(?:#[^ ]+)? +as +([^,\s]+)$}', $reqVersion, $match)) {
@@ -224,7 +224,7 @@ class RootPackageLoader extends ArrayLoader
      * @phpstan-param array<string, BasePackage::STABILITY_*> $stabilityFlags
      * @phpstan-return array<string, BasePackage::STABILITY_*>
      */
-    public static function extractStabilityFlags(array $requires, $minimumStability, array $stabilityFlags)
+    public static function extractStabilityFlags(array $requires, $minimumStability, array $stabilityFlags): array
     {
         $stabilities = BasePackage::$stabilities;
         /** @var int $minimumStability */
@@ -286,7 +286,7 @@ class RootPackageLoader extends ArrayLoader
      *
      * @return array<string, string>
      */
-    public static function extractReferences(array $requires, array $references)
+    public static function extractReferences(array $requires, array $references): array
     {
         foreach ($requires as $reqName => $reqVersion) {
             $reqVersion = Preg::replace('{^([^,\s@]+) as .+$}', '$1', $reqVersion);
diff --git a/src/Composer/Package/Loader/ValidatingArrayLoader.php b/src/Composer/Package/Loader/ValidatingArrayLoader.php
index 7fb2c3b79..2a1278448 100644
--- a/src/Composer/Package/Loader/ValidatingArrayLoader.php
+++ b/src/Composer/Package/Loader/ValidatingArrayLoader.php
@@ -431,7 +431,7 @@ class ValidatingArrayLoader implements LoaderInterface
     /**
      * @return string[]
      */
-    public function getWarnings()
+    public function getWarnings(): array
     {
         return $this->warnings;
     }
@@ -439,7 +439,7 @@ class ValidatingArrayLoader implements LoaderInterface
     /**
      * @return string[]
      */
-    public function getErrors()
+    public function getErrors(): array
     {
         return $this->errors;
     }
@@ -450,7 +450,7 @@ class ValidatingArrayLoader implements LoaderInterface
      *
      * @return string|null
      */
-    public static function hasPackageNamingError($name, $isLink = false)
+    public static function hasPackageNamingError($name, $isLink = false): ?string
     {
         if (PlatformRepository::isPlatformPackage($name)) {
             return null;
@@ -494,7 +494,7 @@ class ValidatingArrayLoader implements LoaderInterface
      * @phpstan-param non-empty-string $property
      * @phpstan-param non-empty-string $regex
      */
-    private function validateRegex($property, $regex, $mandatory = false)
+    private function validateRegex($property, $regex, $mandatory = false): bool
     {
         if (!$this->validateString($property, $mandatory)) {
             return false;
@@ -523,7 +523,7 @@ class ValidatingArrayLoader implements LoaderInterface
      *
      * @phpstan-param non-empty-string $property
      */
-    private function validateString($property, $mandatory = false)
+    private function validateString($property, $mandatory = false): bool
     {
         if (isset($this->config[$property]) && !is_string($this->config[$property])) {
             $this->errors[] = $property.' : should be a string, '.gettype($this->config[$property]).' given';
@@ -552,7 +552,7 @@ class ValidatingArrayLoader implements LoaderInterface
      *
      * @phpstan-param non-empty-string $property
      */
-    private function validateArray($property, $mandatory = false)
+    private function validateArray($property, $mandatory = false): bool
     {
         if (isset($this->config[$property]) && !is_array($this->config[$property])) {
             $this->errors[] = $property.' : should be an array, '.gettype($this->config[$property]).' given';
@@ -583,7 +583,7 @@ class ValidatingArrayLoader implements LoaderInterface
      * @phpstan-param non-empty-string      $property
      * @phpstan-param non-empty-string|null $regex
      */
-    private function validateFlatArray($property, $regex = null, $mandatory = false)
+    private function validateFlatArray($property, $regex = null, $mandatory = false): bool
     {
         if (!$this->validateArray($property, $mandatory)) {
             return false;
@@ -617,7 +617,7 @@ class ValidatingArrayLoader implements LoaderInterface
      *
      * @phpstan-param non-empty-string $property
      */
-    private function validateUrl($property, $mandatory = false)
+    private function validateUrl($property, $mandatory = false): bool
     {
         if (!$this->validateString($property, $mandatory)) {
             return false;
@@ -639,7 +639,7 @@ class ValidatingArrayLoader implements LoaderInterface
      *
      * @return bool
      */
-    private function filterUrl($value, array $schemes = array('http', 'https'))
+    private function filterUrl($value, array $schemes = array('http', 'https')): bool
     {
         if ($value === '') {
             return true;
diff --git a/src/Composer/Package/Locker.php b/src/Composer/Package/Locker.php
index 5a306d14d..1702d2ba9 100644
--- a/src/Composer/Package/Locker.php
+++ b/src/Composer/Package/Locker.php
@@ -78,7 +78,7 @@ class Locker
      *
      * @return string
      */
-    public static function getContentHash($composerFileContents)
+    public static function getContentHash($composerFileContents): string
     {
         $content = JsonFile::parseJson($composerFileContents, 'composer.json');
 
@@ -115,7 +115,7 @@ class Locker
      *
      * @return bool
      */
-    public function isLocked()
+    public function isLocked(): bool
     {
         if (!$this->virtualFileWritten && !$this->lockFile->exists()) {
             return false;
@@ -131,7 +131,7 @@ class Locker
      *
      * @return bool
      */
-    public function isFresh()
+    public function isFresh(): bool
     {
         $lock = $this->lockFile->read();
 
@@ -156,7 +156,7 @@ class Locker
      * @throws \RuntimeException
      * @return \Composer\Repository\LockArrayRepository
      */
-    public function getLockedRepository($withDevReqs = false)
+    public function getLockedRepository($withDevReqs = false): \Composer\Repository\LockArrayRepository
     {
         $lockData = $this->getLockData();
         $packages = new LockArrayRepository();
@@ -205,7 +205,7 @@ class Locker
     /**
      * @return string[] Names of dependencies installed through require-dev
      */
-    public function getDevPackageNames()
+    public function getDevPackageNames(): array
     {
         $names = array();
         $lockData = $this->getLockData();
@@ -224,7 +224,7 @@ class Locker
      * @param  bool                     $withDevReqs if true, the platform requirements from the require-dev block are also returned
      * @return \Composer\Package\Link[]
      */
-    public function getPlatformRequirements($withDevReqs = false)
+    public function getPlatformRequirements($withDevReqs = false): array
     {
         $lockData = $this->getLockData();
         $requirements = array();
@@ -255,7 +255,7 @@ class Locker
     /**
      * @return string
      */
-    public function getMinimumStability()
+    public function getMinimumStability(): string
     {
         $lockData = $this->getLockData();
 
@@ -265,7 +265,7 @@ class Locker
     /**
      * @return array<string, string>
      */
-    public function getStabilityFlags()
+    public function getStabilityFlags(): array
     {
         $lockData = $this->getLockData();
 
@@ -275,7 +275,7 @@ class Locker
     /**
      * @return bool|null
      */
-    public function getPreferStable()
+    public function getPreferStable(): ?bool
     {
         $lockData = $this->getLockData();
 
@@ -287,7 +287,7 @@ class Locker
     /**
      * @return bool|null
      */
-    public function getPreferLowest()
+    public function getPreferLowest(): ?bool
     {
         $lockData = $this->getLockData();
 
@@ -299,7 +299,7 @@ class Locker
     /**
      * @return array<string, string>
      */
-    public function getPlatformOverrides()
+    public function getPlatformOverrides(): array
     {
         $lockData = $this->getLockData();
 
@@ -311,7 +311,7 @@ class Locker
      *
      * @phpstan-return list<array{package: string, version: string, alias: string, alias_normalized: string}>
      */
-    public function getAliases()
+    public function getAliases(): array
     {
         $lockData = $this->getLockData();
 
@@ -321,7 +321,7 @@ class Locker
     /**
      * @return array<string, mixed>
      */
-    public function getLockData()
+    public function getLockData(): array
     {
         if (null !== $this->lockDataCache) {
             return $this->lockDataCache;
@@ -353,11 +353,11 @@ class Locker
      *
      * @phpstan-param list<array{package: string, version: string, alias: string, alias_normalized: string}> $aliases
      */
-    public function setLockData(array $packages, $devPackages, array $platformReqs, $platformDevReqs, array $aliases, $minimumStability, array $stabilityFlags, $preferStable, $preferLowest, array $platformOverrides, $write = true)
+    public function setLockData(array $packages, $devPackages, array $platformReqs, $platformDevReqs, array $aliases, $minimumStability, array $stabilityFlags, $preferStable, $preferLowest, array $platformOverrides, $write = true): bool
     {
         // keep old default branch names normalized to DEFAULT_BRANCH_ALIAS for BC as that is how Composer 1 outputs the lock file
         // when loading the lock file the version is anyway ignored in Composer 2, so it has no adverse effect
-        $aliases = array_map(function ($alias) {
+        $aliases = array_map(function ($alias): array {
             if (in_array($alias['version'], array('dev-master', 'dev-trunk', 'dev-default'), true)) {
                 $alias['version'] = VersionParser::DEFAULT_BRANCH_ALIAS;
             }
@@ -419,7 +419,7 @@ class Locker
      *
      * @phpstan-return list<array<string, mixed>>
      */
-    private function lockPackages(array $packages)
+    private function lockPackages(array $packages): array
     {
         $locked = array();
 
@@ -477,7 +477,7 @@ class Locker
      * @param  PackageInterface $package The package to scan.
      * @return string|null      The formatted datetime or null if none was found.
      */
-    private function getPackageTime(PackageInterface $package)
+    private function getPackageTime(PackageInterface $package): ?string
     {
         if (!function_exists('proc_open')) {
             return null;
diff --git a/src/Composer/Package/Package.php b/src/Composer/Package/Package.php
index a0be96f3d..7549c4008 100644
--- a/src/Composer/Package/Package.php
+++ b/src/Composer/Package/Package.php
@@ -788,7 +788,7 @@ class Package extends BasePackage
      * @param  string $source
      * @return array<string, Link>
      */
-    private function convertLinksToMap(array $links, $source)
+    private function convertLinksToMap(array $links, $source): array
     {
         trigger_error('Package::'.$source.' must be called with a map of lowercased package name => Link object, got a indexed array, this is deprecated and you should fix your usage.');
         $newLinks = array();
diff --git a/src/Composer/Package/RootAliasPackage.php b/src/Composer/Package/RootAliasPackage.php
index 4817653dc..adf69b6b0 100644
--- a/src/Composer/Package/RootAliasPackage.php
+++ b/src/Composer/Package/RootAliasPackage.php
@@ -43,7 +43,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function getAliases()
+    public function getAliases(): array
     {
         return $this->aliasOf->getAliases();
     }
@@ -51,7 +51,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function getMinimumStability()
+    public function getMinimumStability(): string
     {
         return $this->aliasOf->getMinimumStability();
     }
@@ -59,7 +59,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function getStabilityFlags()
+    public function getStabilityFlags(): array
     {
         return $this->aliasOf->getStabilityFlags();
     }
@@ -67,7 +67,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function getReferences()
+    public function getReferences(): array
     {
         return $this->aliasOf->getReferences();
     }
@@ -75,7 +75,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function getPreferStable()
+    public function getPreferStable(): bool
     {
         return $this->aliasOf->getPreferStable();
     }
@@ -83,7 +83,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function getConfig()
+    public function getConfig(): array
     {
         return $this->aliasOf->getConfig();
     }
@@ -91,7 +91,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setRequires(array $require)
+    public function setRequires(array $require): void
     {
         $this->requires = $this->replaceSelfVersionDependencies($require, Link::TYPE_REQUIRE);
 
@@ -101,7 +101,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setDevRequires(array $devRequire)
+    public function setDevRequires(array $devRequire): void
     {
         $this->devRequires = $this->replaceSelfVersionDependencies($devRequire, Link::TYPE_DEV_REQUIRE);
 
@@ -111,7 +111,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setConflicts(array $conflicts)
+    public function setConflicts(array $conflicts): void
     {
         $this->conflicts = $this->replaceSelfVersionDependencies($conflicts, Link::TYPE_CONFLICT);
         $this->aliasOf->setConflicts($conflicts);
@@ -120,7 +120,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setProvides(array $provides)
+    public function setProvides(array $provides): void
     {
         $this->provides = $this->replaceSelfVersionDependencies($provides, Link::TYPE_PROVIDE);
         $this->aliasOf->setProvides($provides);
@@ -129,7 +129,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setReplaces(array $replaces)
+    public function setReplaces(array $replaces): void
     {
         $this->replaces = $this->replaceSelfVersionDependencies($replaces, Link::TYPE_REPLACE);
         $this->aliasOf->setReplaces($replaces);
@@ -138,7 +138,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setAutoload(array $autoload)
+    public function setAutoload(array $autoload): void
     {
         $this->aliasOf->setAutoload($autoload);
     }
@@ -146,7 +146,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setDevAutoload(array $devAutoload)
+    public function setDevAutoload(array $devAutoload): void
     {
         $this->aliasOf->setDevAutoload($devAutoload);
     }
@@ -154,7 +154,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setStabilityFlags(array $stabilityFlags)
+    public function setStabilityFlags(array $stabilityFlags): void
     {
         $this->aliasOf->setStabilityFlags($stabilityFlags);
     }
@@ -162,7 +162,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setMinimumStability($minimumStability)
+    public function setMinimumStability($minimumStability): void
     {
         $this->aliasOf->setMinimumStability($minimumStability);
     }
@@ -170,7 +170,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setPreferStable($preferStable)
+    public function setPreferStable($preferStable): void
     {
         $this->aliasOf->setPreferStable($preferStable);
     }
@@ -178,7 +178,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setConfig(array $config)
+    public function setConfig(array $config): void
     {
         $this->aliasOf->setConfig($config);
     }
@@ -186,7 +186,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setReferences(array $references)
+    public function setReferences(array $references): void
     {
         $this->aliasOf->setReferences($references);
     }
@@ -194,7 +194,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setAliases(array $aliases)
+    public function setAliases(array $aliases): void
     {
         $this->aliasOf->setAliases($aliases);
     }
@@ -202,7 +202,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setSuggests(array $suggests)
+    public function setSuggests(array $suggests): void
     {
         $this->aliasOf->setSuggests($suggests);
     }
@@ -210,7 +210,7 @@ class RootAliasPackage extends CompleteAliasPackage implements RootPackageInterf
     /**
      * @inheritDoc
      */
-    public function setExtra(array $extra)
+    public function setExtra(array $extra): void
     {
         $this->aliasOf->setExtra($extra);
     }
diff --git a/src/Composer/Package/RootPackage.php b/src/Composer/Package/RootPackage.php
index 070e7b6c9..38375dc73 100644
--- a/src/Composer/Package/RootPackage.php
+++ b/src/Composer/Package/RootPackage.php
@@ -37,7 +37,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setMinimumStability($minimumStability)
+    public function setMinimumStability($minimumStability): void
     {
         $this->minimumStability = $minimumStability;
     }
@@ -45,7 +45,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function getMinimumStability()
+    public function getMinimumStability(): string
     {
         return $this->minimumStability;
     }
@@ -53,7 +53,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function setStabilityFlags(array $stabilityFlags)
+    public function setStabilityFlags(array $stabilityFlags): void
     {
         $this->stabilityFlags = $stabilityFlags;
     }
@@ -61,7 +61,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function getStabilityFlags()
+    public function getStabilityFlags(): array
     {
         return $this->stabilityFlags;
     }
@@ -69,7 +69,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setPreferStable($preferStable)
+    public function setPreferStable($preferStable): void
     {
         $this->preferStable = $preferStable;
     }
@@ -77,7 +77,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function getPreferStable()
+    public function getPreferStable(): bool
     {
         return $this->preferStable;
     }
@@ -85,7 +85,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setConfig(array $config)
+    public function setConfig(array $config): void
     {
         $this->config = $config;
     }
@@ -93,7 +93,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function getConfig()
+    public function getConfig(): array
     {
         return $this->config;
     }
@@ -101,7 +101,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setReferences(array $references)
+    public function setReferences(array $references): void
     {
         $this->references = $references;
     }
@@ -109,7 +109,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function getReferences()
+    public function getReferences(): array
     {
         return $this->references;
     }
@@ -117,7 +117,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setAliases(array $aliases)
+    public function setAliases(array $aliases): void
     {
         $this->aliases = $aliases;
     }
@@ -125,7 +125,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function getAliases()
+    public function getAliases(): array
     {
         return $this->aliases;
     }
diff --git a/src/Composer/Package/Version/StabilityFilter.php b/src/Composer/Package/Version/StabilityFilter.php
index 8eda2033b..81f4ec063 100644
--- a/src/Composer/Package/Version/StabilityFilter.php
+++ b/src/Composer/Package/Version/StabilityFilter.php
@@ -30,7 +30,7 @@ class StabilityFilter
      * @param  string   $stability one of 'stable', 'RC', 'beta', 'alpha' or 'dev'
      * @return bool     true if any package name is acceptable
      */
-    public static function isPackageAcceptable(array $acceptableStabilities, array $stabilityFlags, array $names, $stability)
+    public static function isPackageAcceptable(array $acceptableStabilities, array $stabilityFlags, array $names, $stability): bool
     {
         foreach ($names as $name) {
             // allow if package matches the package-specific stability flag
diff --git a/src/Composer/Package/Version/VersionGuesser.php b/src/Composer/Package/Version/VersionGuesser.php
index 7927fa726..d5b0bf907 100644
--- a/src/Composer/Package/Version/VersionGuesser.php
+++ b/src/Composer/Package/Version/VersionGuesser.php
@@ -66,7 +66,7 @@ class VersionGuesser
      * @return array|null
      * @phpstan-return Version|null
      */
-    public function guessVersion(array $packageConfig, $path)
+    public function guessVersion(array $packageConfig, $path): ?array
     {
         if (!function_exists('proc_open')) {
             return null;
@@ -103,7 +103,7 @@ class VersionGuesser
      * @return array
      * @phpstan-return Version
      */
-    private function postprocess(array $versionData)
+    private function postprocess(array $versionData): array
     {
         if (!empty($versionData['feature_version']) && $versionData['feature_version'] === $versionData['version'] && $versionData['feature_pretty_version'] === $versionData['pretty_version']) {
             unset($versionData['feature_version'], $versionData['feature_pretty_version']);
@@ -126,7 +126,7 @@ class VersionGuesser
      *
      * @return array{version: string|null, commit: string|null, pretty_version: string|null, feature_version?: string|null, feature_pretty_version?: string|null}
      */
-    private function guessGitVersion(array $packageConfig, $path)
+    private function guessGitVersion(array $packageConfig, $path): array
     {
         GitUtil::cleanEnv();
         $commit = null;
@@ -211,7 +211,7 @@ class VersionGuesser
      *
      * @return array{version: string, pretty_version: string}|null
      */
-    private function versionFromGitTags($path)
+    private function versionFromGitTags($path): ?array
     {
         // try to fetch current version from git tags
         if (0 === $this->process->execute('git describe --exact-match --tags', $output, $path)) {
@@ -232,7 +232,7 @@ class VersionGuesser
      *
      * @return array{version: string|null, commit: ''|null, pretty_version: string|null, feature_version?: string|null, feature_pretty_version?: string|null}|null
      */
-    private function guessHgVersion(array $packageConfig, $path)
+    private function guessHgVersion(array $packageConfig, $path): ?array
     {
         // try to fetch current version from hg branch
         if (0 === $this->process->execute('hg branch', $output, $path)) {
@@ -276,7 +276,7 @@ class VersionGuesser
      *
      * @return array{version: string|null, pretty_version: string|null}
      */
-    private function guessFeatureVersion(array $packageConfig, $version, array $branches, $scmCmdline, $path)
+    private function guessFeatureVersion(array $packageConfig, $version, array $branches, $scmCmdline, $path): array
     {
         $prettyVersion = $version;
 
@@ -296,7 +296,7 @@ class VersionGuesser
             // sort local branches first then remote ones
             // and sort numeric branches below named ones, to make sure if the branch has the same distance from main and 1.10 and 1.9 for example, main is picked
             // and sort using natural sort so that 1.10 will appear before 1.9
-            usort($branches, function ($a, $b) {
+            usort($branches, function ($a, $b): int {
                 $aRemote = 0 === strpos($a, 'remotes/');
                 $bRemote = 0 === strpos($b, 'remotes/');
 
@@ -340,7 +340,7 @@ class VersionGuesser
      *
      * @return bool
      */
-    private function isFeatureBranch(array $packageConfig, $branchName)
+    private function isFeatureBranch(array $packageConfig, $branchName): bool
     {
         $nonFeatureBranches = '';
         if (!empty($packageConfig['non-feature-branches'])) {
@@ -355,7 +355,7 @@ class VersionGuesser
      *
      * @return array{version: string|null, commit: '', pretty_version: string|null}
      */
-    private function guessFossilVersion($path)
+    private function guessFossilVersion($path): array
     {
         $version = null;
         $prettyVersion = null;
@@ -385,7 +385,7 @@ class VersionGuesser
      *
      * @return array{version: string, commit: '', pretty_version: string}|null
      */
-    private function guessSvnVersion(array $packageConfig, $path)
+    private function guessSvnVersion(array $packageConfig, $path): ?array
     {
         SvnUtil::cleanEnv();
 
diff --git a/src/Composer/Package/Version/VersionSelector.php b/src/Composer/Package/Version/VersionSelector.php
index 8e4b45faa..431bdb851 100644
--- a/src/Composer/Package/Version/VersionSelector.php
+++ b/src/Composer/Package/Version/VersionSelector.php
@@ -87,7 +87,7 @@ class VersionSelector
 
         if ($this->platformConstraints && !($platformRequirementFilter instanceof IgnoreAllPlatformRequirementFilter)) {
             $platformConstraints = $this->platformConstraints;
-            $candidates = array_filter($candidates, function ($pkg) use ($platformConstraints, $platformRequirementFilter) {
+            $candidates = array_filter($candidates, function ($pkg) use ($platformConstraints, $platformRequirementFilter): bool {
                 $reqs = $pkg->getRequires();
 
                 foreach ($reqs as $name => $link) {
@@ -172,7 +172,7 @@ class VersionSelector
      * @param  PackageInterface $package
      * @return string
      */
-    public function findRecommendedRequireVersion(PackageInterface $package)
+    public function findRecommendedRequireVersion(PackageInterface $package): string
     {
         // Extensions which are versioned in sync with PHP should rather be required as "*" to simplify
         // the requires and have only one required version to change when bumping the php requirement
@@ -211,7 +211,7 @@ class VersionSelector
      *
      * @return string
      */
-    private function transformVersion($version, $prettyVersion, $stability)
+    private function transformVersion($version, $prettyVersion, $stability): string
     {
         // attempt to transform 2.1.1 to 2.1
         // this allows you to upgrade through minor versions
@@ -242,7 +242,7 @@ class VersionSelector
     /**
      * @return VersionParser
      */
-    private function getParser()
+    private function getParser(): VersionParser
     {
         if ($this->parser === null) {
             $this->parser = new VersionParser();
diff --git a/src/Composer/PartialComposer.php b/src/Composer/PartialComposer.php
new file mode 100644
index 000000000..891175066
--- /dev/null
+++ b/src/Composer/PartialComposer.php
@@ -0,0 +1,151 @@
+<?php
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer;
+
+use Composer\Package\RootPackageInterface;
+use Composer\Util\Loop;
+use Composer\Repository\RepositoryManager;
+use Composer\Installer\InstallationManager;
+use Composer\EventDispatcher\EventDispatcher;
+
+/**
+ * @author Jordi Boggiano <j.boggiano@seld.be>
+ */
+class PartialComposer
+{
+    /**
+     * @var RootPackageInterface
+     */
+    private $package;
+
+    /**
+     * @var Loop
+     */
+    private $loop;
+
+    /**
+     * @var Repository\RepositoryManager
+     */
+    private $repositoryManager;
+
+    /**
+     * @var Installer\InstallationManager
+     */
+    private $installationManager;
+
+    /**
+     * @var Config
+     */
+    private $config;
+
+    /**
+     * @var EventDispatcher
+     */
+    private $eventDispatcher;
+
+    /**
+     * @return void
+     */
+    public function setPackage(RootPackageInterface $package): void
+    {
+        $this->package = $package;
+    }
+
+    /**
+     * @return RootPackageInterface
+     */
+    public function getPackage(): RootPackageInterface
+    {
+        return $this->package;
+    }
+
+    /**
+     * @return void
+     */
+    public function setConfig(Config $config): void
+    {
+        $this->config = $config;
+    }
+
+    /**
+     * @return Config
+     */
+    public function getConfig(): Config
+    {
+        return $this->config;
+    }
+
+    /**
+     * @return void
+     */
+    public function setLoop(Loop $loop): void
+    {
+        $this->loop = $loop;
+    }
+
+    /**
+     * @return Loop
+     */
+    public function getLoop(): Loop
+    {
+        return $this->loop;
+    }
+
+    /**
+     * @return void
+     */
+    public function setRepositoryManager(RepositoryManager $manager): void
+    {
+        $this->repositoryManager = $manager;
+    }
+
+    /**
+     * @return RepositoryManager
+     */
+    public function getRepositoryManager(): RepositoryManager
+    {
+        return $this->repositoryManager;
+    }
+
+    /**
+     * @return void
+     */
+    public function setInstallationManager(InstallationManager $manager): void
+    {
+        $this->installationManager = $manager;
+    }
+
+    /**
+     * @return InstallationManager
+     */
+    public function getInstallationManager(): InstallationManager
+    {
+        return $this->installationManager;
+    }
+
+    /**
+     * @return void
+     */
+    public function setEventDispatcher(EventDispatcher $eventDispatcher): void
+    {
+        $this->eventDispatcher = $eventDispatcher;
+    }
+
+    /**
+     * @return EventDispatcher
+     */
+    public function getEventDispatcher(): EventDispatcher
+    {
+        return $this->eventDispatcher;
+    }
+}
diff --git a/src/Composer/Platform/HhvmDetector.php b/src/Composer/Platform/HhvmDetector.php
index 7fee999a6..2785d26c4 100644
--- a/src/Composer/Platform/HhvmDetector.php
+++ b/src/Composer/Platform/HhvmDetector.php
@@ -34,7 +34,7 @@ class HhvmDetector
     /**
      * @return void
      */
-    public function reset()
+    public function reset(): void
     {
         self::$hhvmVersion = null;
     }
@@ -42,7 +42,7 @@ class HhvmDetector
     /**
      * @return string|null
      */
-    public function getVersion()
+    public function getVersion(): ?string
     {
         if (null !== self::$hhvmVersion) {
             return self::$hhvmVersion ?: null;
diff --git a/src/Composer/Platform/Runtime.php b/src/Composer/Platform/Runtime.php
index 250a21aa7..a5d228027 100644
--- a/src/Composer/Platform/Runtime.php
+++ b/src/Composer/Platform/Runtime.php
@@ -20,7 +20,7 @@ class Runtime
      *
      * @return bool
      */
-    public function hasConstant($constant, $class = null)
+    public function hasConstant($constant, $class = null): bool
     {
         return defined(ltrim($class.'::'.$constant, ':'));
     }
@@ -41,7 +41,7 @@ class Runtime
      *
      * @return bool
      */
-    public function hasFunction($fn)
+    public function hasFunction($fn): bool
     {
         return function_exists($fn);
     }
@@ -62,7 +62,7 @@ class Runtime
      *
      * @return bool
      */
-    public function hasClass($class)
+    public function hasClass($class): bool
     {
         return class_exists($class, false);
     }
@@ -74,7 +74,7 @@ class Runtime
      * @return object
      * @throws \ReflectionException
      */
-    public function construct($class, array $arguments = array())
+    public function construct($class, array $arguments = array()): object
     {
         if (empty($arguments)) {
             return new $class;
@@ -86,7 +86,7 @@ class Runtime
     }
 
     /** @return string[] */
-    public function getExtensions()
+    public function getExtensions(): array
     {
         return get_loaded_extensions();
     }
@@ -96,7 +96,7 @@ class Runtime
      *
      * @return string
      */
-    public function getExtensionVersion($extension)
+    public function getExtensionVersion($extension): string
     {
         return phpversion($extension);
     }
@@ -107,7 +107,7 @@ class Runtime
      * @return string
      * @throws \ReflectionException
      */
-    public function getExtensionInfo($extension)
+    public function getExtensionInfo($extension): string
     {
         $reflector = new \ReflectionExtension($extension);
 
diff --git a/src/Composer/Platform/Version.php b/src/Composer/Platform/Version.php
index 6c9cd8fa5..d2b4d1171 100644
--- a/src/Composer/Platform/Version.php
+++ b/src/Composer/Platform/Version.php
@@ -24,7 +24,7 @@ class Version
      * @param  bool        $isFips
      * @return string|null
      */
-    public static function parseOpenssl($opensslVersion, &$isFips)
+    public static function parseOpenssl($opensslVersion, &$isFips): ?string
     {
         $isFips = false;
 
@@ -43,7 +43,7 @@ class Version
      * @param  string      $libjpegVersion
      * @return string|null
      */
-    public static function parseLibjpeg($libjpegVersion)
+    public static function parseLibjpeg($libjpegVersion): ?string
     {
         if (!Preg::isMatch('/^(?<major>\d+)(?<minor>[a-z]*)$/', $libjpegVersion, $matches)) {
             return null;
@@ -56,7 +56,7 @@ class Version
      * @param  string      $zoneinfoVersion
      * @return string|null
      */
-    public static function parseZoneinfoVersion($zoneinfoVersion)
+    public static function parseZoneinfoVersion($zoneinfoVersion): ?string
     {
         if (!Preg::isMatch('/^(?<year>\d{4})(?<revision>[a-z]*)$/', $zoneinfoVersion, $matches)) {
             return null;
@@ -71,7 +71,7 @@ class Version
      * @param  string $alpha
      * @return int
      */
-    private static function convertAlphaVersionToIntVersion($alpha)
+    private static function convertAlphaVersionToIntVersion($alpha): int
     {
         return strlen($alpha) * (-ord('a') + 1) + array_sum(array_map('ord', str_split($alpha)));
     }
@@ -80,7 +80,7 @@ class Version
      * @param  int    $versionId
      * @return string
      */
-    public static function convertLibxpmVersionId($versionId)
+    public static function convertLibxpmVersionId($versionId): string
     {
         return self::convertVersionId($versionId, 100);
     }
@@ -89,7 +89,7 @@ class Version
      * @param  int    $versionId
      * @return string
      */
-    public static function convertOpenldapVersionId($versionId)
+    public static function convertOpenldapVersionId($versionId): string
     {
         return self::convertVersionId($versionId, 100);
     }
@@ -100,7 +100,7 @@ class Version
      *
      * @return string
      */
-    private static function convertVersionId($versionId, $base)
+    private static function convertVersionId($versionId, $base): string
     {
         return sprintf(
             '%d.%d.%d',
diff --git a/src/Composer/Plugin/CommandEvent.php b/src/Composer/Plugin/CommandEvent.php
index 6d81ee2cb..650757ff9 100644
--- a/src/Composer/Plugin/CommandEvent.php
+++ b/src/Composer/Plugin/CommandEvent.php
@@ -61,7 +61,7 @@ class CommandEvent extends Event
      *
      * @return InputInterface
      */
-    public function getInput()
+    public function getInput(): InputInterface
     {
         return $this->input;
     }
@@ -71,7 +71,7 @@ class CommandEvent extends Event
      *
      * @return OutputInterface
      */
-    public function getOutput()
+    public function getOutput(): OutputInterface
     {
         return $this->output;
     }
@@ -81,7 +81,7 @@ class CommandEvent extends Event
      *
      * @return string
      */
-    public function getCommandName()
+    public function getCommandName(): string
     {
         return $this->commandName;
     }
diff --git a/src/Composer/Plugin/PluginManager.php b/src/Composer/Plugin/PluginManager.php
index 4f974d9f8..b849de107 100644
--- a/src/Composer/Plugin/PluginManager.php
+++ b/src/Composer/Plugin/PluginManager.php
@@ -13,6 +13,7 @@
 namespace Composer\Plugin;
 
 use Composer\Composer;
+use Composer\Autoload\AutoloadGenerator;
 use Composer\EventDispatcher\EventSubscriberInterface;
 use Composer\Installer\InstallerInterface;
 use Composer\IO\IOInterface;
@@ -20,6 +21,7 @@ use Composer\Package\BasePackage;
 use Composer\Package\CompletePackage;
 use Composer\Package\Package;
 use Composer\Package\Version\VersionParser;
+use Composer\PartialComposer;
 use Composer\Pcre\Preg;
 use Composer\Repository\RepositoryInterface;
 use Composer\Repository\InstalledRepository;
@@ -42,7 +44,7 @@ class PluginManager
     protected $composer;
     /** @var IOInterface */
     protected $io;
-    /** @var ?Composer */
+    /** @var PartialComposer|null */
     protected $globalComposer;
     /** @var VersionParser */
     protected $versionParser;
@@ -67,15 +69,7 @@ class PluginManager
     /** @var int */
     private static $classCounter = 0;
 
-    /**
-     * Initializes plugin manager
-     *
-     * @param IOInterface $io
-     * @param Composer    $composer
-     * @param Composer    $globalComposer
-     * @param bool        $disablePlugins
-     */
-    public function __construct(IOInterface $io, Composer $composer, Composer $globalComposer = null, $disablePlugins = false)
+    public function __construct(IOInterface $io, Composer $composer, PartialComposer $globalComposer = null, bool $disablePlugins = false)
     {
         $this->io = $io;
         $this->composer = $composer;
@@ -92,7 +86,7 @@ class PluginManager
      *
      * @return void
      */
-    public function loadInstalledPlugins()
+    public function loadInstalledPlugins(): void
     {
         if ($this->disablePlugins) {
             return;
@@ -111,17 +105,16 @@ class PluginManager
      *
      * @return void
      */
-    public function deactivateInstalledPlugins()
+    public function deactivateInstalledPlugins(): void
     {
         if ($this->disablePlugins) {
             return;
         }
 
         $repo = $this->composer->getRepositoryManager()->getLocalRepository();
-        $globalRepo = $this->globalComposer ? $this->globalComposer->getRepositoryManager()->getLocalRepository() : null;
         $this->deactivateRepository($repo, false);
-        if ($globalRepo) {
-            $this->deactivateRepository($globalRepo, true);
+        if ($this->globalComposer !== null) {
+            $this->deactivateRepository($this->globalComposer->getRepositoryManager()->getLocalRepository(), true);
         }
     }
 
@@ -130,17 +123,15 @@ class PluginManager
      *
      * @return array<PluginInterface> plugins
      */
-    public function getPlugins()
+    public function getPlugins(): array
     {
         return $this->plugins;
     }
 
     /**
      * Gets global composer or null when main composer is not fully loaded
-     *
-     * @return Composer|null
      */
-    public function getGlobalComposer()
+    public function getGlobalComposer(): ?PartialComposer
     {
         return $this->globalComposer;
     }
@@ -159,7 +150,7 @@ class PluginManager
      *
      * @throws \UnexpectedValueException
      */
-    public function registerPackage(PackageInterface $package, $failOnMissingClasses = false, $isGlobalPlugin = false)
+    public function registerPackage(PackageInterface $package, $failOnMissingClasses = false, $isGlobalPlugin = false): void
     {
         if ($this->disablePlugins) {
             return;
@@ -318,7 +309,7 @@ class PluginManager
      *
      * @throws \UnexpectedValueException
      */
-    public function deactivatePackage(PackageInterface $package)
+    public function deactivatePackage(PackageInterface $package): void
     {
         if ($this->disablePlugins) {
             return;
@@ -349,7 +340,7 @@ class PluginManager
      *
      * @throws \UnexpectedValueException
      */
-    public function uninstallPackage(PackageInterface $package)
+    public function uninstallPackage(PackageInterface $package): void
     {
         if ($this->disablePlugins) {
             return;
@@ -374,7 +365,7 @@ class PluginManager
      *
      * @return string
      */
-    protected function getPluginApiVersion()
+    protected function getPluginApiVersion(): string
     {
         return PluginInterface::PLUGIN_API_VERSION;
     }
@@ -392,7 +383,7 @@ class PluginManager
      *
      * @return void
      */
-    public function addPlugin(PluginInterface $plugin, $isGlobalPlugin = false, PackageInterface $sourcePackage = null)
+    public function addPlugin(PluginInterface $plugin, $isGlobalPlugin = false, PackageInterface $sourcePackage = null): void
     {
         if ($sourcePackage === null) {
             trigger_error('Calling PluginManager::addPlugin without $sourcePackage is deprecated, if you are using this please get in touch with us to explain the use case', E_USER_DEPRECATED);
@@ -429,7 +420,7 @@ class PluginManager
      *
      * @return void
      */
-    public function removePlugin(PluginInterface $plugin)
+    public function removePlugin(PluginInterface $plugin): void
     {
         $index = array_search($plugin, $this->plugins, true);
         if ($index === false) {
@@ -454,7 +445,7 @@ class PluginManager
      *
      * @return void
      */
-    public function uninstallPlugin(PluginInterface $plugin)
+    public function uninstallPlugin(PluginInterface $plugin): void
     {
         $this->io->writeError('Uninstalling plugin '.get_class($plugin), true, IOInterface::DEBUG);
         $plugin->uninstall($this->composer, $this->io);
@@ -476,7 +467,7 @@ class PluginManager
      *
      * @throws \RuntimeException
      */
-    private function loadRepository(RepositoryInterface $repo, $isGlobalRepo)
+    private function loadRepository(RepositoryInterface $repo, $isGlobalRepo): void
     {
         $packages = $repo->getPackages();
         $sortedPackages = PackageSorter::sortPackages($packages);
@@ -503,7 +494,7 @@ class PluginManager
      *
      * @return void
      */
-    private function deactivateRepository(RepositoryInterface $repo, $isGlobalRepo)
+    private function deactivateRepository(RepositoryInterface $repo, $isGlobalRepo): void
     {
         $packages = $repo->getPackages();
         $sortedPackages = array_reverse(PackageSorter::sortPackages($packages));
@@ -530,7 +521,7 @@ class PluginManager
      *
      * @return array<string, PackageInterface> Map of package names to packages
      */
-    private function collectDependencies(InstalledRepository $installedRepo, array $collected, PackageInterface $package)
+    private function collectDependencies(InstalledRepository $installedRepo, array $collected, PackageInterface $package): array
     {
         foreach ($package->getRequires() as $requireLink) {
             foreach ($installedRepo->findPackagesWithReplacersAndProviders($requireLink->getTarget()) as $requiredPackage) {
@@ -552,12 +543,13 @@ class PluginManager
      *
      * @return string Install path
      */
-    private function getInstallPath(PackageInterface $package, $global = false)
+    private function getInstallPath(PackageInterface $package, $global = false): string
     {
         if (!$global) {
             return $this->composer->getInstallationManager()->getInstallPath($package);
         }
 
+        assert(null !== $this->globalComposer);
         return $this->globalComposer->getInstallationManager()->getInstallPath($package);
     }
 
@@ -567,7 +559,7 @@ class PluginManager
      * @throws \RuntimeException On empty or non-string implementation class name value
      * @return null|string       The fully qualified class of the implementation or null if Plugin is not of Capable type or does not provide it
      */
-    protected function getCapabilityImplementationClassName(PluginInterface $plugin, $capability)
+    protected function getCapabilityImplementationClassName(PluginInterface $plugin, $capability): ?string
     {
         if (!($plugin instanceof Capable)) {
             return null;
@@ -600,7 +592,7 @@ class PluginManager
      * @phpstan-param class-string<CapabilityClass> $capabilityClassName
      * @phpstan-return null|CapabilityClass
      */
-    public function getPluginCapability(PluginInterface $plugin, $capabilityClassName, array $ctorArgs = array())
+    public function getPluginCapability(PluginInterface $plugin, $capabilityClassName, array $ctorArgs = array()): ?Capability
     {
         if ($capabilityClass = $this->getCapabilityImplementationClassName($plugin, $capabilityClassName)) {
             if (!class_exists($capabilityClass)) {
@@ -631,7 +623,7 @@ class PluginManager
      *                                                            Keeping it an array will allow future values to be passed w\o changing the signature.
      * @return CapabilityClass[]
      */
-    public function getPluginCapabilities($capabilityClassName, array $ctorArgs = array())
+    public function getPluginCapabilities($capabilityClassName, array $ctorArgs = array()): array
     {
         $capabilities = array();
         foreach ($this->getPlugins() as $plugin) {
@@ -647,7 +639,7 @@ class PluginManager
      * @param  array<string, bool>|bool|null $allowPluginsConfig
      * @return array<non-empty-string, bool>|null
      */
-    private function parseAllowedPlugins($allowPluginsConfig)
+    private function parseAllowedPlugins($allowPluginsConfig): ?array
     {
         if (null === $allowPluginsConfig) {
             return null;
@@ -674,7 +666,7 @@ class PluginManager
      * @param bool $isGlobalPlugin
      * @return bool
      */
-    private function isPluginAllowed($package, $isGlobalPlugin)
+    private function isPluginAllowed($package, $isGlobalPlugin): bool
     {
         static $warned = array();
         $rules = $isGlobalPlugin ? $this->allowGlobalPluginRules : $this->allowPluginRules;
diff --git a/src/Composer/Plugin/PostFileDownloadEvent.php b/src/Composer/Plugin/PostFileDownloadEvent.php
index 7be34d728..80489af9d 100644
--- a/src/Composer/Plugin/PostFileDownloadEvent.php
+++ b/src/Composer/Plugin/PostFileDownloadEvent.php
@@ -81,7 +81,7 @@ class PostFileDownloadEvent extends Event
      *
      * @return string|null
      */
-    public function getFileName()
+    public function getFileName(): ?string
     {
         return $this->fileName;
     }
@@ -91,7 +91,7 @@ class PostFileDownloadEvent extends Event
      *
      * @return string|null
      */
-    public function getChecksum()
+    public function getChecksum(): ?string
     {
         return $this->checksum;
     }
@@ -101,7 +101,7 @@ class PostFileDownloadEvent extends Event
      *
      * @return string
      */
-    public function getUrl()
+    public function getUrl(): string
     {
         return $this->url;
     }
@@ -127,7 +127,7 @@ class PostFileDownloadEvent extends Event
      * @return \Composer\Package\PackageInterface|null The package.
      * @deprecated Use getContext instead
      */
-    public function getPackage()
+    public function getPackage(): ?\Composer\Package\PackageInterface
     {
         trigger_error('PostFileDownloadEvent::getPackage is deprecated since Composer 2.1, use getContext instead.', E_USER_DEPRECATED);
         $context = $this->getContext();
@@ -140,7 +140,7 @@ class PostFileDownloadEvent extends Event
      *
      * @return string
      */
-    public function getType()
+    public function getType(): string
     {
         return $this->type;
     }
diff --git a/src/Composer/Plugin/PreCommandRunEvent.php b/src/Composer/Plugin/PreCommandRunEvent.php
index 60ad05b4a..2c534a68e 100644
--- a/src/Composer/Plugin/PreCommandRunEvent.php
+++ b/src/Composer/Plugin/PreCommandRunEvent.php
@@ -51,7 +51,7 @@ class PreCommandRunEvent extends Event
      *
      * @return InputInterface
      */
-    public function getInput()
+    public function getInput(): InputInterface
     {
         return $this->input;
     }
@@ -61,7 +61,7 @@ class PreCommandRunEvent extends Event
      *
      * @return string
      */
-    public function getCommand()
+    public function getCommand(): string
     {
         return $this->command;
     }
diff --git a/src/Composer/Plugin/PreFileDownloadEvent.php b/src/Composer/Plugin/PreFileDownloadEvent.php
index 9e675d438..5b9b0307a 100644
--- a/src/Composer/Plugin/PreFileDownloadEvent.php
+++ b/src/Composer/Plugin/PreFileDownloadEvent.php
@@ -73,7 +73,7 @@ class PreFileDownloadEvent extends Event
     /**
      * @return HttpDownloader
      */
-    public function getHttpDownloader()
+    public function getHttpDownloader(): HttpDownloader
     {
         return $this->httpDownloader;
     }
@@ -83,7 +83,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return string
      */
-    public function getProcessedUrl()
+    public function getProcessedUrl(): string
     {
         return $this->processedUrl;
     }
@@ -95,7 +95,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return void
      */
-    public function setProcessedUrl($processedUrl)
+    public function setProcessedUrl($processedUrl): void
     {
         $this->processedUrl = $processedUrl;
     }
@@ -105,7 +105,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return string|null
      */
-    public function getCustomCacheKey()
+    public function getCustomCacheKey(): ?string
     {
         return $this->customCacheKey;
     }
@@ -117,7 +117,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return void
      */
-    public function setCustomCacheKey($customCacheKey)
+    public function setCustomCacheKey($customCacheKey): void
     {
         $this->customCacheKey = $customCacheKey;
     }
@@ -127,7 +127,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return string
      */
-    public function getType()
+    public function getType(): string
     {
         return $this->type;
     }
@@ -152,7 +152,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return mixed[]
      */
-    public function getTransportOptions()
+    public function getTransportOptions(): array
     {
         return $this->transportOptions;
     }
@@ -166,7 +166,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return void
      */
-    public function setTransportOptions(array $options)
+    public function setTransportOptions(array $options): void
     {
         $this->transportOptions = $options;
     }
diff --git a/src/Composer/Plugin/PrePoolCreateEvent.php b/src/Composer/Plugin/PrePoolCreateEvent.php
index ff7702553..d64e96d95 100644
--- a/src/Composer/Plugin/PrePoolCreateEvent.php
+++ b/src/Composer/Plugin/PrePoolCreateEvent.php
@@ -93,7 +93,7 @@ class PrePoolCreateEvent extends Event
     /**
      * @return RepositoryInterface[]
      */
-    public function getRepositories()
+    public function getRepositories(): array
     {
         return $this->repositories;
     }
@@ -101,7 +101,7 @@ class PrePoolCreateEvent extends Event
     /**
      * @return Request
      */
-    public function getRequest()
+    public function getRequest(): Request
     {
         return $this->request;
     }
@@ -110,7 +110,7 @@ class PrePoolCreateEvent extends Event
      * @return int[] array of stability => BasePackage::STABILITY_* value
      * @phpstan-return array<string, BasePackage::STABILITY_*>
      */
-    public function getAcceptableStabilities()
+    public function getAcceptableStabilities(): array
     {
         return $this->acceptableStabilities;
     }
@@ -119,7 +119,7 @@ class PrePoolCreateEvent extends Event
      * @return int[] array of package name => BasePackage::STABILITY_* value
      * @phpstan-return array<string, BasePackage::STABILITY_*>
      */
-    public function getStabilityFlags()
+    public function getStabilityFlags(): array
     {
         return $this->stabilityFlags;
     }
@@ -128,7 +128,7 @@ class PrePoolCreateEvent extends Event
      * @return array[] of package => version => [alias, alias_normalized]
      * @phpstan-return array<string, array<string, array{alias: string, alias_normalized: string}>>
      */
-    public function getRootAliases()
+    public function getRootAliases(): array
     {
         return $this->rootAliases;
     }
@@ -137,7 +137,7 @@ class PrePoolCreateEvent extends Event
      * @return string[]
      * @phpstan-return array<string, string>
      */
-    public function getRootReferences()
+    public function getRootReferences(): array
     {
         return $this->rootReferences;
     }
@@ -145,7 +145,7 @@ class PrePoolCreateEvent extends Event
     /**
      * @return BasePackage[]
      */
-    public function getPackages()
+    public function getPackages(): array
     {
         return $this->packages;
     }
@@ -153,7 +153,7 @@ class PrePoolCreateEvent extends Event
     /**
      * @return BasePackage[]
      */
-    public function getUnacceptableFixedPackages()
+    public function getUnacceptableFixedPackages(): array
     {
         return $this->unacceptableFixedPackages;
     }
@@ -163,7 +163,7 @@ class PrePoolCreateEvent extends Event
      *
      * @return void
      */
-    public function setPackages(array $packages)
+    public function setPackages(array $packages): void
     {
         $this->packages = $packages;
     }
@@ -173,7 +173,7 @@ class PrePoolCreateEvent extends Event
      *
      * @return void
      */
-    public function setUnacceptableFixedPackages(array $packages)
+    public function setUnacceptableFixedPackages(array $packages): void
     {
         $this->unacceptableFixedPackages = $packages;
     }
diff --git a/src/Composer/Question/StrictConfirmationQuestion.php b/src/Composer/Question/StrictConfirmationQuestion.php
index 301bb91b0..bcb339bda 100644
--- a/src/Composer/Question/StrictConfirmationQuestion.php
+++ b/src/Composer/Question/StrictConfirmationQuestion.php
@@ -53,7 +53,7 @@ class StrictConfirmationQuestion extends Question
      *
      * @return callable
      */
-    private function getDefaultNormalizer()
+    private function getDefaultNormalizer(): callable
     {
         $default = $this->getDefault();
         $trueRegex = $this->trueAnswerRegex;
@@ -84,9 +84,9 @@ class StrictConfirmationQuestion extends Question
      *
      * @return callable
      */
-    private function getDefaultValidator()
+    private function getDefaultValidator(): callable
     {
-        return function ($answer) {
+        return function ($answer): bool {
             if (!is_bool($answer)) {
                 throw new InvalidArgumentException('Please answer yes, y, no, or n.');
             }
diff --git a/src/Composer/Repository/ArtifactRepository.php b/src/Composer/Repository/ArtifactRepository.php
index e8140162c..7db28bcab 100644
--- a/src/Composer/Repository/ArtifactRepository.php
+++ b/src/Composer/Repository/ArtifactRepository.php
@@ -73,7 +73,7 @@ class ArtifactRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return void
      */
-    private function scanDirectory($path)
+    private function scanDirectory($path): void
     {
         $io = $this->io;
 
@@ -102,7 +102,7 @@ class ArtifactRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return ?BasePackage
      */
-    private function getComposerInformation(\SplFileInfo $file)
+    private function getComposerInformation(\SplFileInfo $file): ?BasePackage
     {
         $json = null;
         $fileType = null;
diff --git a/src/Composer/Repository/ComposerRepository.php b/src/Composer/Repository/ComposerRepository.php
index 8fa6bf03b..9e6adbaf6 100644
--- a/src/Composer/Repository/ComposerRepository.php
+++ b/src/Composer/Repository/ComposerRepository.php
@@ -39,6 +39,7 @@ use Composer\Semver\Constraint\MatchAllConstraint;
 use Composer\Util\Http\Response;
 use Composer\MetadataMinifier\MetadataMinifier;
 use Composer\Util\Url;
+use React\Promise\PromiseInterface;
 
 /**
  * @author Jordi Boggiano <j.boggiano@seld.be>
@@ -349,7 +350,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
              * @param list<string> $results
              * @return list<string>
              */
-            function (array $results) {
+            function (array $results): array {
                 return $results;
             }
         ;
@@ -360,7 +361,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
                  * @param list<string> $results
                  * @return list<string>
                  */
-                function (array $results) use ($packageFilterRegex) {
+                function (array $results) use ($packageFilterRegex): array {
                     /** @var list<string> $results */
                     return Preg::grep($packageFilterRegex, $results);
                 }
@@ -399,7 +400,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return list<string>
      */
-    private function getVendorNames()
+    private function getVendorNames(): array
     {
         $cacheKey = 'vendor-list.txt';
         $cacheAge = $this->cache->getAge($cacheKey);
@@ -430,7 +431,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      * @param string|null $packageFilter
      * @return list<string>
      */
-    private function loadPackageList($packageFilter = null)
+    private function loadPackageList($packageFilter = null): array
     {
         if (null === $this->listUrl) {
             throw new \LogicException('Make sure to call loadRootServerFile before loadPackageList');
@@ -640,7 +641,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return string[]
      */
-    private function getProviderNames()
+    private function getProviderNames(): array
     {
         $this->loadRootServerFile();
 
@@ -677,7 +678,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return bool
      */
-    private function hasProviders()
+    private function hasProviders(): bool
     {
         $this->loadRootServerFile();
 
@@ -694,7 +695,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return array<string, BasePackage>
      */
-    private function whatProvides($name, array $acceptableStabilities = null, array $stabilityFlags = null, array $alreadyLoaded = array())
+    private function whatProvides($name, array $acceptableStabilities = null, array $stabilityFlags = null, array $alreadyLoaded = array()): array
     {
         $packagesSource = null;
         if (!$this->hasPartialPackages() || !isset($this->partialPackagesByName[$name])) {
@@ -862,7 +863,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return array{namesFound: array<string, true>, packages: array<string, BasePackage>}
      */
-    private function loadAsyncPackages(array $packageNames, array $acceptableStabilities = null, array $stabilityFlags = null, array $alreadyLoaded = array())
+    private function loadAsyncPackages(array $packageNames, array $acceptableStabilities = null, array $stabilityFlags = null, array $alreadyLoaded = array()): array
     {
         $this->loadRootServerFile();
 
@@ -904,7 +905,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
             }
 
             $promises[] = $this->asyncFetchFile($url, $cacheKey, $lastModified)
-                ->then(function ($response) use (&$packages, &$namesFound, $url, $cacheKey, $contents, $realName, $constraint, $acceptableStabilities, $stabilityFlags, $alreadyLoaded) {
+                ->then(function ($response) use (&$packages, &$namesFound, $url, $cacheKey, $contents, $realName, $constraint, $acceptableStabilities, $stabilityFlags, $alreadyLoaded): void {
                     $packagesSource = 'downloaded file ('.Url::sanitize($url).')';
 
                     if (true === $response) {
@@ -972,7 +973,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return bool
      */
-    private function isVersionAcceptable($constraint, $name, $versionData, array $acceptableStabilities = null, array $stabilityFlags = null)
+    private function isVersionAcceptable($constraint, $name, $versionData, array $acceptableStabilities = null, array $stabilityFlags = null): bool
     {
         $versions = array($versionData['version_normalized']);
 
@@ -998,7 +999,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return string
      */
-    private function getPackagesJsonUrl()
+    private function getPackagesJsonUrl(): string
     {
         $jsonUrlParts = parse_url($this->url);
 
@@ -1094,7 +1095,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
             // Disables lazy-provider behavior as with available-packages, but may allow much more compact expression of packages covered by this repository.
             // Over-specifying covered packages is safe, but may result in increased traffic to your repository.
             if (!empty($data['available-package-patterns'])) {
-                $this->availablePackagePatterns = array_map(function ($pattern) {
+                $this->availablePackagePatterns = array_map(function ($pattern): string {
                     return BasePackage::packageNameToRegexp($pattern);
                 }, $data['available-package-patterns']);
                 $this->hasAvailablePackageList = true;
@@ -1135,7 +1136,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return string
      */
-    private function canonicalizeUrl($url)
+    private function canonicalizeUrl($url): string
     {
         if ('/' === $url[0]) {
             if (Preg::isMatch('{^[^:]++://[^/]*+}', $this->url, $matches)) {
@@ -1151,7 +1152,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return mixed[]
      */
-    private function loadDataFromServer()
+    private function loadDataFromServer(): array
     {
         $data = $this->loadRootServerFile();
 
@@ -1161,7 +1162,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     /**
      * @return bool
      */
-    private function hasPartialPackages()
+    private function hasPartialPackages(): bool
     {
         if ($this->hasPartialPackages && null === $this->partialPackagesByName) {
             $this->initializePartialPackages();
@@ -1175,7 +1176,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return void
      */
-    private function loadProviderListings($data)
+    private function loadProviderListings($data): void
     {
         if (isset($data['providers'])) {
             if (!is_array($this->providerListing)) {
@@ -1205,7 +1206,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return mixed[]
      */
-    private function loadIncludes($data)
+    private function loadIncludes($data): array
     {
         $packages = array();
 
@@ -1250,7 +1251,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return list<CompletePackage|CompleteAliasPackage>
      */
-    private function createPackages(array $packages, $source = null)
+    private function createPackages(array $packages, $source = null): array
     {
         if (!$packages) {
             return array();
@@ -1457,10 +1458,8 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      * @param string $filename
      * @param string $cacheKey
      * @param string|null $lastModifiedTime
-     *
-     * @return \React\Promise\PromiseInterface
      */
-    private function asyncFetchFile($filename, $cacheKey, $lastModifiedTime = null)
+    private function asyncFetchFile($filename, $cacheKey, $lastModifiedTime = null): PromiseInterface
     {
         if (isset($this->packagesNotFoundCache[$filename])) {
             return \React\Promise\resolve(array('packages' => array()));
@@ -1494,6 +1493,9 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
         $degradedMode = &$this->degradedMode;
         $eventDispatcher = $this->eventDispatcher;
 
+        /**
+         * @return array<mixed>|true true if the response was a 304 and the cache is fresh
+         */
         $accept = function ($response) use ($io, $url, $filename, $cache, $cacheKey, $eventDispatcher) {
             // package not found is acceptable for a v2 protocol repository
             if ($response->getStatusCode() === 404) {
@@ -1566,7 +1568,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
      *
      * @return void
      */
-    private function initializePartialPackages()
+    private function initializePartialPackages(): void
     {
         $rootData = $this->loadRootServerFile();
 
diff --git a/src/Composer/Repository/CompositeRepository.php b/src/Composer/Repository/CompositeRepository.php
index 83603081c..e34ce7357 100644
--- a/src/Composer/Repository/CompositeRepository.php
+++ b/src/Composer/Repository/CompositeRepository.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Repository;
 
+use Composer\Package\BasePackage;
 use Composer\Package\PackageInterface;
 
 /**
@@ -39,9 +40,9 @@ class CompositeRepository implements RepositoryInterface
         }
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
-        return 'composite repo ('.implode(', ', array_map(function ($repo) {
+        return 'composite repo ('.implode(', ', array_map(function ($repo): string {
             return $repo->getRepoName();
         }, $this->repositories)).')';
     }
@@ -51,7 +52,7 @@ class CompositeRepository implements RepositoryInterface
      *
      * @return RepositoryInterface[]
      */
-    public function getRepositories()
+    public function getRepositories(): array
     {
         return $this->repositories;
     }
@@ -59,7 +60,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function hasPackage(PackageInterface $package)
+    public function hasPackage(PackageInterface $package): bool
     {
         foreach ($this->repositories as $repository) {
             /* @var $repository RepositoryInterface */
@@ -74,7 +75,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function findPackage($name, $constraint)
+    public function findPackage($name, $constraint): ?BasePackage
     {
         foreach ($this->repositories as $repository) {
             /* @var $repository RepositoryInterface */
@@ -90,7 +91,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function findPackages($name, $constraint = null)
+    public function findPackages($name, $constraint = null): array
     {
         $packages = array();
         foreach ($this->repositories as $repository) {
@@ -104,7 +105,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function loadPackages(array $packageMap, array $acceptableStabilities, array $stabilityFlags, array $alreadyLoaded = array())
+    public function loadPackages(array $packageMap, array $acceptableStabilities, array $stabilityFlags, array $alreadyLoaded = array()): array
     {
         $packages = array();
         $namesFound = array();
@@ -124,7 +125,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function search($query, $mode = 0, $type = null)
+    public function search($query, $mode = 0, $type = null): array
     {
         $matches = array();
         foreach ($this->repositories as $repository) {
@@ -138,7 +139,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function getPackages()
+    public function getPackages(): array
     {
         $packages = array();
         foreach ($this->repositories as $repository) {
@@ -152,7 +153,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function getProviders($packageName)
+    public function getProviders($packageName): array
     {
         $results = array();
         foreach ($this->repositories as $repository) {
@@ -166,7 +167,7 @@ class CompositeRepository implements RepositoryInterface
     /**
      * @return void
      */
-    public function removePackage(PackageInterface $package)
+    public function removePackage(PackageInterface $package): void
     {
         foreach ($this->repositories as $repository) {
             if ($repository instanceof WritableRepositoryInterface) {
@@ -195,7 +196,7 @@ class CompositeRepository implements RepositoryInterface
      *
      * @return void
      */
-    public function addRepository(RepositoryInterface $repository)
+    public function addRepository(RepositoryInterface $repository): void
     {
         if ($repository instanceof self) {
             foreach ($repository->getRepositories() as $repo) {
diff --git a/src/Composer/Repository/FilesystemRepository.php b/src/Composer/Repository/FilesystemRepository.php
index 6138514fc..ea9a0eeb1 100644
--- a/src/Composer/Repository/FilesystemRepository.php
+++ b/src/Composer/Repository/FilesystemRepository.php
@@ -150,7 +150,7 @@ class FilesystemRepository extends WritableArrayRepository
         }
 
         sort($data['dev-package-names']);
-        usort($data['packages'], function ($a, $b) {
+        usort($data['packages'], function ($a, $b): int {
             return strcmp($a['name'], $b['name']);
         });
 
@@ -173,7 +173,7 @@ class FilesystemRepository extends WritableArrayRepository
      *
      * @return string
      */
-    private function dumpToPhpCode(array $array = array(), $level = 0)
+    private function dumpToPhpCode(array $array = array(), $level = 0): string
     {
         $lines = "array(\n";
         $level++;
@@ -211,7 +211,7 @@ class FilesystemRepository extends WritableArrayRepository
      *
      * @return ?array<mixed>
      */
-    private function generateInstalledVersions(InstallationManager $installationManager, array $installPaths, $devMode, $repoDir)
+    private function generateInstalledVersions(InstallationManager $installationManager, array $installPaths, $devMode, $repoDir): ?array
     {
         if (!$this->dumpVersions) {
             return null;
diff --git a/src/Composer/Repository/FilterRepository.php b/src/Composer/Repository/FilterRepository.php
index 078f66f8f..fc5acd776 100644
--- a/src/Composer/Repository/FilterRepository.php
+++ b/src/Composer/Repository/FilterRepository.php
@@ -62,7 +62,7 @@ class FilterRepository implements RepositoryInterface
         $this->repo = $repo;
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return $this->repo->getRepoName();
     }
@@ -72,7 +72,7 @@ class FilterRepository implements RepositoryInterface
      *
      * @return RepositoryInterface
      */
-    public function getRepository()
+    public function getRepository(): RepositoryInterface
     {
         return $this->repo;
     }
@@ -80,7 +80,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function hasPackage(PackageInterface $package)
+    public function hasPackage(PackageInterface $package): bool
     {
         return $this->repo->hasPackage($package);
     }
@@ -88,7 +88,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function findPackage($name, $constraint)
+    public function findPackage($name, $constraint): ?BasePackage
     {
         if (!$this->isAllowed($name)) {
             return null;
@@ -100,7 +100,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function findPackages($name, $constraint = null)
+    public function findPackages($name, $constraint = null): array
     {
         if (!$this->isAllowed($name)) {
             return array();
@@ -112,7 +112,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function loadPackages(array $packageMap, array $acceptableStabilities, array $stabilityFlags, array $alreadyLoaded = array())
+    public function loadPackages(array $packageMap, array $acceptableStabilities, array $stabilityFlags, array $alreadyLoaded = array()): array
     {
         foreach ($packageMap as $name => $constraint) {
             if (!$this->isAllowed($name)) {
@@ -135,7 +135,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function search($query, $mode = 0, $type = null)
+    public function search($query, $mode = 0, $type = null): array
     {
         $result = array();
 
@@ -151,7 +151,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function getPackages()
+    public function getPackages(): array
     {
         $result = array();
         foreach ($this->repo->getPackages() as $package) {
@@ -166,7 +166,7 @@ class FilterRepository implements RepositoryInterface
     /**
      * @inheritDoc
      */
-    public function getProviders($packageName)
+    public function getProviders($packageName): array
     {
         $result = array();
         foreach ($this->repo->getProviders($packageName) as $name => $provider) {
@@ -195,7 +195,7 @@ class FilterRepository implements RepositoryInterface
      *
      * @return bool
      */
-    private function isAllowed($name)
+    private function isAllowed($name): bool
     {
         if (!$this->only && !$this->exclude) {
             return true;
diff --git a/src/Composer/Repository/InstalledArrayRepository.php b/src/Composer/Repository/InstalledArrayRepository.php
index 40673e7f7..d046a9973 100644
--- a/src/Composer/Repository/InstalledArrayRepository.php
+++ b/src/Composer/Repository/InstalledArrayRepository.php
@@ -21,7 +21,7 @@ namespace Composer\Repository;
  */
 class InstalledArrayRepository extends WritableArrayRepository implements InstalledRepositoryInterface
 {
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return 'installed '.parent::getRepoName();
     }
@@ -29,7 +29,7 @@ class InstalledArrayRepository extends WritableArrayRepository implements Instal
     /**
      * @inheritDoc
      */
-    public function isFresh()
+    public function isFresh(): bool
     {
         // this is not a completely correct implementation but there is no way to
         // distinguish an empty repo and a newly created one given this is all in-memory
diff --git a/src/Composer/Repository/InstalledRepository.php b/src/Composer/Repository/InstalledRepository.php
index db0967ba7..e99ed95db 100644
--- a/src/Composer/Repository/InstalledRepository.php
+++ b/src/Composer/Repository/InstalledRepository.php
@@ -38,7 +38,7 @@ class InstalledRepository extends CompositeRepository
      *
      * @return BasePackage[]
      */
-    public function findPackagesWithReplacersAndProviders($name, $constraint = null)
+    public function findPackagesWithReplacersAndProviders($name, $constraint = null): array
     {
         $name = strtolower($name);
 
@@ -87,7 +87,7 @@ class InstalledRepository extends CompositeRepository
      * @return array[] An associative array of arrays as described above.
      * @phpstan-return array<array{0: PackageInterface, 1: Link, 2: mixed[]|bool}>
      */
-    public function getDependents($needle, $constraint = null, $invert = false, $recurse = true, $packagesFound = null)
+    public function getDependents($needle, $constraint = null, $invert = false, $recurse = true, $packagesFound = null): array
     {
         $needles = array_map('strtolower', (array) $needle);
         $results = array();
@@ -249,9 +249,9 @@ class InstalledRepository extends CompositeRepository
         return $results;
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
-        return 'installed repo ('.implode(', ', array_map(function ($repo) {
+        return 'installed repo ('.implode(', ', array_map(function ($repo): string {
             return $repo->getRepoName();
         }, $this->getRepositories())).')';
     }
@@ -259,7 +259,7 @@ class InstalledRepository extends CompositeRepository
     /**
      * @inheritDoc
      */
-    public function addRepository(RepositoryInterface $repository)
+    public function addRepository(RepositoryInterface $repository): void
     {
         if (
             $repository instanceof LockArrayRepository
diff --git a/src/Composer/Repository/LockArrayRepository.php b/src/Composer/Repository/LockArrayRepository.php
index 7e3b92b03..ddccdd0b1 100644
--- a/src/Composer/Repository/LockArrayRepository.php
+++ b/src/Composer/Repository/LockArrayRepository.php
@@ -21,7 +21,7 @@ namespace Composer\Repository;
  */
 class LockArrayRepository extends ArrayRepository
 {
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return 'lock repo';
     }
diff --git a/src/Composer/Repository/PackageRepository.php b/src/Composer/Repository/PackageRepository.php
index b787931ca..f0ed385ee 100644
--- a/src/Composer/Repository/PackageRepository.php
+++ b/src/Composer/Repository/PackageRepository.php
@@ -45,7 +45,7 @@ class PackageRepository extends ArrayRepository
     /**
      * Initializes repository (reads file, or remote address).
      */
-    protected function initialize()
+    protected function initialize(): void
     {
         parent::initialize();
 
@@ -61,7 +61,7 @@ class PackageRepository extends ArrayRepository
         }
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return Preg::replace('{^array }', 'package ', parent::getRepoName());
     }
diff --git a/src/Composer/Repository/PathRepository.php b/src/Composer/Repository/PathRepository.php
index 725a50360..dcb9c82d5 100644
--- a/src/Composer/Repository/PathRepository.php
+++ b/src/Composer/Repository/PathRepository.php
@@ -129,12 +129,12 @@ class PathRepository extends ArrayRepository implements ConfigurableRepositoryIn
         parent::__construct();
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return 'path repo ('.Url::sanitize($this->repoConfig['url']).')';
     }
 
-    public function getRepoConfig()
+    public function getRepoConfig(): array
     {
         return $this->repoConfig;
     }
@@ -144,7 +144,7 @@ class PathRepository extends ArrayRepository implements ConfigurableRepositoryIn
      *
      * This method will basically read the folder and add the found package.
      */
-    protected function initialize()
+    protected function initialize(): void
     {
         parent::initialize();
 
@@ -240,7 +240,7 @@ class PathRepository extends ArrayRepository implements ConfigurableRepositoryIn
      *
      * @return string[]
      */
-    private function getUrlMatches()
+    private function getUrlMatches(): array
     {
         $flags = GLOB_MARK | GLOB_ONLYDIR;
 
@@ -251,7 +251,7 @@ class PathRepository extends ArrayRepository implements ConfigurableRepositoryIn
         }
 
         // Ensure environment-specific path separators are normalized to URL separators
-        return array_map(function ($val) {
+        return array_map(function ($val): string {
             return rtrim(str_replace(DIRECTORY_SEPARATOR, '/', $val), '/');
         }, glob($this->url, $flags));
     }
diff --git a/src/Composer/Repository/PlatformRepository.php b/src/Composer/Repository/PlatformRepository.php
index b1c98b61c..4722f9899 100644
--- a/src/Composer/Repository/PlatformRepository.php
+++ b/src/Composer/Repository/PlatformRepository.php
@@ -84,7 +84,7 @@ class PlatformRepository extends ArrayRepository
         parent::__construct($packages);
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return 'platform repo';
     }
@@ -93,7 +93,7 @@ class PlatformRepository extends ArrayRepository
      * @param  string  $name
      * @return bool
      */
-    public function isPlatformPackageDisabled($name)
+    public function isPlatformPackageDisabled($name): bool
     {
         return isset($this->disabledPackages[$name]);
     }
@@ -101,12 +101,12 @@ class PlatformRepository extends ArrayRepository
     /**
      * @return array<string, CompletePackageInterface>
      */
-    public function getDisabledPackages()
+    public function getDisabledPackages(): array
     {
         return $this->disabledPackages;
     }
 
-    protected function initialize()
+    protected function initialize(): void
     {
         parent::initialize();
 
@@ -365,7 +365,7 @@ class PlatformRepository extends ArrayRepository
 
                 case 'libxml':
                     // ext/dom, ext/simplexml, ext/xmlreader and ext/xmlwriter use the same libxml as the ext/libxml
-                    $libxmlProvides = array_map(function ($extension) {
+                    $libxmlProvides = array_map(function ($extension): string {
                         return $extension . '-libxml';
                     }, array_intersect($loadedExtensions, array('dom', 'simplexml', 'xml', 'xmlreader', 'xmlwriter')));
                     $this->addLibrary($name, $this->runtime->getConstant('LIBXML_DOTTED_VERSION'), 'libxml library version', array(), $libxmlProvides);
@@ -531,7 +531,7 @@ class PlatformRepository extends ArrayRepository
     /**
      * @inheritDoc
      */
-    public function addPackage(PackageInterface $package)
+    public function addPackage(PackageInterface $package): void
     {
         if (!$package instanceof CompletePackage) {
             throw new \UnexpectedValueException('Expected CompletePackage but got '.get_class($package));
@@ -586,7 +586,7 @@ class PlatformRepository extends ArrayRepository
      *
      * @return CompletePackage
      */
-    private function addOverriddenPackage(array $override, $name = null)
+    private function addOverriddenPackage(array $override, $name = null): CompletePackage
     {
         $version = $this->versionParser->normalize($override['version']);
         $package = new CompletePackage($name ?: $override['name'], $version, $override['version']);
@@ -604,7 +604,7 @@ class PlatformRepository extends ArrayRepository
     /**
      * @return void
      */
-    private function addDisabledPackage(CompletePackage $package)
+    private function addDisabledPackage(CompletePackage $package): void
     {
         $package->setDescription($package->getDescription().'. <warning>Package disabled via config.platform</warning>');
         $package->setExtra(array('config.platform' => true));
@@ -620,7 +620,7 @@ class PlatformRepository extends ArrayRepository
      *
      * @return void
      */
-    private function addExtension($name, $prettyVersion)
+    private function addExtension($name, $prettyVersion): void
     {
         $extraDescription = null;
 
@@ -653,7 +653,7 @@ class PlatformRepository extends ArrayRepository
      * @param  string $name
      * @return string
      */
-    private function buildPackageName($name)
+    private function buildPackageName($name): string
     {
         return 'ext-' . str_replace(' ', '-', strtolower($name));
     }
@@ -667,7 +667,7 @@ class PlatformRepository extends ArrayRepository
      *
      * @return void
      */
-    private function addLibrary($name, $prettyVersion, $description = null, array $replaces = array(), array $provides = array())
+    private function addLibrary($name, $prettyVersion, $description = null, array $replaces = array(), array $provides = array()): void
     {
         try {
             $version = $this->versionParser->normalize($prettyVersion);
@@ -704,7 +704,7 @@ class PlatformRepository extends ArrayRepository
      * @param  string $name
      * @return bool
      */
-    public static function isPlatformPackage($name)
+    public static function isPlatformPackage($name): bool
     {
         static $cache = array();
 
@@ -725,12 +725,12 @@ class PlatformRepository extends ArrayRepository
      * @internal
      * @return string|null
      */
-    public static function getPlatformPhpVersion()
+    public static function getPlatformPhpVersion(): ?string
     {
         return self::$lastSeenPlatformPhp;
     }
 
-    public function search($query, $mode = 0, $type = null)
+    public function search($query, $mode = 0, $type = null): array
     {
         // suppress vendor search as there are no vendors to match in platform packages
         if ($mode === self::SEARCH_VENDOR) {
diff --git a/src/Composer/Repository/RepositoryFactory.php b/src/Composer/Repository/RepositoryFactory.php
index 5891d0f7a..9dbfcb1bb 100644
--- a/src/Composer/Repository/RepositoryFactory.php
+++ b/src/Composer/Repository/RepositoryFactory.php
@@ -64,7 +64,7 @@ class RepositoryFactory
      * @param  bool                $allowFilesystem
      * @return RepositoryInterface
      */
-    public static function fromString(IOInterface $io, Config $config, $repository, $allowFilesystem = false, RepositoryManager $rm = null)
+    public static function fromString(IOInterface $io, Config $config, $repository, $allowFilesystem = false, RepositoryManager $rm = null): RepositoryInterface
     {
         $repoConfig = static::configFromString($io, $config, $repository, $allowFilesystem);
 
@@ -77,7 +77,7 @@ class RepositoryFactory
      * @param  array<string, mixed> $repoConfig
      * @return RepositoryInterface
      */
-    public static function createRepo(IOInterface $io, Config $config, array $repoConfig, RepositoryManager $rm = null)
+    public static function createRepo(IOInterface $io, Config $config, array $repoConfig, RepositoryManager $rm = null): RepositoryInterface
     {
         if (!$rm) {
             $rm = static::manager($io, $config, Factory::createHttpDownloader($io, $config));
@@ -93,7 +93,7 @@ class RepositoryFactory
      * @param  RepositoryManager|null $rm
      * @return RepositoryInterface[]
      */
-    public static function defaultRepos(IOInterface $io = null, Config $config = null, RepositoryManager $rm = null)
+    public static function defaultRepos(IOInterface $io = null, Config $config = null, RepositoryManager $rm = null): array
     {
         if (!$config) {
             $config = Factory::createConfig($io);
@@ -118,7 +118,7 @@ class RepositoryFactory
      * @param  HttpDownloader    $httpDownloader
      * @return RepositoryManager
      */
-    public static function manager(IOInterface $io, Config $config, HttpDownloader $httpDownloader, EventDispatcher $eventDispatcher = null, ProcessExecutor $process = null)
+    public static function manager(IOInterface $io, Config $config, HttpDownloader $httpDownloader, EventDispatcher $eventDispatcher = null, ProcessExecutor $process = null): RepositoryManager
     {
         $rm = new RepositoryManager($io, $config, $httpDownloader, $eventDispatcher, $process);
         $rm->setRepositoryClass('composer', 'Composer\Repository\ComposerRepository');
@@ -145,7 +145,7 @@ class RepositoryFactory
      *
      * @return RepositoryInterface[]
      */
-    private static function createRepos(RepositoryManager $rm, array $repoConfigs)
+    private static function createRepos(RepositoryManager $rm, array $repoConfigs): array
     {
         $repos = array();
 
@@ -178,9 +178,9 @@ class RepositoryFactory
      *
      * @return string
      */
-    public static function generateRepositoryName($index, array $repo, array $existingRepos)
+    public static function generateRepositoryName($index, array $repo, array $existingRepos): string
     {
-        $name = is_int($index) && isset($repo['url']) ? Preg::replace('{^https?://}i', '', $repo['url']) : $index;
+        $name = is_int($index) && isset($repo['url']) ? Preg::replace('{^https?://}i', '', $repo['url']) : (string) $index;
         while (isset($existingRepos[$name])) {
             $name .= '2';
         }
diff --git a/src/Composer/Repository/RepositorySet.php b/src/Composer/Repository/RepositorySet.php
index eb640df2b..b01ce59f9 100644
--- a/src/Composer/Repository/RepositorySet.php
+++ b/src/Composer/Repository/RepositorySet.php
@@ -118,7 +118,7 @@ class RepositorySet
      *
      * @return void
      */
-    public function allowInstalledRepositories($allow = true)
+    public function allowInstalledRepositories($allow = true): void
     {
         $this->allowInstalledRepositories = $allow;
     }
@@ -127,7 +127,7 @@ class RepositorySet
      * @return ConstraintInterface[] an array of package name => constraint from the root package, platform requirements excluded
      * @phpstan-return array<string, ConstraintInterface>
      */
-    public function getRootRequires()
+    public function getRootRequires(): array
     {
         return $this->rootRequires;
     }
@@ -142,7 +142,7 @@ class RepositorySet
      *
      * @return void
      */
-    public function addRepository(RepositoryInterface $repo)
+    public function addRepository(RepositoryInterface $repo): void
     {
         if ($this->locked) {
             throw new \RuntimeException("Pool has already been created from this repository set, it cannot be modified anymore.");
@@ -169,7 +169,7 @@ class RepositorySet
      * @param  int                      $flags      any of the ALLOW_* constants from this class to tweak what is returned
      * @return BasePackage[]
      */
-    public function findPackages($name, ConstraintInterface $constraint = null, $flags = 0)
+    public function findPackages($name, ConstraintInterface $constraint = null, $flags = 0): array
     {
         $ignoreStability = ($flags & self::ALLOW_UNACCEPTABLE_STABILITIES) !== 0;
         $loadFromAllRepos = ($flags & self::ALLOW_SHADOWED_REPOSITORIES) !== 0;
@@ -216,7 +216,7 @@ class RepositorySet
      * @return array[] an array with the provider name as key and value of array('name' => '...', 'description' => '...', 'type' => '...')
      * @phpstan-return array<string, array{name: string, description: string, type: string}>
      */
-    public function getProviders($packageName)
+    public function getProviders($packageName): array
     {
         $providers = array();
         foreach ($this->repositories as $repository) {
@@ -235,7 +235,7 @@ class RepositorySet
      * @param  string   $stability one of 'stable', 'RC', 'beta', 'alpha' or 'dev'
      * @return bool
      */
-    public function isPackageAcceptable($names, $stability)
+    public function isPackageAcceptable($names, $stability): bool
     {
         return StabilityFilter::isPackageAcceptable($this->acceptableStabilities, $this->stabilityFlags, $names, $stability);
     }
@@ -245,7 +245,7 @@ class RepositorySet
      *
      * @return Pool
      */
-    public function createPool(Request $request, IOInterface $io, EventDispatcher $eventDispatcher = null, PoolOptimizer $poolOptimizer = null)
+    public function createPool(Request $request, IOInterface $io, EventDispatcher $eventDispatcher = null, PoolOptimizer $poolOptimizer = null): Pool
     {
         $poolBuilder = new PoolBuilder($this->acceptableStabilities, $this->stabilityFlags, $this->rootAliases, $this->rootReferences, $io, $eventDispatcher, $poolOptimizer);
 
@@ -265,7 +265,7 @@ class RepositorySet
      *
      * @return Pool
      */
-    public function createPoolWithAllPackages()
+    public function createPoolWithAllPackages(): Pool
     {
         foreach ($this->repositories as $repo) {
             if (($repo instanceof InstalledRepositoryInterface || $repo instanceof InstalledRepository) && !$this->allowInstalledRepositories) {
@@ -304,7 +304,7 @@ class RepositorySet
      *
      * @return Pool
      */
-    public function createPoolForPackage($packageName, LockArrayRepository $lockedRepo = null)
+    public function createPoolForPackage($packageName, LockArrayRepository $lockedRepo = null): Pool
     {
         // TODO unify this with above in some simpler version without "request"?
         return $this->createPoolForPackages(array($packageName), $lockedRepo);
@@ -315,7 +315,7 @@ class RepositorySet
      *
      * @return Pool
      */
-    public function createPoolForPackages($packageNames, LockArrayRepository $lockedRepo = null)
+    public function createPoolForPackages($packageNames, LockArrayRepository $lockedRepo = null): Pool
     {
         $request = new Request($lockedRepo);
 
@@ -336,7 +336,7 @@ class RepositorySet
      *
      * @return array<string, array<string, array{alias: string, alias_normalized: string}>>
      */
-    private static function getRootAliasesPerPackage(array $aliases)
+    private static function getRootAliasesPerPackage(array $aliases): array
     {
         $normalizedAliases = array();
 
diff --git a/src/Composer/Repository/RootPackageRepository.php b/src/Composer/Repository/RootPackageRepository.php
index 9652282f7..e1aa37443 100644
--- a/src/Composer/Repository/RootPackageRepository.php
+++ b/src/Composer/Repository/RootPackageRepository.php
@@ -28,7 +28,7 @@ class RootPackageRepository extends ArrayRepository
         parent::__construct(array($package));
     }
 
-    public function getRepoName()
+    public function getRepoName(): string
     {
         return 'root package repo';
     }
diff --git a/src/Composer/Repository/Vcs/FossilDriver.php b/src/Composer/Repository/Vcs/FossilDriver.php
index 66a50bdff..8304c61c8 100644
--- a/src/Composer/Repository/Vcs/FossilDriver.php
+++ b/src/Composer/Repository/Vcs/FossilDriver.php
@@ -38,7 +38,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function initialize()
+    public function initialize(): void
     {
         // Make sure fossil is installed and reachable.
         $this->checkFossil();
@@ -71,7 +71,7 @@ class FossilDriver extends VcsDriver
      *
      * @return void
      */
-    protected function checkFossil()
+    protected function checkFossil(): void
     {
         if (0 !== $this->process->execute('fossil version', $ignoredOutput)) {
             throw new \RuntimeException("fossil was not found, check that it is installed and in your PATH env.\n\n" . $this->process->getErrorOutput());
@@ -83,7 +83,7 @@ class FossilDriver extends VcsDriver
      *
      * @return void
      */
-    protected function updateLocalRepo()
+    protected function updateLocalRepo(): void
     {
         $fs = new Filesystem();
         $fs->ensureDirectoryExists($this->checkoutDir);
@@ -121,7 +121,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getRootIdentifier()
+    public function getRootIdentifier(): string
     {
         if (null === $this->rootIdentifier) {
             $this->rootIdentifier = 'trunk';
@@ -133,7 +133,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getUrl()
+    public function getUrl(): string
     {
         return $this->url;
     }
@@ -141,7 +141,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getSource($identifier)
+    public function getSource($identifier): array
     {
         return array('type' => 'fossil', 'url' => $this->getUrl(), 'reference' => $identifier);
     }
@@ -149,7 +149,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getDist($identifier)
+    public function getDist($identifier): ?array
     {
         return null;
     }
@@ -157,7 +157,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getFileContent($file, $identifier)
+    public function getFileContent($file, $identifier): ?string
     {
         $command = sprintf('fossil cat -r %s -- %s', ProcessExecutor::escape($identifier), ProcessExecutor::escape($file));
         $this->process->execute($command, $content, $this->checkoutDir);
@@ -172,7 +172,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getChangeDate($identifier)
+    public function getChangeDate($identifier): ?\DateTime
     {
         $this->process->execute('fossil finfo -b -n 1 composer.json', $output, $this->checkoutDir);
         list(, $date) = explode(' ', trim($output), 3);
@@ -183,7 +183,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getTags()
+    public function getTags(): array
     {
         if (null === $this->tags) {
             $tags = array();
@@ -202,7 +202,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getBranches()
+    public function getBranches(): array
     {
         if (null === $this->branches) {
             $branches = array();
@@ -222,7 +222,7 @@ class FossilDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public static function supports(IOInterface $io, Config $config, $url, $deep = false)
+    public static function supports(IOInterface $io, Config $config, $url, $deep = false): bool
     {
         if (Preg::isMatch('#(^(?:https?|ssh)://(?:[^@]@)?(?:chiselapp\.com|fossil\.))#i', $url)) {
             return true;
diff --git a/src/Composer/Repository/Vcs/GitDriver.php b/src/Composer/Repository/Vcs/GitDriver.php
index b91508acd..cd69d5743 100644
--- a/src/Composer/Repository/Vcs/GitDriver.php
+++ b/src/Composer/Repository/Vcs/GitDriver.php
@@ -234,7 +234,7 @@ class GitDriver extends VcsDriver
         GitUtil::cleanEnv();
 
         try {
-            $gitUtil->runCommand(function ($url) {
+            $gitUtil->runCommand(function ($url): string {
                 return 'git ls-remote --heads -- ' . ProcessExecutor::escape($url);
             }, $url, sys_get_temp_dir());
         } catch (\RuntimeException $e) {
diff --git a/src/Composer/Repository/Vcs/GitLabDriver.php b/src/Composer/Repository/Vcs/GitLabDriver.php
index 2e7d36cc2..85ac75049 100644
--- a/src/Composer/Repository/Vcs/GitLabDriver.php
+++ b/src/Composer/Repository/Vcs/GitLabDriver.php
@@ -91,7 +91,7 @@ class GitLabDriver extends VcsDriver
      *
      * @inheritDoc
      */
-    public function initialize()
+    public function initialize(): void
     {
         if (!Preg::isMatch(self::URL_REGEX, $this->url, $match)) {
             throw new \InvalidArgumentException(sprintf('The GitLab repository URL %s is invalid. It must be the HTTP URL of a GitLab project.', $this->url));
@@ -136,7 +136,7 @@ class GitLabDriver extends VcsDriver
      *
      * @return void
      */
-    public function setHttpDownloader(HttpDownloader $httpDownloader)
+    public function setHttpDownloader(HttpDownloader $httpDownloader): void
     {
         $this->httpDownloader = $httpDownloader;
     }
@@ -144,7 +144,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getComposerInformation($identifier)
+    public function getComposerInformation($identifier): array
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getComposerInformation($identifier);
@@ -184,7 +184,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getFileContent($file, $identifier)
+    public function getFileContent($file, $identifier): ?string
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getFileContent($file, $identifier);
@@ -216,7 +216,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getChangeDate($identifier)
+    public function getChangeDate($identifier): ?\DateTime
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getChangeDate($identifier);
@@ -232,7 +232,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @return string
      */
-    public function getRepositoryUrl()
+    public function getRepositoryUrl(): string
     {
         if ($this->protocol) {
             return $this->project["{$this->protocol}_url_to_repo"];
@@ -244,7 +244,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getUrl()
+    public function getUrl(): string
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getUrl();
@@ -256,7 +256,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getDist($identifier)
+    public function getDist($identifier): ?array
     {
         $url = $this->getApiUrl().'/repository/archive.zip?sha='.$identifier;
 
@@ -266,7 +266,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getSource($identifier)
+    public function getSource($identifier): array
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getSource($identifier);
@@ -278,7 +278,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getRootIdentifier()
+    public function getRootIdentifier(): string
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getRootIdentifier();
@@ -290,7 +290,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getBranches()
+    public function getBranches(): array
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getBranches();
@@ -306,7 +306,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @inheritDoc
      */
-    public function getTags()
+    public function getTags(): array
     {
         if ($this->gitDriver) {
             return $this->gitDriver->getTags();
@@ -322,7 +322,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @return string Base URL for GitLab API v3
      */
-    public function getApiUrl()
+    public function getApiUrl(): string
     {
         return $this->scheme.'://'.$this->originUrl.'/api/v4/projects/'.$this->urlEncodeAll($this->namespace).'%2F'.$this->urlEncodeAll($this->repository);
     }
@@ -333,7 +333,7 @@ class GitLabDriver extends VcsDriver
      * @param  string $string
      * @return string
      */
-    private function urlEncodeAll($string)
+    private function urlEncodeAll($string): string
     {
         $encoded = '';
         for ($i = 0; isset($string[$i]); $i++) {
@@ -352,7 +352,7 @@ class GitLabDriver extends VcsDriver
      *
      * @return string[] where keys are named references like tags or branches and the value a sha
      */
-    protected function getReferences($type)
+    protected function getReferences($type): array
     {
         $perPage = 100;
         $resource = $this->getApiUrl().'/repository/'.$type.'?per_page='.$perPage;
@@ -383,7 +383,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @return void
      */
-    protected function fetchProject()
+    protected function fetchProject(): void
     {
         // we need to fetch the default branch from the api
         $resource = $this->getApiUrl();
@@ -402,7 +402,7 @@ class GitLabDriver extends VcsDriver
      * @return true
      * @throws \RuntimeException
      */
-    protected function attemptCloneFallback()
+    protected function attemptCloneFallback(): bool
     {
         if ($this->isPrivate === false) {
             $url = $this->generatePublicUrl();
@@ -430,7 +430,7 @@ class GitLabDriver extends VcsDriver
      *
      * @return string
      */
-    protected function generateSshUrl()
+    protected function generateSshUrl(): string
     {
         if ($this->hasNonstandardOrigin) {
             return 'ssh://git@'.$this->originUrl.'/'.$this->namespace.'/'.$this->repository.'.git';
@@ -442,7 +442,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @return string
      */
-    protected function generatePublicUrl()
+    protected function generatePublicUrl(): string
     {
         return $this->scheme . '://' . $this->originUrl . '/'.$this->namespace.'/'.$this->repository.'.git';
     }
@@ -452,7 +452,7 @@ class GitLabDriver extends VcsDriver
      *
      * @return void
      */
-    protected function setupGitDriver($url)
+    protected function setupGitDriver($url): void
     {
         $this->gitDriver = new GitDriver(
             array('url' => $url),
@@ -469,7 +469,7 @@ class GitLabDriver extends VcsDriver
      *
      * @param bool $fetchingRepoData
      */
-    protected function getContents($url, $fetchingRepoData = false)
+    protected function getContents($url, $fetchingRepoData = false): Response
     {
         try {
             $response = parent::getContents($url);
@@ -568,7 +568,7 @@ class GitLabDriver extends VcsDriver
      *
      * @inheritDoc
      */
-    public static function supports(IOInterface $io, Config $config, $url, $deep = false)
+    public static function supports(IOInterface $io, Config $config, $url, $deep = false): bool
     {
         if (!Preg::isMatch(self::URL_REGEX, $url, $match)) {
             return false;
@@ -594,7 +594,7 @@ class GitLabDriver extends VcsDriver
     /**
      * @return string|null
      */
-    protected function getNextPage(Response $response)
+    protected function getNextPage(Response $response): ?string
     {
         $header = $response->getHeader('link');
 
diff --git a/src/Composer/Repository/Vcs/HgDriver.php b/src/Composer/Repository/Vcs/HgDriver.php
index 73bc426e5..e9b83d6c5 100644
--- a/src/Composer/Repository/Vcs/HgDriver.php
+++ b/src/Composer/Repository/Vcs/HgDriver.php
@@ -71,7 +71,7 @@ class HgDriver extends VcsDriver
                 $fs->removeDirectory($this->repoDir);
 
                 $repoDir = $this->repoDir;
-                $command = function ($url) use ($repoDir) {
+                $command = function ($url) use ($repoDir): string {
                     return sprintf('hg clone --noupdate -- %s %s', ProcessExecutor::escape($url), ProcessExecutor::escape($repoDir));
                 };
 
diff --git a/src/Composer/Repository/Vcs/PerforceDriver.php b/src/Composer/Repository/Vcs/PerforceDriver.php
index 75a61037c..b178257f2 100644
--- a/src/Composer/Repository/Vcs/PerforceDriver.php
+++ b/src/Composer/Repository/Vcs/PerforceDriver.php
@@ -55,7 +55,7 @@ class PerforceDriver extends VcsDriver
      *
      * @return void
      */
-    private function initPerforce($repoConfig)
+    private function initPerforce($repoConfig): void
     {
         if (!empty($this->perforce)) {
             return;
diff --git a/src/Composer/Repository/VcsRepository.php b/src/Composer/Repository/VcsRepository.php
index b5b85cd14..11d8df0e9 100644
--- a/src/Composer/Repository/VcsRepository.php
+++ b/src/Composer/Repository/VcsRepository.php
@@ -542,7 +542,7 @@ class VcsRepository extends ArrayRepository implements ConfigurableRepositoryInt
     /**
      * @return bool
      */
-    private function shouldRethrowTransportException(TransportException $e)
+    private function shouldRethrowTransportException(TransportException $e): bool
     {
         return in_array($e->getCode(), array(401, 403, 429), true) || $e->getCode() >= 500;
     }
diff --git a/src/Composer/Repository/WritableArrayRepository.php b/src/Composer/Repository/WritableArrayRepository.php
index 722cc68f1..eb6b595db 100644
--- a/src/Composer/Repository/WritableArrayRepository.php
+++ b/src/Composer/Repository/WritableArrayRepository.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Repository;
 
+use Composer\Package\PackageInterface;
 use Composer\Package\AliasPackage;
 use Composer\Installer\InstallationManager;
 
diff --git a/src/Composer/Script/Event.php b/src/Composer/Script/Event.php
index 83aef267a..3ddf2eae0 100644
--- a/src/Composer/Script/Event.php
+++ b/src/Composer/Script/Event.php
@@ -121,7 +121,7 @@ class Event extends BaseEvent
      * @param  BaseEvent $event
      * @return BaseEvent
      */
-    private function calculateOriginatingEvent(BaseEvent $event)
+    private function calculateOriginatingEvent(BaseEvent $event): BaseEvent
     {
         if ($event instanceof Event && $event->getOriginatingEvent()) {
             return $this->calculateOriginatingEvent($event->getOriginatingEvent());
diff --git a/src/Composer/SelfUpdate/Keys.php b/src/Composer/SelfUpdate/Keys.php
index 64666f251..8c33a436f 100644
--- a/src/Composer/SelfUpdate/Keys.php
+++ b/src/Composer/SelfUpdate/Keys.php
@@ -24,7 +24,7 @@ class Keys
      *
      * @return string
      */
-    public static function fingerprint($path)
+    public static function fingerprint($path): string
     {
         $hash = strtoupper(hash('sha256', Preg::replace('{\s}', '', file_get_contents($path))));
 
diff --git a/src/Composer/SelfUpdate/Versions.php b/src/Composer/SelfUpdate/Versions.php
index 9e617fb13..ce959479d 100644
--- a/src/Composer/SelfUpdate/Versions.php
+++ b/src/Composer/SelfUpdate/Versions.php
@@ -41,7 +41,7 @@ class Versions
     /**
      * @return string
      */
-    public function getChannel()
+    public function getChannel(): string
     {
         if ($this->channel) {
             return $this->channel;
@@ -63,7 +63,7 @@ class Versions
      *
      * @return void
      */
-    public function setChannel($channel)
+    public function setChannel($channel): void
     {
         if (!in_array($channel, self::$channels, true)) {
             throw new \InvalidArgumentException('Invalid channel '.$channel.', must be one of: ' . implode(', ', self::$channels));
@@ -79,7 +79,7 @@ class Versions
      *
      * @return array{path: string, version: string, min-php: int}
      */
-    public function getLatest($channel = null)
+    public function getLatest($channel = null): array
     {
         $versions = $this->getVersionsData();
 
@@ -95,7 +95,7 @@ class Versions
     /**
      * @return array<string, array<int, array{path: string, version: string, min-php: int}>>
      */
-    private function getVersionsData()
+    private function getVersionsData(): array
     {
         if (!$this->versionsData) {
             if ($this->config->get('disable-tls') === true) {
diff --git a/src/Composer/Util/AuthHelper.php b/src/Composer/Util/AuthHelper.php
index 28c4aba68..26eab08b2 100644
--- a/src/Composer/Util/AuthHelper.php
+++ b/src/Composer/Util/AuthHelper.php
@@ -41,7 +41,7 @@ class AuthHelper
      *
      * @return void
      */
-    public function storeAuth($origin, $storeAuth)
+    public function storeAuth($origin, $storeAuth): void
     {
         $store = false;
         $configSource = $this->config->getAuthConfigSource();
@@ -50,7 +50,7 @@ class AuthHelper
         } elseif ($storeAuth === 'prompt') {
             $answer = $this->io->askAndValidate(
                 'Do you want to store credentials for '.$origin.' in '.$configSource->getName().' ? [Yn] ',
-                function ($value) {
+                function ($value): string {
                     $input = strtolower(substr(trim($value), 0, 1));
                     if (in_array($input, array('y','n'))) {
                         return $input;
@@ -83,7 +83,7 @@ class AuthHelper
      *                                retried, if storeAuth is true then on a successful retry the authentication should be persisted to auth.json
      * @phpstan-return ?array{retry: bool, storeAuth: string|bool}
      */
-    public function promptAuthIfNeeded($url, $origin, $statusCode, $reason = null, $headers = array())
+    public function promptAuthIfNeeded($url, $origin, $statusCode, $reason = null, $headers = array()): ?array
     {
         $storeAuth = false;
 
@@ -215,7 +215,7 @@ class AuthHelper
      *
      * @return string[] updated headers array
      */
-    public function addAuthenticationHeader(array $headers, $origin, $url)
+    public function addAuthenticationHeader(array $headers, $origin, $url): array
     {
         if ($this->io->hasAuthentication($origin)) {
             $authenticationDisplayMessage = null;
@@ -272,7 +272,7 @@ class AuthHelper
      *
      * @return bool Whether the given URL is a public BitBucket download which requires no authentication.
      */
-    public function isPublicBitBucketDownload($urlToBitBucketFile)
+    public function isPublicBitBucketDownload($urlToBitBucketFile): bool
     {
         $domain = parse_url($urlToBitBucketFile, PHP_URL_HOST);
         if (strpos($domain, 'bitbucket.org') === false) {
diff --git a/src/Composer/Util/Bitbucket.php b/src/Composer/Util/Bitbucket.php
index 62a46c1c5..5eb6b8fe6 100644
--- a/src/Composer/Util/Bitbucket.php
+++ b/src/Composer/Util/Bitbucket.php
@@ -58,7 +58,7 @@ class Bitbucket
     /**
      * @return string
      */
-    public function getToken()
+    public function getToken(): string
     {
         if (!isset($this->token['access_token'])) {
             return '';
@@ -73,7 +73,7 @@ class Bitbucket
      * @param  string $originUrl The host this Bitbucket instance is located at
      * @return bool   true on success
      */
-    public function authorizeOAuth($originUrl)
+    public function authorizeOAuth($originUrl): bool
     {
         if ($originUrl !== 'bitbucket.org') {
             return false;
@@ -92,7 +92,7 @@ class Bitbucket
     /**
      * @return bool
      */
-    private function requestAccessToken()
+    private function requestAccessToken(): bool
     {
         try {
             $response = $this->httpDownloader->get(self::OAUTH2_ACCESS_TOKEN_URL, array(
@@ -140,7 +140,7 @@ class Bitbucket
      * @throws TransportException|\Exception
      * @return bool                          true on success
      */
-    public function authorizeOAuthInteractively($originUrl, $message = null)
+    public function authorizeOAuthInteractively($originUrl, $message = null): bool
     {
         if ($message) {
             $this->io->writeError($message);
@@ -194,7 +194,7 @@ class Bitbucket
      * @param  string $consumerSecret
      * @return string
      */
-    public function requestToken($originUrl, $consumerKey, $consumerSecret)
+    public function requestToken($originUrl, $consumerKey, $consumerSecret): string
     {
         if ($this->token !== null || $this->getTokenFromConfig($originUrl)) {
             return $this->token['access_token'];
@@ -223,7 +223,7 @@ class Bitbucket
      *
      * @return void
      */
-    private function storeInAuthConfig($originUrl, $consumerKey, $consumerSecret)
+    private function storeInAuthConfig($originUrl, $consumerKey, $consumerSecret): void
     {
         $this->config->getConfigSource()->removeConfigSetting('bitbucket-oauth.'.$originUrl);
 
@@ -246,7 +246,7 @@ class Bitbucket
      * @param  string $originUrl
      * @return bool
      */
-    private function getTokenFromConfig($originUrl)
+    private function getTokenFromConfig($originUrl): bool
     {
         $authConfig = $this->config->get('bitbucket-oauth');
 
diff --git a/src/Composer/Util/ComposerMirror.php b/src/Composer/Util/ComposerMirror.php
index f06668518..948bccee3 100644
--- a/src/Composer/Util/ComposerMirror.php
+++ b/src/Composer/Util/ComposerMirror.php
@@ -31,7 +31,7 @@ class ComposerMirror
      *
      * @return string
      */
-    public static function processUrl($mirrorUrl, $packageName, $version, $reference, $type, $prettyVersion = null)
+    public static function processUrl($mirrorUrl, $packageName, $version, $reference, $type, $prettyVersion = null): string
     {
         if ($reference) {
             $reference = Preg::isMatch('{^([a-f0-9]*|%reference%)$}', $reference) ? $reference : md5($reference);
@@ -56,7 +56,7 @@ class ComposerMirror
      *
      * @return string
      */
-    public static function processGitUrl($mirrorUrl, $packageName, $url, $type)
+    public static function processGitUrl($mirrorUrl, $packageName, $url, $type): string
     {
         if (Preg::isMatch('#^(?:(?:https?|git)://github\.com/|git@github\.com:)([^/]+)/(.+?)(?:\.git)?$#', $url, $match)) {
             $url = 'gh-'.$match[1].'/'.$match[2];
@@ -81,7 +81,7 @@ class ComposerMirror
      *
      * @return string
      */
-    public static function processHgUrl($mirrorUrl, $packageName, $url, $type)
+    public static function processHgUrl($mirrorUrl, $packageName, $url, $type): string
     {
         return self::processGitUrl($mirrorUrl, $packageName, $url, $type);
     }
diff --git a/src/Composer/Util/ConfigValidator.php b/src/Composer/Util/ConfigValidator.php
index c7447a2a8..b0dd79fad 100644
--- a/src/Composer/Util/ConfigValidator.php
+++ b/src/Composer/Util/ConfigValidator.php
@@ -48,7 +48,7 @@ class ConfigValidator
      *
      * @return array{list<string>, list<string>, list<string>} a triple containing the errors, publishable errors, and warnings
      */
-    public function validate($file, $arrayLoaderValidationFlags = ValidatingArrayLoader::CHECK_ALL, $flags = self::CHECK_VERSION)
+    public function validate($file, $arrayLoaderValidationFlags = ValidatingArrayLoader::CHECK_ALL, $flags = self::CHECK_VERSION): array
     {
         $errors = array();
         $publishErrors = array();
diff --git a/src/Composer/Util/ErrorHandler.php b/src/Composer/Util/ErrorHandler.php
index 5e2bc1790..257f7ce29 100644
--- a/src/Composer/Util/ErrorHandler.php
+++ b/src/Composer/Util/ErrorHandler.php
@@ -36,7 +36,7 @@ class ErrorHandler
      * @throws \ErrorException
      * @return bool
      */
-    public static function handle($level, $message, $file, $line)
+    public static function handle($level, $message, $file, $line): bool
     {
         // error code is not included in error_reporting
         if (!(error_reporting() & $level)) {
@@ -56,7 +56,7 @@ class ErrorHandler
             self::$io->writeError('<warning>Deprecation Notice: '.$message.' in '.$file.':'.$line.'</warning>');
             if (self::$io->isVerbose()) {
                 self::$io->writeError('<warning>Stack trace:</warning>');
-                self::$io->writeError(array_filter(array_map(function ($a) {
+                self::$io->writeError(array_filter(array_map(function ($a): ?string {
                     if (isset($a['line'], $a['file'])) {
                         return '<warning> '.$a['file'].':'.$a['line'].'</warning>';
                     }
@@ -76,7 +76,7 @@ class ErrorHandler
      *
      * @return void
      */
-    public static function register(IOInterface $io = null)
+    public static function register(IOInterface $io = null): void
     {
         set_error_handler(array(__CLASS__, 'handle'));
         error_reporting(E_ALL | E_STRICT);
diff --git a/src/Composer/Util/Filesystem.php b/src/Composer/Util/Filesystem.php
index 3bd1c86fa..99aa803c9 100644
--- a/src/Composer/Util/Filesystem.php
+++ b/src/Composer/Util/Filesystem.php
@@ -177,7 +177,7 @@ class Filesystem
      *
      * @return bool|null Returns null, when no edge case was hit. Otherwise a bool whether removal was successful
      */
-    private function removeEdgeCases($directory, $fallbackToPhp = true)
+    private function removeEdgeCases($directory, $fallbackToPhp = true): ?bool
     {
         if ($this->isSymlinkedDirectory($directory)) {
             return $this->unlinkSymlinkedDirectory($directory);
@@ -711,7 +711,7 @@ class Filesystem
      *
      * @return bool
      */
-    private function unlinkImplementation($path)
+    private function unlinkImplementation($path): bool
     {
         if (Platform::isWindows() && is_dir($path) && is_link($path)) {
             return rmdir($path);
@@ -767,7 +767,7 @@ class Filesystem
      *
      * @return bool
      */
-    private function unlinkSymlinkedDirectory($directory)
+    private function unlinkSymlinkedDirectory($directory): bool
     {
         $resolved = $this->resolveSymlinkedDirectorySymlink($directory);
 
@@ -781,7 +781,7 @@ class Filesystem
      *
      * @return string resolved path to symbolic link or original pathname (unresolved)
      */
-    private function resolveSymlinkedDirectorySymlink($pathname)
+    private function resolveSymlinkedDirectorySymlink($pathname): string
     {
         if (!is_dir($pathname)) {
             return $pathname;
@@ -926,7 +926,7 @@ class Filesystem
      *
      * @return bool
      */
-    private function filesAreEqual($a, $b)
+    private function filesAreEqual($a, $b): bool
     {
         // Check if filesize is different
         if (filesize($a) !== filesize($b)) {
diff --git a/src/Composer/Util/Git.php b/src/Composer/Util/Git.php
index f2ea4dc24..27dc75a0d 100644
--- a/src/Composer/Util/Git.php
+++ b/src/Composer/Util/Git.php
@@ -283,7 +283,7 @@ class Git
         // update the repo if it is a valid git repository
         if (is_dir($dir) && 0 === $this->process->execute('git rev-parse --git-dir', $output, $dir) && trim($output) === '.') {
             try {
-                $commandCallable = function ($url) {
+                $commandCallable = function ($url): string {
                     $sanitizedUrl = Preg::replace('{://([^@]+?):(.+?)@}', '://', $url);
 
                     return sprintf('git remote set-url origin -- %s && git remote update --prune origin && git remote set-url origin -- %s && git gc --auto', ProcessExecutor::escape($url), ProcessExecutor::escape($sanitizedUrl));
@@ -301,7 +301,7 @@ class Git
         // clean up directory and do a fresh clone into it
         $this->filesystem->removeDirectory($dir);
 
-        $commandCallable = function ($url) use ($dir) {
+        $commandCallable = function ($url) use ($dir): string {
             return sprintf('git clone --mirror -- %s %s', ProcessExecutor::escape($url), ProcessExecutor::escape($dir));
         };
 
@@ -349,7 +349,7 @@ class Git
      *
      * @return bool
      */
-    private function checkRefIsInMirror($dir, $ref)
+    private function checkRefIsInMirror($dir, $ref): bool
     {
         if (is_dir($dir) && 0 === $this->process->execute('git rev-parse --git-dir', $output, $dir) && trim($output) === '.') {
             $escapedRef = ProcessExecutor::escape($ref.'^{commit}');
@@ -368,7 +368,7 @@ class Git
      *
      * @return bool
      */
-    private function isAuthenticationFailure($url, &$match)
+    private function isAuthenticationFailure($url, &$match): bool
     {
         if (!Preg::isMatch('{^(https?://)([^/]+)(.*)$}i', $url, $match)) {
             return false;
@@ -441,7 +441,7 @@ class Git
      *
      * @return never
      */
-    private function throwException($message, $url)
+    private function throwException($message, $url): void
     {
         // git might delete a directory when it fails and php will not know
         clearstatcache();
@@ -476,7 +476,7 @@ class Git
      *
      * @return string
      */
-    private function maskCredentials($error, array $credentials)
+    private function maskCredentials($error, array $credentials): string
     {
         $maskedCredentials = array();
 
diff --git a/src/Composer/Util/GitHub.php b/src/Composer/Util/GitHub.php
index 2dd53e350..f20cb2376 100644
--- a/src/Composer/Util/GitHub.php
+++ b/src/Composer/Util/GitHub.php
@@ -54,7 +54,7 @@ class GitHub
      * @param  string $originUrl The host this GitHub instance is located at
      * @return bool   true on success
      */
-    public function authorizeOAuth($originUrl)
+    public function authorizeOAuth($originUrl): bool
     {
         if (!in_array($originUrl, $this->config->get('github-domains'))) {
             return false;
@@ -79,7 +79,7 @@ class GitHub
      * @throws TransportException|\Exception
      * @return bool                          true on success
      */
-    public function authorizeOAuthInteractively($originUrl, $message = null)
+    public function authorizeOAuthInteractively($originUrl, $message = null): bool
     {
         if ($message) {
             $this->io->writeError($message);
@@ -145,7 +145,7 @@ class GitHub
      *
      * @return array{limit: int|'?', reset: string}
      */
-    public function getRateLimit(array $headers)
+    public function getRateLimit(array $headers): array
     {
         $rateLimit = array(
             'limit' => '?',
@@ -178,7 +178,7 @@ class GitHub
      *
      * @return string|null
      */
-    public function getSsoUrl(array $headers)
+    public function getSsoUrl(array $headers): ?string
     {
         foreach ($headers as $header) {
             $header = trim($header);
@@ -200,7 +200,7 @@ class GitHub
      *
      * @return bool
      */
-    public function isRateLimited(array $headers)
+    public function isRateLimited(array $headers): bool
     {
         foreach ($headers as $header) {
             if (Preg::isMatch('{^X-RateLimit-Remaining: *0$}i', trim($header))) {
@@ -220,7 +220,7 @@ class GitHub
      *
      * @return bool
      */
-    public function requiresSso(array $headers)
+    public function requiresSso(array $headers): bool
     {
         foreach ($headers as $header) {
             if (Preg::isMatch('{^X-GitHub-SSO: required}i', trim($header))) {
diff --git a/src/Composer/Util/GitLab.php b/src/Composer/Util/GitLab.php
index 4c1376ad4..4bd61d792 100644
--- a/src/Composer/Util/GitLab.php
+++ b/src/Composer/Util/GitLab.php
@@ -55,7 +55,7 @@ class GitLab
      *
      * @return bool true on success
      */
-    public function authorizeOAuth($originUrl)
+    public function authorizeOAuth($originUrl): bool
     {
         // before composer 1.9, origin URLs had no port number in them
         $bcOriginUrl = Preg::replace('{:\d+}', '', $originUrl);
@@ -112,7 +112,7 @@ class GitLab
      *
      * @return bool true on success
      */
-    public function authorizeOAuthInteractively($scheme, $originUrl, $message = null)
+    public function authorizeOAuthInteractively($scheme, $originUrl, $message = null): bool
     {
         if ($message) {
             $this->io->writeError($message);
@@ -169,7 +169,7 @@ class GitLab
      *
      * @see https://docs.gitlab.com/ee/api/oauth2.html#resource-owner-password-credentials-flow
      */
-    private function createToken($scheme, $originUrl)
+    private function createToken($scheme, $originUrl): array
     {
         $username = $this->io->ask('Username: ');
         $password = $this->io->askAndHideAnswer('Password: ');
diff --git a/src/Composer/Util/Hg.php b/src/Composer/Util/Hg.php
index 73e3a8e52..da8c3329a 100644
--- a/src/Composer/Util/Hg.php
+++ b/src/Composer/Util/Hg.php
@@ -53,7 +53,7 @@ class Hg
      *
      * @return void
      */
-    public function runCommand($commandCallable, $url, $cwd)
+    public function runCommand($commandCallable, $url, $cwd): void
     {
         $this->config->prohibitUrlByConfig($url, $this->io);
 
@@ -89,7 +89,7 @@ class Hg
      *
      * @return never
      */
-    private function throwException($message, $url)
+    private function throwException($message, $url): void
     {
         if (null === self::getVersion($this->process)) {
             throw new \RuntimeException(Url::sanitize('Failed to clone ' . $url . ', hg was not found, check that it is installed and in your PATH env.' . "\n\n" . $this->process->getErrorOutput()));
@@ -103,7 +103,7 @@ class Hg
      *
      * @return string|null The hg version number, if present.
      */
-    public static function getVersion(ProcessExecutor $process)
+    public static function getVersion(ProcessExecutor $process): ?string
     {
         if (false === self::$version) {
             self::$version = null;
diff --git a/src/Composer/Util/Http/CurlDownloader.php b/src/Composer/Util/Http/CurlDownloader.php
index a53fd7fb7..eeff04e1f 100644
--- a/src/Composer/Util/Http/CurlDownloader.php
+++ b/src/Composer/Util/Http/CurlDownloader.php
@@ -133,7 +133,7 @@ class CurlDownloader
      *
      * @return int internal job id
      */
-    public function download($resolve, $reject, $origin, $url, $options, $copyTo = null)
+    public function download($resolve, $reject, $origin, $url, $options, $copyTo = null): int
     {
         $attributes = array();
         if (isset($options['retry-auth-failure'])) {
@@ -156,7 +156,7 @@ class CurlDownloader
      *
      * @return int internal job id
      */
-    private function initDownload($resolve, $reject, $origin, $url, $options, $copyTo = null, array $attributes = array())
+    private function initDownload($resolve, $reject, $origin, $url, $options, $copyTo = null, array $attributes = array()): int
     {
         // set defaults in a PHPStan-happy way (array_merge is not well supported)
         $attributes['retryAuthFailure'] = $attributes['retryAuthFailure'] ?? true;
@@ -180,7 +180,7 @@ class CurlDownloader
         if ($copyTo) {
             $errorMessage = '';
             // @phpstan-ignore-next-line
-            set_error_handler(function ($code, $msg) use (&$errorMessage) {
+            set_error_handler(function ($code, $msg) use (&$errorMessage): void {
                 if ($errorMessage) {
                     $errorMessage .= "\n";
                 }
@@ -287,7 +287,7 @@ class CurlDownloader
      * @param  int  $id
      * @return void
      */
-    public function abortRequest($id)
+    public function abortRequest($id): void
     {
         if (isset($this->jobs[$id], $this->jobs[$id]['curlHandle'])) {
             $job = $this->jobs[$id];
@@ -309,7 +309,7 @@ class CurlDownloader
     /**
      * @return void
      */
-    public function tick()
+    public function tick(): void
     {
         static $timeoutWarning = false;
 
@@ -489,7 +489,7 @@ class CurlDownloader
      * @param  Job    $job
      * @return string
      */
-    private function handleRedirect(array $job, Response $response)
+    private function handleRedirect(array $job, Response $response): string
     {
         if ($locationHeader = $response->getHeader('location')) {
             if (parse_url($locationHeader, PHP_URL_SCHEME)) {
@@ -524,7 +524,7 @@ class CurlDownloader
      * @param  Job                                        $job
      * @return array{retry: bool, storeAuth: string|bool}
      */
-    private function isAuthenticatedRetryNeeded(array $job, Response $response)
+    private function isAuthenticatedRetryNeeded(array $job, Response $response): array
     {
         if (in_array($response->getStatusCode(), array(401, 403)) && $job['attributes']['retryAuthFailure']) {
             $result = $this->authHelper->promptAuthIfNeeded($job['url'], $job['origin'], $response->getStatusCode(), $response->getStatusMessage(), $response->getHeaders());
@@ -579,7 +579,7 @@ class CurlDownloader
      *
      * @return void
      */
-    private function restartJob(array $job, $url, array $attributes = array())
+    private function restartJob(array $job, $url, array $attributes = array()): void
     {
         if (null !== $job['filename']) {
             @unlink($job['filename'].'~');
@@ -596,7 +596,7 @@ class CurlDownloader
      * @param  string             $errorMessage
      * @return TransportException
      */
-    private function failResponse(array $job, Response $response, $errorMessage)
+    private function failResponse(array $job, Response $response, $errorMessage): TransportException
     {
         if (null !== $job['filename']) {
             @unlink($job['filename'].'~');
@@ -614,7 +614,7 @@ class CurlDownloader
      * @param  Job                $job
      * @return void
      */
-    private function rejectJob(array $job, \Exception $e)
+    private function rejectJob(array $job, \Exception $e): void
     {
         if (is_resource($job['headerHandle'])) {
             fclose($job['headerHandle']);
@@ -632,7 +632,7 @@ class CurlDownloader
      * @param  int  $code
      * @return void
      */
-    private function checkCurlResult($code)
+    private function checkCurlResult($code): void
     {
         if ($code != CURLM_OK && $code != CURLM_CALL_MULTI_PERFORM) {
             throw new \RuntimeException(
diff --git a/src/Composer/Util/Http/CurlResponse.php b/src/Composer/Util/Http/CurlResponse.php
index 495a73b85..c51ff5069 100644
--- a/src/Composer/Util/Http/CurlResponse.php
+++ b/src/Composer/Util/Http/CurlResponse.php
@@ -19,12 +19,13 @@ class CurlResponse extends Response
 {
     /**
      * @see https://www.php.net/curl_getinfo
-     * @var CurlInfo
+     * @var array
+     * @phpstan-var CurlInfo
      */
     private $curlInfo;
 
     /**
-     * @param CurlInfo $curlInfo
+     * @phpstan-param CurlInfo $curlInfo
      */
     public function __construct(array $request, $code, array $headers, $body, array $curlInfo)
     {
@@ -33,9 +34,9 @@ class CurlResponse extends Response
     }
 
     /**
-     * @return CurlInfo
+     * @phpstan-return CurlInfo
      */
-    public function getCurlInfo()
+    public function getCurlInfo(): array
     {
         return $this->curlInfo;
     }
diff --git a/src/Composer/Util/Http/ProxyHelper.php b/src/Composer/Util/Http/ProxyHelper.php
index febcfc73d..db19db62d 100644
--- a/src/Composer/Util/Http/ProxyHelper.php
+++ b/src/Composer/Util/Http/ProxyHelper.php
@@ -27,7 +27,7 @@ class ProxyHelper
      *
      * @throws \RuntimeException on malformed url
      */
-    public static function getProxyData()
+    public static function getProxyData(): array
     {
         $httpProxy = null;
         $httpsProxy = null;
@@ -64,7 +64,7 @@ class ProxyHelper
      *
      * @return array{http: array{proxy: string, header?: string}}
      */
-    public static function getContextOptions($proxyUrl)
+    public static function getContextOptions($proxyUrl): array
     {
         $proxy = parse_url($proxyUrl);
 
@@ -97,7 +97,7 @@ class ProxyHelper
      *
      * @return void
      */
-    public static function setRequestFullUri($requestUrl, array &$options)
+    public static function setRequestFullUri($requestUrl, array &$options): void
     {
         if ('http' === parse_url($requestUrl, PHP_URL_SCHEME)) {
             $options['http']['request_fulluri'] = true;
@@ -114,7 +114,7 @@ class ProxyHelper
      *
      * @return string|null The found value
      */
-    private static function getProxyEnv(array $names, &$name)
+    private static function getProxyEnv(array $names, &$name): ?string
     {
         foreach ($names as $name) {
             if (!empty($_SERVER[$name])) {
@@ -133,7 +133,7 @@ class ProxyHelper
      * @throws \RuntimeException on malformed url
      * @return string            The formatted proxy url
      */
-    private static function checkProxy($proxyUrl, $envName)
+    private static function checkProxy($proxyUrl, $envName): string
     {
         $error = sprintf('malformed %s url', $envName);
         $proxy = parse_url($proxyUrl);
@@ -161,7 +161,7 @@ class ProxyHelper
      *
      * @return string The formatted value
      */
-    private static function formatParsedUrl(array $proxy, $includeAuth)
+    private static function formatParsedUrl(array $proxy, $includeAuth): string
     {
         $proxyUrl = isset($proxy['scheme']) ? strtolower($proxy['scheme']) . '://' : '';
 
diff --git a/src/Composer/Util/Http/ProxyManager.php b/src/Composer/Util/Http/ProxyManager.php
index 352ab8d35..5f11c8242 100644
--- a/src/Composer/Util/Http/ProxyManager.php
+++ b/src/Composer/Util/Http/ProxyManager.php
@@ -58,7 +58,7 @@ class ProxyManager
     /**
      * @return ProxyManager
      */
-    public static function getInstance()
+    public static function getInstance(): ProxyManager
     {
         if (!self::$instance) {
             self::$instance = new self();
@@ -72,7 +72,7 @@ class ProxyManager
      *
      * @return void
      */
-    public static function reset()
+    public static function reset(): void
     {
         self::$instance = null;
     }
@@ -83,7 +83,7 @@ class ProxyManager
      * @param  string       $requestUrl
      * @return RequestProxy
      */
-    public function getProxyForRequest($requestUrl)
+    public function getProxyForRequest($requestUrl): RequestProxy
     {
         if ($this->error) {
             throw new TransportException('Unable to use a proxy: '.$this->error);
@@ -113,7 +113,7 @@ class ProxyManager
      *
      * @return bool If false any error will be in $message
      */
-    public function isProxying()
+    public function isProxying(): bool
     {
         return $this->hasProxy;
     }
@@ -123,7 +123,7 @@ class ProxyManager
      *
      * @return string|null Safe proxy URL or an error message if setting up proxy failed or null if no proxy was configured
      */
-    public function getFormattedProxy()
+    public function getFormattedProxy(): ?string
     {
         return $this->hasProxy ? $this->info : $this->error;
     }
@@ -133,7 +133,7 @@ class ProxyManager
      *
      * @return void
      */
-    private function initProxyData()
+    private function initProxyData(): void
     {
         try {
             list($httpProxy, $httpsProxy, $noProxy) = ProxyHelper::getProxyData();
@@ -167,7 +167,7 @@ class ProxyManager
      *
      * @return non-empty-string
      */
-    private function setData($url, $scheme)
+    private function setData($url, $scheme): string
     {
         $safeProxy = Url::sanitize($url);
         $this->fullProxy[$scheme] = $url;
@@ -184,7 +184,7 @@ class ProxyManager
      * @param  string $requestUrl
      * @return bool
      */
-    private function noProxy($requestUrl)
+    private function noProxy($requestUrl): bool
     {
         return $this->noProxyHandler && $this->noProxyHandler->test($requestUrl);
     }
diff --git a/src/Composer/Util/Http/RequestProxy.php b/src/Composer/Util/Http/RequestProxy.php
index 5a832706f..e60c1a19c 100644
--- a/src/Composer/Util/Http/RequestProxy.php
+++ b/src/Composer/Util/Http/RequestProxy.php
@@ -47,7 +47,7 @@ class RequestProxy
      *
      * @return mixed[]
      */
-    public function getContextOptions()
+    public function getContextOptions(): array
     {
         return $this->contextOptions;
     }
@@ -58,7 +58,7 @@ class RequestProxy
      * @param  string|null $format Output format specifier
      * @return string      Safe proxy, no proxy or empty
      */
-    public function getFormattedUrl($format = '')
+    public function getFormattedUrl($format = ''): string
     {
         $result = '';
         if ($this->formattedUrl) {
@@ -74,7 +74,7 @@ class RequestProxy
      *
      * @return string Proxy url or empty
      */
-    public function getUrl()
+    public function getUrl(): string
     {
         return $this->url;
     }
@@ -84,7 +84,7 @@ class RequestProxy
      *
      * @return bool False if not secure or there is no proxy
      */
-    public function isSecure()
+    public function isSecure(): bool
     {
         return $this->isSecure;
     }
diff --git a/src/Composer/Util/Http/Response.php b/src/Composer/Util/Http/Response.php
index 39c68d6f2..4c9072fac 100644
--- a/src/Composer/Util/Http/Response.php
+++ b/src/Composer/Util/Http/Response.php
@@ -50,7 +50,7 @@ class Response
     /**
      * @return int
      */
-    public function getStatusCode()
+    public function getStatusCode(): int
     {
         return $this->code;
     }
@@ -58,7 +58,7 @@ class Response
     /**
      * @return string|null
      */
-    public function getStatusMessage()
+    public function getStatusMessage(): ?string
     {
         $value = null;
         foreach ($this->headers as $header) {
@@ -75,7 +75,7 @@ class Response
     /**
      * @return string[]
      */
-    public function getHeaders()
+    public function getHeaders(): array
     {
         return $this->headers;
     }
@@ -84,7 +84,7 @@ class Response
      * @param  string  $name
      * @return ?string
      */
-    public function getHeader($name)
+    public function getHeader($name): ?string
     {
         return self::findHeaderValue($this->headers, $name);
     }
@@ -92,7 +92,7 @@ class Response
     /**
      * @return ?string
      */
-    public function getBody()
+    public function getBody(): ?string
     {
         return $this->body;
     }
@@ -109,7 +109,7 @@ class Response
      * @return void
      * @phpstan-impure
      */
-    public function collect()
+    public function collect(): void
     {
         /** @phpstan-ignore-next-line */
         $this->request = $this->code = $this->headers = $this->body = null;
@@ -120,7 +120,7 @@ class Response
      * @param  string      $name    header name (case insensitive)
      * @return string|null
      */
-    public static function findHeaderValue(array $headers, $name)
+    public static function findHeaderValue(array $headers, $name): ?string
     {
         $value = null;
         foreach ($headers as $header) {
diff --git a/src/Composer/Util/HttpDownloader.php b/src/Composer/Util/HttpDownloader.php
index d1c8c1fbd..b919680f2 100644
--- a/src/Composer/Util/HttpDownloader.php
+++ b/src/Composer/Util/HttpDownloader.php
@@ -191,7 +191,7 @@ class HttpDownloader
      *
      * @return array{Job, PromiseInterface}
      */
-    private function addJob($request, $sync = false)
+    private function addJob($request, $sync = false): array
     {
         $request['options'] = array_replace_recursive($this->options, $request['options']);
 
@@ -216,13 +216,13 @@ class HttpDownloader
         $rfs = $this->rfs;
 
         if ($this->canUseCurl($job)) {
-            $resolver = function ($resolve, $reject) use (&$job) {
+            $resolver = function ($resolve, $reject) use (&$job): void {
                 $job['status'] = HttpDownloader::STATUS_QUEUED;
                 $job['resolve'] = $resolve;
                 $job['reject'] = $reject;
             };
         } else {
-            $resolver = function ($resolve, $reject) use (&$job, $rfs) {
+            $resolver = function ($resolve, $reject) use (&$job, $rfs): void {
                 // start job
                 $url = $job['request']['url'];
                 $options = $job['request']['options'];
@@ -248,7 +248,7 @@ class HttpDownloader
 
         $curl = $this->curl;
 
-        $canceler = function () use (&$job, $curl) {
+        $canceler = function () use (&$job, $curl): void {
             if ($job['status'] === HttpDownloader::STATUS_QUEUED) {
                 $job['status'] = HttpDownloader::STATUS_ABORTED;
             }
@@ -270,7 +270,7 @@ class HttpDownloader
             $this->markJobDone();
 
             return $response;
-        }, function ($e) use (&$job) {
+        }, function ($e) use (&$job): void {
             $job['status'] = HttpDownloader::STATUS_FAILED;
             $job['exception'] = $e;
 
@@ -291,7 +291,7 @@ class HttpDownloader
      * @param  int  $id
      * @return void
      */
-    private function startJob($id)
+    private function startJob($id): void
     {
         $job = &$this->jobs[$id];
         if ($job['status'] !== self::STATUS_QUEUED) {
@@ -355,7 +355,7 @@ class HttpDownloader
      *
      * @return void
      */
-    public function enableAsync()
+    public function enableAsync(): void
     {
         $this->allowAsync = true;
     }
@@ -366,7 +366,7 @@ class HttpDownloader
      * @param  int|null $index For internal use only, the job id
      * @return int      number of active (queued or started) jobs
      */
-    public function countActiveJobs($index = null)
+    public function countActiveJobs($index = null): int
     {
         if ($this->runningJobs < $this->maxJobs) {
             foreach ($this->jobs as $job) {
@@ -400,7 +400,7 @@ class HttpDownloader
      * @param  int $index Job id
      * @return Response
      */
-    private function getResponse($index)
+    private function getResponse($index): Response
     {
         if (!isset($this->jobs[$index])) {
             throw new \LogicException('Invalid request id');
@@ -428,7 +428,7 @@ class HttpDownloader
      * @param  array{warning?: string, info?: string, warning-versions?: string, info-versions?: string, warnings?: array<array{versions: string, message: string}>, infos?: array<array{versions: string, message: string}>} $data
      * @return void
      */
-    public static function outputWarnings(IOInterface $io, $url, $data)
+    public static function outputWarnings(IOInterface $io, $url, $data): void
     {
         // legacy warning/info keys
         foreach (array('warning', 'info') as $type) {
@@ -473,7 +473,7 @@ class HttpDownloader
      *
      * @return ?string[]
      */
-    public static function getExceptionHints(\Exception $e)
+    public static function getExceptionHints(\Exception $e): ?array
     {
         if (!$e instanceof TransportException) {
             return null;
@@ -507,7 +507,7 @@ class HttpDownloader
      * @param  Job  $job
      * @return bool
      */
-    private function canUseCurl(array $job)
+    private function canUseCurl(array $job): bool
     {
         if (!$this->curl) {
             return false;
@@ -528,7 +528,7 @@ class HttpDownloader
      * @internal
      * @return bool
      */
-    public static function isCurlEnabled()
+    public static function isCurlEnabled(): bool
     {
         return \extension_loaded('curl') && \function_exists('curl_multi_exec') && \function_exists('curl_multi_init');
     }
diff --git a/src/Composer/Util/IniHelper.php b/src/Composer/Util/IniHelper.php
index 8076f82ed..699f22513 100644
--- a/src/Composer/Util/IniHelper.php
+++ b/src/Composer/Util/IniHelper.php
@@ -31,7 +31,7 @@ class IniHelper
      *
      * @return string[]
      */
-    public static function getAll()
+    public static function getAll(): array
     {
         return XdebugHandler::getAllIniFiles();
     }
@@ -41,7 +41,7 @@ class IniHelper
      *
      * @return string
      */
-    public static function getMessage()
+    public static function getMessage(): string
     {
         $paths = self::getAll();
 
diff --git a/src/Composer/Util/Loop.php b/src/Composer/Util/Loop.php
index b103011a4..48edc1e16 100644
--- a/src/Composer/Util/Loop.php
+++ b/src/Composer/Util/Loop.php
@@ -44,7 +44,7 @@ class Loop
     /**
      * @return HttpDownloader
      */
-    public function getHttpDownloader()
+    public function getHttpDownloader(): HttpDownloader
     {
         return $this->httpDownloader;
     }
@@ -52,7 +52,7 @@ class Loop
     /**
      * @return ProcessExecutor|null
      */
-    public function getProcessExecutor()
+    public function getProcessExecutor(): ?ProcessExecutor
     {
         return $this->processExecutor;
     }
@@ -62,15 +62,15 @@ class Loop
      * @param  ?ProgressBar       $progress
      * @return void
      */
-    public function wait(array $promises, ProgressBar $progress = null)
+    public function wait(array $promises, ProgressBar $progress = null): void
     {
         /** @var \Exception|null */
         $uncaught = null;
 
         \React\Promise\all($promises)->then(
-            function () {
+            function (): void {
             },
-            function ($e) use (&$uncaught) {
+            function ($e) use (&$uncaught): void {
                 $uncaught = $e;
             }
         );
@@ -122,7 +122,7 @@ class Loop
     /**
      * @return void
      */
-    public function abortJobs()
+    public function abortJobs(): void
     {
         foreach ($this->currentPromises as $promiseGroup) {
             foreach ($promiseGroup as $promise) {
diff --git a/src/Composer/Util/NoProxyPattern.php b/src/Composer/Util/NoProxyPattern.php
index e8f14997b..a81a88a5b 100644
--- a/src/Composer/Util/NoProxyPattern.php
+++ b/src/Composer/Util/NoProxyPattern.php
@@ -51,7 +51,7 @@ class NoProxyPattern
      *
      * @return bool
      */
-    public function test($url)
+    public function test($url): bool
     {
         if ($this->noproxy) {
             return true;
@@ -115,7 +115,7 @@ class NoProxyPattern
      *
      * @return bool
      */
-    protected function match($index, $hostName, $url)
+    protected function match($index, $hostName, $url): bool
     {
         if (!$rule = $this->getRule($index, $hostName)) {
             // Data must have been misformatted
@@ -154,7 +154,7 @@ class NoProxyPattern
      *
      * @return bool
      */
-    protected function matchRange(stdClass $network, stdClass $target)
+    protected function matchRange(stdClass $network, stdClass $target): bool
     {
         $net = unpack('C*', $network->ip);
         $mask = unpack('C*', $network->netmask);
@@ -186,7 +186,7 @@ class NoProxyPattern
      *
      * @return null|stdClass Null if the hostname is invalid
      */
-    private function getRule($index, $hostName)
+    private function getRule($index, $hostName): ?stdClass
     {
         if (array_key_exists($index, $this->rules)) {
             return $this->rules[$index];
@@ -213,7 +213,7 @@ class NoProxyPattern
      *
      * @return bool False if the host contains invalid data
      */
-    private function ipCheckData($host, &$ipdata, $allowPrefix = false)
+    private function ipCheckData($host, &$ipdata, $allowPrefix = false): bool
     {
         $ipdata = null;
         $netmask = null;
@@ -262,7 +262,7 @@ class NoProxyPattern
      *
      * @return mixed[] in_addr, size
      */
-    private function ipGetAddr($host)
+    private function ipGetAddr($host): array
     {
         $ip = inet_pton($host);
         $size = strlen($ip);
@@ -279,7 +279,7 @@ class NoProxyPattern
      *
      * @return string
      */
-    private function ipGetMask($prefix, $size)
+    private function ipGetMask($prefix, $size): string
     {
         $mask = '';
 
@@ -305,7 +305,7 @@ class NoProxyPattern
      *
      * @return string[] network in_addr, binary mask
      */
-    private function ipGetNetwork($rangeIp, $size, $prefix)
+    private function ipGetNetwork($rangeIp, $size, $prefix): array
     {
         $netmask = $this->ipGetMask($prefix, $size);
 
@@ -335,7 +335,7 @@ class NoProxyPattern
      *
      * @return string Mapped or existing in_addr
      */
-    private function ipMapTo6($binary, $size)
+    private function ipMapTo6($binary, $size): string
     {
         if ($size === 4) {
             $prefix = str_repeat(chr(0), 10) . str_repeat(chr(255), 2);
@@ -354,7 +354,7 @@ class NoProxyPattern
      *
      * @return stdClass
      */
-    private function makeData($host, $port, $ipdata)
+    private function makeData($host, $port, $ipdata): stdClass
     {
         return (object) array(
             'host' => $host,
@@ -373,7 +373,7 @@ class NoProxyPattern
      *
      * @return stdClass
      */
-    private function makeIpData($ip, $size, $netmask)
+    private function makeIpData($ip, $size, $netmask): stdClass
     {
         return (object) array(
             'ip' => $ip,
@@ -389,7 +389,7 @@ class NoProxyPattern
      *
      * @return mixed[] host, port, if there was error
      */
-    private function splitHostPort($hostName)
+    private function splitHostPort($hostName): array
     {
         // host, port, err
         $error = array('', '', true);
@@ -439,7 +439,7 @@ class NoProxyPattern
      *
      * @return bool
      */
-    private function validateInt($int, $min, $max)
+    private function validateInt($int, $min, $max): bool
     {
         $options = array(
             'options' => array(
diff --git a/src/Composer/Util/PackageSorter.php b/src/Composer/Util/PackageSorter.php
index df2d89a4b..62889961a 100644
--- a/src/Composer/Util/PackageSorter.php
+++ b/src/Composer/Util/PackageSorter.php
@@ -24,7 +24,7 @@ class PackageSorter
      * @param  PackageInterface[] $packages
      * @return PackageInterface[] sorted array
      */
-    public static function sortPackages(array $packages)
+    public static function sortPackages(array $packages): array
     {
         $usageList = array();
 
@@ -69,17 +69,17 @@ class PackageSorter
             $weightList[$index] = $weight;
         }
 
-        $stable_sort = function (&$array) {
+        $stable_sort = function (&$array): void {
             static $transform, $restore;
 
             $i = 0;
 
             if (!$transform) {
-                $transform = function (&$v, $k) use (&$i) {
+                $transform = function (&$v, $k) use (&$i): void {
                     $v = array($v, ++$i, $k, $v);
                 };
 
-                $restore = function (&$v) {
+                $restore = function (&$v): void {
                     $v = $v[3];
                 };
             }
diff --git a/src/Composer/Util/Platform.php b/src/Composer/Util/Platform.php
index 756c11506..d237f8ea1 100644
--- a/src/Composer/Util/Platform.php
+++ b/src/Composer/Util/Platform.php
@@ -51,7 +51,7 @@ class Platform
      * @param  string $value
      * @return void
      */
-    public static function putEnv($name, $value)
+    public static function putEnv($name, $value): void
     {
         $value = (string) $value;
         putenv($name . '=' . $value);
@@ -64,7 +64,7 @@ class Platform
      * @param  string $name
      * @return void
      */
-    public static function clearEnv($name)
+    public static function clearEnv($name): void
     {
         putenv($name);
         unset($_SERVER[$name], $_ENV[$name]);
@@ -76,13 +76,13 @@ class Platform
      * @param  string $path
      * @return string
      */
-    public static function expandPath($path)
+    public static function expandPath($path): string
     {
         if (Preg::isMatch('#^~[\\/]#', $path)) {
             return self::getUserDirectory() . substr($path, 1);
         }
 
-        return Preg::replaceCallback('#^(\$|(?P<percent>%))(?P<var>\w++)(?(percent)%)(?P<path>.*)#', function ($matches) {
+        return Preg::replaceCallback('#^(\$|(?P<percent>%))(?P<var>\w++)(?(percent)%)(?P<path>.*)#', function ($matches): string {
             // Treat HOME as an alias for USERPROFILE on Windows for legacy reasons
             if (Platform::isWindows() && $matches['var'] == 'HOME') {
                 return (Platform::getEnv('HOME') ?: Platform::getEnv('USERPROFILE')) . $matches['path'];
@@ -96,7 +96,7 @@ class Platform
      * @throws \RuntimeException If the user home could not reliably be determined
      * @return string            The formal user home as detected from environment parameters
      */
-    public static function getUserDirectory()
+    public static function getUserDirectory(): string
     {
         if (false !== ($home = self::getEnv('HOME'))) {
             return $home;
@@ -118,7 +118,7 @@ class Platform
     /**
      * @return bool Whether the host machine is running on the Windows Subsystem for Linux (WSL)
      */
-    public static function isWindowsSubsystemForLinux()
+    public static function isWindowsSubsystemForLinux(): bool
     {
         if (null === self::$isWindowsSubsystemForLinux) {
             self::$isWindowsSubsystemForLinux = false;
@@ -144,7 +144,7 @@ class Platform
     /**
      * @return bool Whether the host machine is running a Windows OS
      */
-    public static function isWindows()
+    public static function isWindows(): bool
     {
         return \defined('PHP_WINDOWS_VERSION_BUILD');
     }
@@ -153,7 +153,7 @@ class Platform
      * @param  string $str
      * @return int    return a guaranteed binary length of the string, regardless of silly mbstring configs
      */
-    public static function strlen($str)
+    public static function strlen($str): int
     {
         static $useMbString = null;
         if (null === $useMbString) {
@@ -171,7 +171,7 @@ class Platform
      * @param  ?resource $fd Open file descriptor or null to default to STDOUT
      * @return bool
      */
-    public static function isTty($fd = null)
+    public static function isTty($fd = null): bool
     {
         if ($fd === null) {
             $fd = defined('STDOUT') ? STDOUT : fopen('php://stdout', 'w');
@@ -202,7 +202,7 @@ class Platform
     /**
      * @return void
      */
-    public static function workaroundFilesystemIssues()
+    public static function workaroundFilesystemIssues(): void
     {
         if (self::isVirtualBoxGuest()) {
             usleep(200000);
@@ -216,7 +216,7 @@ class Platform
      *
      * @return bool
      */
-    private static function isVirtualBoxGuest()
+    private static function isVirtualBoxGuest(): bool
     {
         if (null === self::$isVirtualBoxGuest) {
             self::$isVirtualBoxGuest = false;
diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index cb9cdd225..fb661bac4 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -100,7 +100,7 @@ class ProcessExecutor
      * @param  mixed   $output
      * @return int
      */
-    private function doExecute($command, $cwd, $tty, &$output = null)
+    private function doExecute($command, $cwd, $tty, &$output = null): int
     {
         $this->outputCommandRun($command, $cwd, false);
 
@@ -153,13 +153,13 @@ class ProcessExecutor
             'cwd' => $cwd,
         );
 
-        $resolver = function ($resolve, $reject) use (&$job) {
+        $resolver = function ($resolve, $reject) use (&$job): void {
             $job['status'] = ProcessExecutor::STATUS_QUEUED;
             $job['resolve'] = $resolve;
             $job['reject'] = $reject;
         };
 
-        $canceler = function () use (&$job) {
+        $canceler = function () use (&$job): void {
             if ($job['status'] === ProcessExecutor::STATUS_QUEUED) {
                 $job['status'] = ProcessExecutor::STATUS_ABORTED;
             }
@@ -190,7 +190,7 @@ class ProcessExecutor
             $this->markJobDone();
 
             return $job['process'];
-        }, function ($e) use (&$job) {
+        }, function ($e) use (&$job): void {
             $job['status'] = ProcessExecutor::STATUS_FAILED;
 
             $this->markJobDone();
@@ -210,7 +210,7 @@ class ProcessExecutor
      * @param  int  $id
      * @return void
      */
-    private function startJob($id)
+    private function startJob($id): void
     {
         $job = &$this->jobs[$id];
         if ($job['status'] !== self::STATUS_QUEUED) {
@@ -269,7 +269,7 @@ class ProcessExecutor
      *
      * @return void
      */
-    public function enableAsync()
+    public function enableAsync(): void
     {
         $this->allowAsync = true;
     }
@@ -280,7 +280,7 @@ class ProcessExecutor
      * @param  ?int $index job id
      * @return int         number of active (queued or started) jobs
      */
-    public function countActiveJobs($index = null)
+    public function countActiveJobs($index = null): int
     {
         // tick
         foreach ($this->jobs as $job) {
@@ -407,7 +407,7 @@ class ProcessExecutor
         }
 
         $commandString = is_string($command) ? $command : implode(' ', array_map(self::class.'::escape', $command));
-        $safeCommand = Preg::replaceCallback('{://(?P<user>[^:/\s]+):(?P<password>[^@\s/]+)@}i', function ($m) {
+        $safeCommand = Preg::replaceCallback('{://(?P<user>[^:/\s]+):(?P<password>[^@\s/]+)@}i', function ($m): string {
             // if the username looks like a long (12char+) hex string, or a modern github token (e.g. ghp_xxx) we obfuscate that
             if (Preg::isMatch('{^([a-f0-9]{12,}|gh[a-z]_[a-zA-Z0-9_]+)$}', $m['user'])) {
                 return '://***:***@';
@@ -437,7 +437,7 @@ class ProcessExecutor
      *
      * @return string
      */
-    private static function escapeArgument($argument)
+    private static function escapeArgument($argument): string
     {
         if ('' === ($argument = (string) $argument)) {
             return escapeshellarg($argument);
diff --git a/src/Composer/Util/RemoteFilesystem.php b/src/Composer/Util/RemoteFilesystem.php
index 28982a8e9..1d6866f7a 100644
--- a/src/Composer/Util/RemoteFilesystem.php
+++ b/src/Composer/Util/RemoteFilesystem.php
@@ -106,7 +106,7 @@ class RemoteFilesystem
      *
      * @return bool true
      */
-    public function copy($originUrl, $fileUrl, $fileName, $progress = true, $options = array())
+    public function copy($originUrl, $fileUrl, $fileName, $progress = true, $options = array()): bool
     {
         return $this->get($originUrl, $fileUrl, $options, $fileName, $progress);
     }
@@ -131,7 +131,7 @@ class RemoteFilesystem
      *
      * @return mixed[] Options
      */
-    public function getOptions()
+    public function getOptions(): array
     {
         return $this->options;
     }
@@ -142,7 +142,7 @@ class RemoteFilesystem
      * @param  mixed[] $options
      * @return void
      */
-    public function setOptions(array $options)
+    public function setOptions(array $options): void
     {
         $this->options = array_replace_recursive($this->options, $options);
     }
@@ -152,7 +152,7 @@ class RemoteFilesystem
      *
      * @return bool
      */
-    public function isTlsDisabled()
+    public function isTlsDisabled(): bool
     {
         return $this->disableTls === true;
     }
@@ -162,7 +162,7 @@ class RemoteFilesystem
      *
      * @return string[]
      */
-    public function getLastHeaders()
+    public function getLastHeaders(): array
     {
         return $this->lastHeaders;
     }
@@ -171,7 +171,7 @@ class RemoteFilesystem
      * @param  string[] $headers array of returned headers like from getLastHeaders()
      * @return int|null
      */
-    public static function findStatusCode(array $headers)
+    public static function findStatusCode(array $headers): ?int
     {
         $value = null;
         foreach ($headers as $header) {
@@ -189,7 +189,7 @@ class RemoteFilesystem
      * @param  string[]    $headers array of returned headers like from getLastHeaders()
      * @return string|null
      */
-    public function findStatusMessage(array $headers)
+    public function findStatusMessage(array $headers): ?string
     {
         $value = null;
         foreach ($headers as $header) {
@@ -290,7 +290,7 @@ class RemoteFilesystem
         $errorMessage = '';
         $errorCode = 0;
         $result = false;
-        set_error_handler(function ($code, $msg) use (&$errorMessage) {
+        set_error_handler(function ($code, $msg) use (&$errorMessage): bool {
             if ($errorMessage) {
                 $errorMessage .= "\n";
             }
@@ -445,7 +445,7 @@ class RemoteFilesystem
             }
 
             $errorMessage = '';
-            set_error_handler(function ($code, $msg) use (&$errorMessage) {
+            set_error_handler(function ($code, $msg) use (&$errorMessage): bool {
                 if ($errorMessage) {
                     $errorMessage .= "\n";
                 }
@@ -554,7 +554,7 @@ class RemoteFilesystem
      *
      * @throws TransportException
      */
-    protected function callbackGet($notificationCode, $severity, $message, $messageCode, $bytesTransferred, $bytesMax)
+    protected function callbackGet($notificationCode, $severity, $message, $messageCode, $bytesTransferred, $bytesMax): void
     {
         switch ($notificationCode) {
             case STREAM_NOTIFY_FAILURE:
@@ -592,7 +592,7 @@ class RemoteFilesystem
      *
      * @return void
      */
-    protected function promptAuthAndRetry($httpStatus, $reason = null, $headers = array())
+    protected function promptAuthAndRetry($httpStatus, $reason = null, $headers = array()): void
     {
         $result = $this->authHelper->promptAuthIfNeeded($this->fileUrl, $this->originUrl, $httpStatus, $reason, $headers);
 
@@ -610,7 +610,7 @@ class RemoteFilesystem
      *
      * @return mixed[]
      */
-    protected function getOptionsForUrl($originUrl, $additionalOptions)
+    protected function getOptionsForUrl($originUrl, $additionalOptions): array
     {
         $tlsOptions = array();
         $headers = array();
@@ -698,7 +698,7 @@ class RemoteFilesystem
      *
      * @return string|null
      */
-    private function decodeResult($result, $http_response_header)
+    private function decodeResult($result, $http_response_header): ?string
     {
         // decode gzip
         if ($result && extension_loaded('zlib')) {
@@ -722,7 +722,7 @@ class RemoteFilesystem
      *
      * @return string|null
      */
-    private function normalizeResult($result)
+    private function normalizeResult($result): ?string
     {
         if ($result === false) {
             return null;
diff --git a/src/Composer/Util/Silencer.php b/src/Composer/Util/Silencer.php
index 361ef41f6..75a97575b 100644
--- a/src/Composer/Util/Silencer.php
+++ b/src/Composer/Util/Silencer.php
@@ -30,7 +30,7 @@ class Silencer
      * @param  int|null $mask Error levels to suppress, default value NULL indicates all warnings and below.
      * @return int      The old error reporting level.
      */
-    public static function suppress($mask = null)
+    public static function suppress($mask = null): int
     {
         if (!isset($mask)) {
             $mask = E_WARNING | E_NOTICE | E_USER_WARNING | E_USER_NOTICE | E_DEPRECATED | E_USER_DEPRECATED | E_STRICT;
@@ -47,7 +47,7 @@ class Silencer
      *
      * @return void
      */
-    public static function restore()
+    public static function restore(): void
     {
         if (!empty(self::$stack)) {
             error_reporting(array_pop(self::$stack));
diff --git a/src/Composer/Util/StreamContextFactory.php b/src/Composer/Util/StreamContextFactory.php
index 852d72946..7448990d3 100644
--- a/src/Composer/Util/StreamContextFactory.php
+++ b/src/Composer/Util/StreamContextFactory.php
@@ -63,7 +63,7 @@ final class StreamContextFactory
      * @phpstan-return array{http: array{header: string[], proxy?: string, request_fulluri: bool}, ssl?: mixed[]}
      * @return array formatted as a stream context array
      */
-    public static function initOptions($url, array $options, $forCurl = false)
+    public static function initOptions($url, array $options, $forCurl = false): array
     {
         // Make sure the headers are in an array form
         if (!isset($options['http']['header'])) {
@@ -134,7 +134,7 @@ final class StreamContextFactory
      *
      * @return mixed[]
      */
-    public static function getTlsDefaults(array $options, LoggerInterface $logger = null)
+    public static function getTlsDefaults(array $options, LoggerInterface $logger = null): array
     {
         $ciphers = implode(':', array(
             'ECDHE-RSA-AES128-GCM-SHA256',
@@ -240,12 +240,12 @@ final class StreamContextFactory
      * @param  string|string[] $header
      * @return string[]
      */
-    private static function fixHttpHeaderField($header)
+    private static function fixHttpHeaderField($header): array
     {
         if (!is_array($header)) {
             $header = explode("\r\n", $header);
         }
-        uasort($header, function ($el) {
+        uasort($header, function ($el): int {
             return stripos($el, 'content-type') === 0 ? 1 : -1;
         });
 
diff --git a/src/Composer/Util/Svn.php b/src/Composer/Util/Svn.php
index 865479e9a..5c47d408c 100644
--- a/src/Composer/Util/Svn.php
+++ b/src/Composer/Util/Svn.php
@@ -86,7 +86,7 @@ class Svn
     /**
      * @return void
      */
-    public static function cleanEnv()
+    public static function cleanEnv(): void
     {
         // clean up env for OSX, see https://github.com/composer/composer/issues/2146#issuecomment-35478940
         Platform::clearEnv('DYLD_LIBRARY_PATH');
@@ -105,7 +105,7 @@ class Svn
      * @throws \RuntimeException
      * @return string
      */
-    public function execute($command, $url, $cwd = null, $path = null, $verbose = false)
+    public function execute($command, $url, $cwd = null, $path = null, $verbose = false): string
     {
         // Ensure we are allowed to use this URL by config
         $this->config->prohibitUrlByConfig($url, $this->io);
@@ -125,7 +125,7 @@ class Svn
      * @throws \RuntimeException
      * @return string
      */
-    public function executeLocal($command, $path, $cwd = null, $verbose = false)
+    public function executeLocal($command, $path, $cwd = null, $verbose = false): string
     {
         // A local command has no remote url
         return $this->executeWithAuthRetry($command, $cwd, '', $path, $verbose);
@@ -140,7 +140,7 @@ class Svn
      *
      * @return ?string
      */
-    private function executeWithAuthRetry($svnCommand, $cwd, $url, $path, $verbose)
+    private function executeWithAuthRetry($svnCommand, $cwd, $url, $path, $verbose): ?string
     {
         // Regenerate the command at each try, to use the newly user-provided credentials
         $command = $this->getCommand($svnCommand, $url, $path);
@@ -194,7 +194,7 @@ class Svn
      * @param  bool $cacheCredentials
      * @return void
      */
-    public function setCacheCredentials($cacheCredentials)
+    public function setCacheCredentials($cacheCredentials): void
     {
         $this->cacheCredentials = $cacheCredentials;
     }
@@ -205,7 +205,7 @@ class Svn
      * @throws \RuntimeException
      * @return \Composer\Util\Svn
      */
-    protected function doAuthDance()
+    protected function doAuthDance(): \Composer\Util\Svn
     {
         // cannot ask for credentials in non interactive mode
         if (!$this->io->isInteractive()) {
@@ -236,7 +236,7 @@ class Svn
      *
      * @return string
      */
-    protected function getCommand($cmd, $url, $path = null)
+    protected function getCommand($cmd, $url, $path = null): string
     {
         $cmd = sprintf(
             '%s %s%s -- %s',
@@ -260,7 +260,7 @@ class Svn
      *
      * @return string
      */
-    protected function getCredentialString()
+    protected function getCredentialString(): string
     {
         if (!$this->hasAuth()) {
             return '';
@@ -280,7 +280,7 @@ class Svn
      * @throws \LogicException
      * @return string
      */
-    protected function getPassword()
+    protected function getPassword(): string
     {
         if ($this->credentials === null) {
             throw new \LogicException("No svn auth detected.");
@@ -295,7 +295,7 @@ class Svn
      * @throws \LogicException
      * @return string
      */
-    protected function getUsername()
+    protected function getUsername(): string
     {
         if ($this->credentials === null) {
             throw new \LogicException("No svn auth detected.");
@@ -309,7 +309,7 @@ class Svn
      *
      * @return bool
      */
-    protected function hasAuth()
+    protected function hasAuth(): bool
     {
         if (null !== $this->hasAuth) {
             return $this->hasAuth;
@@ -327,7 +327,7 @@ class Svn
      *
      * @return string
      */
-    protected function getAuthCache()
+    protected function getAuthCache(): string
     {
         return $this->cacheCredentials ? '' : '--no-auth-cache ';
     }
@@ -337,7 +337,7 @@ class Svn
      *
      * @return bool
      */
-    private function createAuthFromConfig()
+    private function createAuthFromConfig(): bool
     {
         if (!$this->config->has('http-basic')) {
             return $this->hasAuth = false;
@@ -363,7 +363,7 @@ class Svn
      *
      * @return bool
      */
-    private function createAuthFromUrl()
+    private function createAuthFromUrl(): bool
     {
         $uri = parse_url($this->url);
         if (empty($uri['user'])) {
@@ -383,7 +383,7 @@ class Svn
      *
      * @return string|null
      */
-    public function binaryVersion()
+    public function binaryVersion(): ?string
     {
         if (!self::$version) {
             if (0 === $this->process->execute('svn --version', $output)) {
diff --git a/src/Composer/Util/SyncHelper.php b/src/Composer/Util/SyncHelper.php
index 9e8c5d710..f61d02dbe 100644
--- a/src/Composer/Util/SyncHelper.php
+++ b/src/Composer/Util/SyncHelper.php
@@ -31,7 +31,7 @@ class SyncHelper
      *
      * @return void
      */
-    public static function downloadAndInstallPackageSync(Loop $loop, DownloaderInterface $downloader, $path, PackageInterface $package, PackageInterface $prevPackage = null)
+    public static function downloadAndInstallPackageSync(Loop $loop, DownloaderInterface $downloader, $path, PackageInterface $package, PackageInterface $prevPackage = null): void
     {
         $type = $prevPackage ? 'update' : 'install';
 
@@ -61,7 +61,7 @@ class SyncHelper
      *
      * @return void
      */
-    public static function await(Loop $loop, PromiseInterface $promise = null)
+    public static function await(Loop $loop, PromiseInterface $promise = null): void
     {
         if ($promise) {
             $loop->wait(array($promise));
diff --git a/src/Composer/Util/Tar.php b/src/Composer/Util/Tar.php
index cae69c906..9d4f7f112 100644
--- a/src/Composer/Util/Tar.php
+++ b/src/Composer/Util/Tar.php
@@ -22,7 +22,7 @@ class Tar
      *
      * @return string|null
      */
-    public static function getComposerJson($pathToArchive)
+    public static function getComposerJson($pathToArchive): ?string
     {
         $phar = new \PharData($pathToArchive);
 
@@ -40,7 +40,7 @@ class Tar
      *
      * @return string
      */
-    private static function extractComposerJsonFromFolder(\PharData $phar)
+    private static function extractComposerJsonFromFolder(\PharData $phar): string
     {
         if (isset($phar['composer.json'])) {
             return $phar['composer.json']->getContent();
diff --git a/src/Composer/Util/TlsHelper.php b/src/Composer/Util/TlsHelper.php
index 3fe329d4a..697a66b17 100644
--- a/src/Composer/Util/TlsHelper.php
+++ b/src/Composer/Util/TlsHelper.php
@@ -30,7 +30,7 @@ final class TlsHelper
      *
      * @return bool
      */
-    public static function checkCertificateHost($certificate, $hostname, &$cn = null)
+    public static function checkCertificateHost($certificate, $hostname, &$cn = null): bool
     {
         $names = self::getCertificateNames($certificate);
 
@@ -61,7 +61,7 @@ final class TlsHelper
      *
      * @return array{cn: string, san: string[]}|null
      */
-    public static function getCertificateNames($certificate)
+    public static function getCertificateNames($certificate): ?array
     {
         if (is_array($certificate)) {
             $info = $certificate;
@@ -78,7 +78,7 @@ final class TlsHelper
 
         if (isset($info['extensions']['subjectAltName'])) {
             $subjectAltNames = Preg::split('{\s*,\s*}', $info['extensions']['subjectAltName']);
-            $subjectAltNames = array_filter(array_map(function ($name) {
+            $subjectAltNames = array_filter(array_map(function ($name): ?string {
                 if (0 === strpos($name, 'DNS:')) {
                     return strtolower(ltrim(substr($name, 4)));
                 }
@@ -136,7 +136,7 @@ final class TlsHelper
      * @param string $certificate
      * @return string
      */
-    public static function getCertificateFingerprint($certificate)
+    public static function getCertificateFingerprint($certificate): string
     {
         $pubkey = openssl_get_publickey($certificate);
         if ($pubkey === false) {
@@ -161,7 +161,7 @@ final class TlsHelper
      *
      * @return bool
      */
-    public static function isOpensslParseSafe()
+    public static function isOpensslParseSafe(): bool
     {
         return CaBundle::isOpensslParseSafe();
     }
@@ -173,13 +173,13 @@ final class TlsHelper
      *
      * @return callable|null
      */
-    private static function certNameMatcher($certName)
+    private static function certNameMatcher($certName): ?callable
     {
         $wildcards = substr_count($certName, '*');
 
         if (0 === $wildcards) {
             // Literal match.
-            return function ($hostname) use ($certName) {
+            return function ($hostname) use ($certName): bool {
                 return $hostname === $certName;
             };
         }
@@ -203,7 +203,7 @@ final class TlsHelper
             $wildcardRegex = str_replace('\\*', '[a-z0-9-]+', $wildcardRegex);
             $wildcardRegex = "{^{$wildcardRegex}$}";
 
-            return function ($hostname) use ($wildcardRegex) {
+            return function ($hostname) use ($wildcardRegex): bool {
                 return Preg::isMatch($wildcardRegex, $hostname);
             };
         }
diff --git a/src/Composer/Util/Url.php b/src/Composer/Util/Url.php
index c2ee92465..d118054d2 100644
--- a/src/Composer/Util/Url.php
+++ b/src/Composer/Util/Url.php
@@ -26,7 +26,7 @@ class Url
      * @param  string $ref
      * @return string the updated URL
      */
-    public static function updateDistReference(Config $config, $url, $ref)
+    public static function updateDistReference(Config $config, $url, $ref): string
     {
         $host = parse_url($url, PHP_URL_HOST);
 
@@ -64,7 +64,7 @@ class Url
      * @param  string $url
      * @return string
      */
-    public static function getOrigin(Config $config, $url)
+    public static function getOrigin(Config $config, $url): string
     {
         if (0 === strpos($url, 'file://')) {
             return $url;
@@ -108,13 +108,13 @@ class Url
      * @param  string $url
      * @return string
      */
-    public static function sanitize($url)
+    public static function sanitize($url): string
     {
         // GitHub repository rename result in redirect locations containing the access_token as GET parameter
         // e.g. https://api.github.com/repositories/9999999999?access_token=github_token
         $url = Preg::replace('{([&?]access_token=)[^&]+}', '$1***', $url);
 
-        $url = Preg::replaceCallback('{^(?P<prefix>[a-z0-9]+://)?(?P<user>[^:/\s@]+):(?P<password>[^@\s/]+)@}i', function ($m) {
+        $url = Preg::replaceCallback('{^(?P<prefix>[a-z0-9]+://)?(?P<user>[^:/\s@]+):(?P<password>[^@\s/]+)@}i', function ($m): string {
             // if the username looks like a long (12char+) hex string, or a modern github token (e.g. ghp_xxx) we obfuscate that
             if (Preg::isMatch('{^([a-f0-9]{12,}|gh[a-z]_[a-zA-Z0-9_]+)$}', $m['user'])) {
                 return $m['prefix'].'***:***@';
diff --git a/src/Composer/Util/Zip.php b/src/Composer/Util/Zip.php
index 7b1f6285c..3e448048d 100644
--- a/src/Composer/Util/Zip.php
+++ b/src/Composer/Util/Zip.php
@@ -24,7 +24,7 @@ class Zip
      *
      * @return string|null
      */
-    public static function getComposerJson($pathToZip)
+    public static function getComposerJson($pathToZip): ?string
     {
         if (!extension_loaded('zip')) {
             throw new \RuntimeException('The Zip Util requires PHP\'s zip extension');
@@ -65,7 +65,7 @@ class Zip
      *
      * @return int
      */
-    private static function locateFile(\ZipArchive $zip, $filename)
+    private static function locateFile(\ZipArchive $zip, $filename): int
     {
         // return root composer.json if it is there and is a file
         if (false !== ($index = $zip->locateName($filename)) && $zip->getFromIndex($index) !== false) {
diff --git a/tests/Composer/Test/AllFunctionalTest.php b/tests/Composer/Test/AllFunctionalTest.php
index d1f6394e4..12bd95e3d 100644
--- a/tests/Composer/Test/AllFunctionalTest.php
+++ b/tests/Composer/Test/AllFunctionalTest.php
@@ -63,7 +63,7 @@ class AllFunctionalTest extends TestCase
         $fs->removeDirectory(dirname(self::$pharPath));
     }
 
-    public function testBuildPhar()
+    public function testBuildPhar(): void
     {
         if (defined('HHVM_VERSION')) {
             $this->markTestSkipped('Building the phar does not work on HHVM.');
@@ -100,7 +100,7 @@ class AllFunctionalTest extends TestCase
      * @depends testBuildPhar
      * @param string $testFile
      */
-    public function testIntegration($testFile)
+    public function testIntegration($testFile): void
     {
         $testData = $this->parseTestFile($testFile);
         $this->testDir = self::getUniqueTmpDirectory();
@@ -121,7 +121,7 @@ class AllFunctionalTest extends TestCase
         $proc = Process::fromShellCommandline(escapeshellcmd(PHP_BINARY).' '.escapeshellarg(self::$pharPath).' --no-ansi '.$testData['RUN'], $this->testDir, $env, null, 300);
         $output = '';
 
-        $exitCode = $proc->run(function ($type, $buffer) use (&$output) {
+        $exitCode = $proc->run(function ($type, $buffer) use (&$output): void {
             $output .= $buffer;
         });
 
@@ -179,7 +179,7 @@ class AllFunctionalTest extends TestCase
     /**
      * @return array<string, array<string>>
      */
-    public function getTestFiles()
+    public function getTestFiles(): array
     {
         $tests = array();
         foreach (Finder::create()->in(__DIR__.'/Fixtures/functional')->name('*.test')->files() as $file) {
@@ -193,7 +193,7 @@ class AllFunctionalTest extends TestCase
      * @param string $file
      * @return array{RUN: string, EXPECT?: string, EXPECT-EXIT-CODE?: int, EXPECT-REGEX?: string, EXPECT-REGEXES?: string, TEST?: string}
      */
-    private function parseTestFile($file)
+    private function parseTestFile($file): array
     {
         $tokens = Preg::split('#(?:^|\n*)--([A-Z-]+)--\n#', file_get_contents($file), -1, PREG_SPLIT_DELIM_CAPTURE);
         $data = array();
@@ -255,7 +255,7 @@ class AllFunctionalTest extends TestCase
      * @param string $output
      * @return string
      */
-    private function cleanOutput($output)
+    private function cleanOutput($output): string
     {
         $processed = '';
 
diff --git a/tests/Composer/Test/ApplicationTest.php b/tests/Composer/Test/ApplicationTest.php
index ed182360e..30aab4988 100644
--- a/tests/Composer/Test/ApplicationTest.php
+++ b/tests/Composer/Test/ApplicationTest.php
@@ -25,7 +25,7 @@ class ApplicationTest extends TestCase
         putenv('COMPOSER_NO_INTERACTION');
     }
 
-    public function testDevWarning()
+    public function testDevWarning(): void
     {
         $application = new Application;
 
@@ -84,7 +84,7 @@ class ApplicationTest extends TestCase
      * @param  string $command
      * @return void
      */
-    public function ensureNoDevWarning($command)
+    public function ensureNoDevWarning($command): void
     {
         $application = new Application;
 
@@ -133,12 +133,12 @@ class ApplicationTest extends TestCase
         $application->doRun($inputMock, $outputMock);
     }
 
-    public function testDevWarningPrevented()
+    public function testDevWarningPrevented(): void
     {
         $this->ensureNoDevWarning('self-update');
     }
 
-    public function testDevWarningPreventedAlias()
+    public function testDevWarningPreventedAlias(): void
     {
         $this->ensureNoDevWarning('self-up');
     }
diff --git a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
index 96f8d4584..06c12cfb5 100644
--- a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
+++ b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
@@ -100,10 +100,10 @@ class AutoloadGeneratorTest extends TestCase
         $this->config = $this->getMockBuilder('Composer\Config')->getMock();
 
         $this->configValueMap = array(
-            'vendor-dir' => function () {
+            'vendor-dir' => function (): string {
                 return $this->vendorDir;
             },
-            'platform-check' => function () {
+            'platform-check' => function (): bool {
                 return true;
             },
         );
@@ -130,7 +130,7 @@ class AutoloadGeneratorTest extends TestCase
             ->getMock();
         $this->im->expects($this->any())
             ->method('getInstallPath')
-            ->will($this->returnCallback(function ($package) {
+            ->will($this->returnCallback(function ($package): string {
                 $targetDir = $package->getTargetDir();
 
                 return $this->vendorDir.'/'.$package->getName() . ($targetDir ? '/'.$targetDir : '');
@@ -161,7 +161,7 @@ class AutoloadGeneratorTest extends TestCase
         }
     }
 
-    public function testRootPackageAutoloading()
+    public function testRootPackageAutoloading(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -206,7 +206,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('classmap', $this->vendorDir.'/composer', 'classmap');
     }
 
-    public function testRootPackageDevAutoloading()
+    public function testRootPackageDevAutoloading(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -244,7 +244,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('files2', $this->vendorDir.'/composer', 'files');
     }
 
-    public function testRootPackageDevAutoloadingDisabledByDefault()
+    public function testRootPackageDevAutoloadingDisabledByDefault(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -277,7 +277,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertFalse(is_file($this->vendorDir.'/composer/autoload_files.php'));
     }
 
-    public function testVendorDirSameAsWorkingDir()
+    public function testVendorDirSameAsWorkingDir(): void
     {
         $this->vendorDir = $this->workingDir;
 
@@ -308,7 +308,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('classmap3', $this->vendorDir.'/composer', 'classmap');
     }
 
-    public function testRootPackageAutoloadingAlternativeVendorDir()
+    public function testRootPackageAutoloadingAlternativeVendorDir(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -337,7 +337,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('classmap2', $this->vendorDir.'/composer', 'classmap');
     }
 
-    public function testRootPackageAutoloadingWithTargetDir()
+    public function testRootPackageAutoloadingWithTargetDir(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -368,7 +368,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('classmap6', $this->vendorDir.'/composer', 'classmap');
     }
 
-    public function testVendorsAutoloading()
+    public function testVendorsAutoloading(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -397,7 +397,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertFileExists($this->vendorDir.'/composer/autoload_classmap.php', "ClassMap file needs to be generated, even if empty.");
     }
 
-    public function testNonDevAutoloadExclusionWithRecursion()
+    public function testNonDevAutoloadExclusionWithRecursion(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -430,7 +430,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertFileExists($this->vendorDir.'/composer/autoload_classmap.php', "ClassMap file needs to be generated, even if empty.");
     }
 
-    public function testNonDevAutoloadShouldIncludeReplacedPackages()
+    public function testNonDevAutoloadShouldIncludeReplacedPackages(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array('a/a' => new Link('a', 'a/a', new MatchAllConstraint())));
@@ -464,7 +464,7 @@ class AutoloadGeneratorTest extends TestCase
         );
     }
 
-    public function testNonDevAutoloadExclusionWithRecursionReplace()
+    public function testNonDevAutoloadExclusionWithRecursionReplace(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -497,7 +497,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertFileExists($this->vendorDir.'/composer/autoload_classmap.php', "ClassMap file needs to be generated, even if empty.");
     }
 
-    public function testNonDevAutoloadReplacesNestedRequirements()
+    public function testNonDevAutoloadReplacesNestedRequirements(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -549,7 +549,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('classmap9', $this->vendorDir.'/composer', 'classmap');
     }
 
-    public function testPharAutoload()
+    public function testPharAutoload(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -590,7 +590,7 @@ class AutoloadGeneratorTest extends TestCase
         $this->assertAutoloadFiles('phar_static', $this->vendorDir . '/composer', 'static');
     }
 
-    public function testPSRToClassMapIgnoresNonExistingDir()
+    public function testPSRToClassMapIgnoresNonExistingDir(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
 
@@ -613,7 +613,7 @@ class AutoloadGeneratorTest extends TestCase
         );
     }
 
-    public function testPSRToClassMapIgnoresNonPSRClasses()
+    public function testPSRToClassMapIgnoresNonPSRClasses(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
 
@@ -654,7 +654,7 @@ EOF;
         $this->assertStringEqualsFile($this->vendorDir.'/composer/autoload_classmap.php', $expectedClassmap);
     }
 
-    public function testVendorsClassMapAutoloading()
+    public function testVendorsClassMapAutoloading(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -694,7 +694,7 @@ EOF;
         $this->assertAutoloadFiles('classmap4', $this->vendorDir.'/composer', 'classmap');
     }
 
-    public function testVendorsClassMapAutoloadingWithTargetDir()
+    public function testVendorsClassMapAutoloadingWithTargetDir(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -734,7 +734,7 @@ EOF;
         );
     }
 
-    public function testClassMapAutoloadingEmptyDirAndExactFile()
+    public function testClassMapAutoloadingEmptyDirAndExactFile(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -779,7 +779,7 @@ EOF;
         $this->assertStringNotContainsString('$loader->setApcuPrefix(', file_get_contents($this->vendorDir.'/composer/autoload_real.php'));
     }
 
-    public function testClassMapAutoloadingAuthoritativeAndApcu()
+    public function testClassMapAutoloadingAuthoritativeAndApcu(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -828,7 +828,7 @@ EOF;
         $this->assertStringContainsString('$loader->setApcuPrefix(', file_get_contents($this->vendorDir.'/composer/autoload_real.php'));
     }
 
-    public function testClassMapAutoloadingAuthoritativeAndApcuPrefix()
+    public function testClassMapAutoloadingAuthoritativeAndApcuPrefix(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires(array(
@@ -877,7 +877,7 @@ EOF;
         $this->assertStringContainsString('$loader->setApcuPrefix(\'custom\\\'Prefix\');', file_get_contents($this->vendorDir.'/composer/autoload_real.php'));
     }
 
-    public function testFilesAutoloadGeneration()
+    public function testFilesAutoloadGeneration(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array('files' => array('root.php')));
@@ -924,7 +924,7 @@ EOF;
         $this->assertTrue(function_exists('testFilesAutoloadGenerationRoot'));
     }
 
-    public function testFilesAutoloadGenerationRemoveExtraEntitiesFromAutoloadFiles()
+    public function testFilesAutoloadGenerationRemoveExtraEntitiesFromAutoloadFiles(): void
     {
         $autoloadPackage = new RootPackage('root/a', '1.0', '1.0');
         $autoloadPackage->setAutoload(array('files' => array('root.php')));
@@ -995,7 +995,7 @@ EOF;
         $this->assertFileDoesNotExist($this->vendorDir.'/composer/include_paths.php');
     }
 
-    public function testFilesAutoloadOrderByDependencies()
+    public function testFilesAutoloadOrderByDependencies(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array('files' => array('root2.php')));
@@ -1065,7 +1065,7 @@ EOF;
      * - The main package has priority over other packages.
      * - Longer namespaces have priority over shorter namespaces.
      */
-    public function testOverrideVendorsAutoloading()
+    public function testOverrideVendorsAutoloading(): void
     {
         $rootPackage = new RootPackage('root/z', '1.0', '1.0');
         $rootPackage->setAutoload(array(
@@ -1160,7 +1160,7 @@ EOF;
         $this->assertStringEqualsFile($this->vendorDir.'/composer/autoload_classmap.php', $expectedClassmap);
     }
 
-    public function testIncludePathFileGeneration()
+    public function testIncludePathFileGeneration(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $packages = array();
@@ -1197,7 +1197,7 @@ EOF;
         );
     }
 
-    public function testIncludePathsArePrependedInAutoloadFile()
+    public function testIncludePathsArePrependedInAutoloadFile(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $packages = array();
@@ -1228,7 +1228,7 @@ EOF;
         set_include_path($oldIncludePath);
     }
 
-    public function testIncludePathsInRootPackage()
+    public function testIncludePathsInRootPackage(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setIncludePaths(array('/lib', '/src'));
@@ -1257,7 +1257,7 @@ EOF;
         set_include_path($oldIncludePath);
     }
 
-    public function testIncludePathFileWithoutPathsIsSkipped()
+    public function testIncludePathFileWithoutPathsIsSkipped(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $packages = array();
@@ -1276,7 +1276,7 @@ EOF;
         $this->assertFileDoesNotExist($this->vendorDir."/composer/include_paths.php");
     }
 
-    public function testPreAndPostEventsAreDispatchedDuringAutoloadDump()
+    public function testPreAndPostEventsAreDispatchedDuringAutoloadDump(): void
     {
         $this->eventDispatcher
             ->expects($this->exactly(2))
@@ -1297,7 +1297,7 @@ EOF;
         $this->generator->dump($this->config, $this->repository, $package, $this->im, 'composer', true, '_8');
     }
 
-    public function testUseGlobalIncludePath()
+    public function testUseGlobalIncludePath(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -1318,7 +1318,7 @@ EOF;
         $this->assertFileContentEquals(__DIR__.'/Fixtures/autoload_static_include_path.php', $this->vendorDir.'/composer/autoload_static.php');
     }
 
-    public function testVendorDirExcludedFromWorkingDir()
+    public function testVendorDirExcludedFromWorkingDir(): void
     {
         $workingDir = $this->vendorDir.'/working-dir';
         $vendorDir = $workingDir.'/../vendor';
@@ -1354,7 +1354,7 @@ EOF;
             ->getMock();
         $im->expects($this->any())
             ->method('getInstallPath')
-            ->will($this->returnCallback(function ($package) use ($vendorDir) {
+            ->will($this->returnCallback(function ($package) use ($vendorDir): string {
                 $targetDir = $package->getTargetDir();
 
                 return $vendorDir.'/'.$package->getName() . ($targetDir ? '/'.$targetDir : '');
@@ -1432,7 +1432,7 @@ EOF;
         $this->assertStringContainsString("\$baseDir . '/test.php',\n", file_get_contents($vendorDir.'/composer/autoload_files.php'));
     }
 
-    public function testUpLevelRelativePaths()
+    public function testUpLevelRelativePaths(): void
     {
         $workingDir = $this->workingDir.'/working-dir';
         mkdir($workingDir, 0777, true);
@@ -1516,7 +1516,7 @@ EOF;
         $this->assertStringContainsString("\$baseDir . '/../test.php',\n", file_get_contents($this->vendorDir.'/composer/autoload_files.php'));
     }
 
-    public function testEmptyPaths()
+    public function testEmptyPaths(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -1584,7 +1584,7 @@ EOF;
         $this->assertStringEqualsFile($this->vendorDir.'/composer/autoload_classmap.php', $expectedClassmap);
     }
 
-    public function testVendorSubstringPath()
+    public function testVendorSubstringPath(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -1631,7 +1631,7 @@ EOF;
         $this->assertStringEqualsFile($this->vendorDir.'/composer/autoload_psr4.php', $expectedPsr4);
     }
 
-    public function testExcludeFromClassmap()
+    public function testExcludeFromClassmap(): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setAutoload(array(
@@ -1709,7 +1709,7 @@ EOF;
      *
      * @dataProvider platformCheckProvider
      */
-    public function testGeneratesPlatformCheck(array $requires, $expectedFixture, array $provides = array(), array $replaces = array(), $ignorePlatformReqs = false)
+    public function testGeneratesPlatformCheck(array $requires, $expectedFixture, array $provides = array(), array $replaces = array(), $ignorePlatformReqs = false): void
     {
         $package = new RootPackage('root/a', '1.0', '1.0');
         $package->setRequires($requires);
@@ -1741,7 +1741,7 @@ EOF;
     /**
      * @return array<string, mixed[]>
      */
-    public function platformCheckProvider()
+    public function platformCheckProvider(): array
     {
         $versionParser = new VersionParser();
 
@@ -1850,7 +1850,7 @@ EOF;
      *
      * @return void
      */
-    private function assertAutoloadFiles($name, $dir, $type = 'namespaces')
+    private function assertAutoloadFiles($name, $dir, $type = 'namespaces'): void
     {
         $a = __DIR__.'/Fixtures/autoload_'.$name.'.php';
         $b = $dir.'/autoload_'.$type.'.php';
@@ -1864,7 +1864,7 @@ EOF;
      *
      * @return void
      */
-    public static function assertFileContentEquals(string $expected, string $actual, ?string $message = null)
+    public static function assertFileContentEquals(string $expected, string $actual, ?string $message = null): void
     {
         self::assertSame(
             str_replace("\r", '', (string) file_get_contents($expected)),
diff --git a/tests/Composer/Test/Autoload/ClassLoaderTest.php b/tests/Composer/Test/Autoload/ClassLoaderTest.php
index f20c764d2..d11c522a9 100644
--- a/tests/Composer/Test/Autoload/ClassLoaderTest.php
+++ b/tests/Composer/Test/Autoload/ClassLoaderTest.php
@@ -27,7 +27,7 @@ class ClassLoaderTest extends TestCase
      *
      * @param string $class The fully-qualified class name to test, without preceding namespace separator.
      */
-    public function testLoadClass($class)
+    public function testLoadClass($class): void
     {
         $loader = new ClassLoader();
         $loader->add('Namespaced\\', __DIR__ . '/Fixtures');
@@ -42,7 +42,7 @@ class ClassLoaderTest extends TestCase
      *
      * @return array<array<string>> Array of parameter sets to test with.
      */
-    public function getLoadClassTests()
+    public function getLoadClassTests(): array
     {
         return array(
             array('Namespaced\\Foo'),
@@ -54,7 +54,7 @@ class ClassLoaderTest extends TestCase
     /**
      * getPrefixes method should return empty array if ClassLoader does not have any psr-0 configuration
      */
-    public function testGetPrefixesWithNoPSR0Configuration()
+    public function testGetPrefixesWithNoPSR0Configuration(): void
     {
         $loader = new ClassLoader();
         $this->assertEmpty($loader->getPrefixes());
diff --git a/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php b/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php
index 6316ceb26..5315c4b33 100644
--- a/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php
+++ b/tests/Composer/Test/Autoload/ClassMapGeneratorTest.php
@@ -30,7 +30,7 @@ class ClassMapGeneratorTest extends TestCase
      * @param string $directory
      * @param array<string, string> $expected
      */
-    public function testCreateMap($directory, $expected)
+    public function testCreateMap($directory, $expected): void
     {
         $this->assertEqualsNormalized($expected, ClassMapGenerator::createMap($directory));
     }
@@ -38,7 +38,7 @@ class ClassMapGeneratorTest extends TestCase
     /**
      * @return array<array<string|array<string>>>
      */
-    public function getTestCreateMapTests()
+    public function getTestCreateMapTests(): array
     {
         $classmap = array(
             'Foo\\Bar\\A' => realpath(__DIR__) . '/Fixtures/classmap/sameNsMultipleClasses.php',
@@ -120,7 +120,7 @@ class ClassMapGeneratorTest extends TestCase
         return $data;
     }
 
-    public function testCreateMapFinderSupport()
+    public function testCreateMapFinderSupport(): void
     {
         $this->checkIfFinderIsAvailable();
 
@@ -133,7 +133,7 @@ class ClassMapGeneratorTest extends TestCase
         ), ClassMapGenerator::createMap($finder));
     }
 
-    public function testFindClassesThrowsWhenFileDoesNotExist()
+    public function testFindClassesThrowsWhenFileDoesNotExist(): void
     {
         $r = new \ReflectionClass('Composer\\Autoload\\ClassMapGenerator');
         $find = $r->getMethod('findClasses');
@@ -144,7 +144,7 @@ class ClassMapGeneratorTest extends TestCase
         $find->invoke(null, __DIR__ . '/no-file');
     }
 
-    public function testAmbiguousReference()
+    public function testAmbiguousReference(): void
     {
         $this->checkIfFinderIsAvailable();
 
@@ -167,7 +167,7 @@ class ClassMapGeneratorTest extends TestCase
 
         $io->expects($this->once())
             ->method('writeError')
-            ->will($this->returnCallback(function ($text) use (&$msg) {
+            ->will($this->returnCallback(function ($text) use (&$msg): void {
                 $msg = $text;
             }));
 
@@ -188,7 +188,7 @@ class ClassMapGeneratorTest extends TestCase
      * If one file has a class or interface defined more than once,
      * an ambiguous reference warning should not be produced
      */
-    public function testUnambiguousReference()
+    public function testUnambiguousReference(): void
     {
         $tempDir = $this->getUniqueTmpDirectory();
 
@@ -224,14 +224,14 @@ class ClassMapGeneratorTest extends TestCase
         $fs->removeDirectory($tempDir);
     }
 
-    public function testCreateMapThrowsWhenDirectoryDoesNotExist()
+    public function testCreateMapThrowsWhenDirectoryDoesNotExist(): void
     {
         self::expectException('RuntimeException');
         self::expectExceptionMessage('Could not scan for classes inside');
         ClassMapGenerator::createMap(__DIR__ . '/no-file.no-foler');
     }
 
-    public function testDump()
+    public function testDump(): void
     {
         $tempDir = self::getUniqueTmpDirectory();
 
@@ -249,7 +249,7 @@ class ClassMapGeneratorTest extends TestCase
         $fs->removeDirectory($tempDir);
     }
 
-    public function testCreateMapDoesNotHitRegexBacktraceLimit()
+    public function testCreateMapDoesNotHitRegexBacktraceLimit(): void
     {
         $expected = array(
             'Foo\\StripNoise' => realpath(__DIR__) . '/Fixtures/pcrebacktracelimit/StripNoise.php',
@@ -274,7 +274,7 @@ class ClassMapGeneratorTest extends TestCase
      * @param string $message
      * @return  void
      */
-    protected function assertEqualsNormalized($expected, $actual, $message = '')
+    protected function assertEqualsNormalized($expected, $actual, $message = ''): void
     {
         foreach ($expected as $ns => $path) {
             $expected[$ns] = strtr($path, '\\', '/');
@@ -286,7 +286,7 @@ class ClassMapGeneratorTest extends TestCase
     }
 
     /** @return void */
-    private function checkIfFinderIsAvailable()
+    private function checkIfFinderIsAvailable(): void
     {
         if (!class_exists('Symfony\\Component\\Finder\\Finder')) {
             $this->markTestSkipped('Finder component is not available');
diff --git a/tests/Composer/Test/CacheTest.php b/tests/Composer/Test/CacheTest.php
index 4bc0c3994..6b83ac961 100644
--- a/tests/Composer/Test/CacheTest.php
+++ b/tests/Composer/Test/CacheTest.php
@@ -62,7 +62,7 @@ class CacheTest extends TestCase
         }
     }
 
-    public function testRemoveOutdatedFiles()
+    public function testRemoveOutdatedFiles(): void
     {
         $outdated = array_slice($this->files, 1);
         $this->finder
@@ -82,7 +82,7 @@ class CacheTest extends TestCase
         $this->assertFileExists("{$this->root}/cached.file0.zip");
     }
 
-    public function testRemoveFilesWhenCacheIsTooLarge()
+    public function testRemoveFilesWhenCacheIsTooLarge(): void
     {
         $emptyFinder = $this->getMockBuilder('Symfony\Component\Finder\Finder')->disableOriginalConstructor()->getMock();
         $emptyFinder
@@ -111,7 +111,7 @@ class CacheTest extends TestCase
         $this->assertFileExists("{$this->root}/cached.file3.zip");
     }
 
-    public function testClearCache()
+    public function testClearCache(): void
     {
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $cache = new Cache($io, $this->root, 'a-z0-9.', $this->filesystem);
diff --git a/tests/Composer/Test/Command/ArchiveCommandTest.php b/tests/Composer/Test/Command/ArchiveCommandTest.php
index fda64ba7a..88cc15fe1 100644
--- a/tests/Composer/Test/Command/ArchiveCommandTest.php
+++ b/tests/Composer/Test/Command/ArchiveCommandTest.php
@@ -20,7 +20,7 @@ use Symfony\Component\Console\Input\ArrayInput;
 
 class ArchiveCommandTest extends TestCase
 {
-    public function testUsesConfigFromComposerObject()
+    public function testUsesConfigFromComposerObject(): void
     {
         $input = new ArrayInput(array());
 
@@ -64,7 +64,7 @@ class ArchiveCommandTest extends TestCase
         $command->run($input, $output);
     }
 
-    public function testUsesConfigFromFactoryWhenComposerIsNotDefined()
+    public function testUsesConfigFromFactoryWhenComposerIsNotDefined(): void
     {
         $input = new ArrayInput(array());
 
diff --git a/tests/Composer/Test/Command/InitCommandTest.php b/tests/Composer/Test/Command/InitCommandTest.php
index 22950158e..69af6cfbc 100644
--- a/tests/Composer/Test/Command/InitCommandTest.php
+++ b/tests/Composer/Test/Command/InitCommandTest.php
@@ -17,7 +17,7 @@ use Composer\Test\TestCase;
 
 class InitCommandTest extends TestCase
 {
-    public function testParseValidAuthorString()
+    public function testParseValidAuthorString(): void
     {
         $command = new InitCommand;
         $author = $command->parseAuthorString('John Smith <john@example.com>');
@@ -25,7 +25,7 @@ class InitCommandTest extends TestCase
         $this->assertEquals('john@example.com', $author['email']);
     }
 
-    public function testParseValidAuthorStringWithoutEmail()
+    public function testParseValidAuthorStringWithoutEmail(): void
     {
         $command = new InitCommand;
         $author = $command->parseAuthorString('John Smith');
@@ -33,7 +33,7 @@ class InitCommandTest extends TestCase
         $this->assertNull($author['email']);
     }
 
-    public function testParseValidUtf8AuthorString()
+    public function testParseValidUtf8AuthorString(): void
     {
         $command = new InitCommand;
         $author = $command->parseAuthorString('Matti Meikäläinen <matti@example.com>');
@@ -41,7 +41,7 @@ class InitCommandTest extends TestCase
         $this->assertEquals('matti@example.com', $author['email']);
     }
 
-    public function testParseValidUtf8AuthorStringWithNonSpacingMarks()
+    public function testParseValidUtf8AuthorStringWithNonSpacingMarks(): void
     {
         // \xCC\x88 is UTF-8 for U+0308 diaeresis (umlaut) combining mark
         $utf8_expected = "Matti Meika\xCC\x88la\xCC\x88inen";
@@ -51,7 +51,7 @@ class InitCommandTest extends TestCase
         $this->assertEquals('matti@example.com', $author['email']);
     }
 
-    public function testParseNumericAuthorString()
+    public function testParseNumericAuthorString(): void
     {
         $command = new InitCommand;
         $author = $command->parseAuthorString('h4x0r <h4x@example.com>');
@@ -63,7 +63,7 @@ class InitCommandTest extends TestCase
      * Test scenario for issue #5631
      * @link https://github.com/composer/composer/issues/5631 Issue #5631
      */
-    public function testParseValidAlias1AuthorString()
+    public function testParseValidAlias1AuthorString(): void
     {
         $command = new InitCommand;
         $author = $command->parseAuthorString(
@@ -77,7 +77,7 @@ class InitCommandTest extends TestCase
      * Test scenario for issue #5631
      * @link https://github.com/composer/composer/issues/5631 Issue #5631
      */
-    public function testParseValidAlias2AuthorString()
+    public function testParseValidAlias2AuthorString(): void
     {
         $command = new InitCommand;
         $author = $command->parseAuthorString(
@@ -87,35 +87,35 @@ class InitCommandTest extends TestCase
         $this->assertEquals('john@example.com', $author['email']);
     }
 
-    public function testParseEmptyAuthorString()
+    public function testParseEmptyAuthorString(): void
     {
         $command = new InitCommand;
         self::expectException('InvalidArgumentException');
         $command->parseAuthorString('');
     }
 
-    public function testParseAuthorStringWithInvalidEmail()
+    public function testParseAuthorStringWithInvalidEmail(): void
     {
         $command = new InitCommand;
         self::expectException('InvalidArgumentException');
         $command->parseAuthorString('John Smith <john>');
     }
 
-    public function testNamespaceFromValidPackageName()
+    public function testNamespaceFromValidPackageName(): void
     {
         $command = new InitCommand;
         $namespace = $command->namespaceFromPackageName('new_projects.acme-extra/package-name');
         $this->assertEquals('NewProjectsAcmeExtra\PackageName', $namespace);
     }
 
-    public function testNamespaceFromInvalidPackageName()
+    public function testNamespaceFromInvalidPackageName(): void
     {
         $command = new InitCommand;
         $namespace = $command->namespaceFromPackageName('invalid-package-name');
         $this->assertNull($namespace);
     }
 
-    public function testNamespaceFromMissingPackageName()
+    public function testNamespaceFromMissingPackageName(): void
     {
         $command = new InitCommand;
         $namespace = $command->namespaceFromPackageName('');
diff --git a/tests/Composer/Test/Command/RunScriptCommandTest.php b/tests/Composer/Test/Command/RunScriptCommandTest.php
index eaffdbd92..7a9d7aef8 100644
--- a/tests/Composer/Test/Command/RunScriptCommandTest.php
+++ b/tests/Composer/Test/Command/RunScriptCommandTest.php
@@ -24,7 +24,7 @@ class RunScriptCommandTest extends TestCase
      * @param bool $dev
      * @param bool $noDev
      */
-    public function testDetectAndPassDevModeToEventAndToDispatching($dev, $noDev)
+    public function testDetectAndPassDevModeToEventAndToDispatching($dev, $noDev): void
     {
         $scriptName = 'testScript';
 
@@ -61,7 +61,7 @@ class RunScriptCommandTest extends TestCase
 
         $ed->expects($this->once())
             ->method('hasEventListeners')
-            ->with($this->callback(function (ScriptEvent $event) use ($scriptName, $expectedDevMode) {
+            ->with($this->callback(function (ScriptEvent $event) use ($scriptName, $expectedDevMode): bool {
                 return $event->getName() === $scriptName
                 && $event->isDevMode() === $expectedDevMode;
             }))
@@ -89,7 +89,7 @@ class RunScriptCommandTest extends TestCase
     }
 
     /** @return bool[][] **/
-    public function getDevOptions()
+    public function getDevOptions(): array
     {
         return array(
             array(true, true),
@@ -100,7 +100,7 @@ class RunScriptCommandTest extends TestCase
     }
 
     /** @return Composer **/
-    private function createComposerInstance()
+    private function createComposerInstance(): Composer
     {
         $composer = new Composer;
         $config = new Config;
diff --git a/tests/Composer/Test/ComposerTest.php b/tests/Composer/Test/ComposerTest.php
index 1ba78abea..216245a49 100644
--- a/tests/Composer/Test/ComposerTest.php
+++ b/tests/Composer/Test/ComposerTest.php
@@ -16,7 +16,7 @@ use Composer\Composer;
 
 class ComposerTest extends TestCase
 {
-    public function testSetGetPackage()
+    public function testSetGetPackage(): void
     {
         $composer = new Composer();
         $package = $this->getMockBuilder('Composer\Package\RootPackageInterface')->getMock();
@@ -25,7 +25,7 @@ class ComposerTest extends TestCase
         $this->assertSame($package, $composer->getPackage());
     }
 
-    public function testSetGetLocker()
+    public function testSetGetLocker(): void
     {
         $composer = new Composer();
         $locker = $this->getMockBuilder('Composer\Package\Locker')->disableOriginalConstructor()->getMock();
@@ -34,7 +34,7 @@ class ComposerTest extends TestCase
         $this->assertSame($locker, $composer->getLocker());
     }
 
-    public function testSetGetRepositoryManager()
+    public function testSetGetRepositoryManager(): void
     {
         $composer = new Composer();
         $manager = $this->getMockBuilder('Composer\Repository\RepositoryManager')->disableOriginalConstructor()->getMock();
@@ -43,7 +43,7 @@ class ComposerTest extends TestCase
         $this->assertSame($manager, $composer->getRepositoryManager());
     }
 
-    public function testSetGetDownloadManager()
+    public function testSetGetDownloadManager(): void
     {
         $composer = new Composer();
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
@@ -53,7 +53,7 @@ class ComposerTest extends TestCase
         $this->assertSame($manager, $composer->getDownloadManager());
     }
 
-    public function testSetGetInstallationManager()
+    public function testSetGetInstallationManager(): void
     {
         $composer = new Composer();
         $manager = $this->getMockBuilder('Composer\Installer\InstallationManager')->disableOriginalConstructor()->getMock();
diff --git a/tests/Composer/Test/Config/JsonConfigSourceTest.php b/tests/Composer/Test/Config/JsonConfigSourceTest.php
index 3d9e68980..fd0e1f3e4 100644
--- a/tests/Composer/Test/Config/JsonConfigSourceTest.php
+++ b/tests/Composer/Test/Config/JsonConfigSourceTest.php
@@ -29,7 +29,7 @@ class JsonConfigSourceTest extends TestCase
      *
      * @return string
      */
-    protected function fixturePath($name)
+    protected function fixturePath($name): string
     {
         return __DIR__.'/Fixtures/'.$name;
     }
@@ -48,7 +48,7 @@ class JsonConfigSourceTest extends TestCase
         }
     }
 
-    public function testAddRepository()
+    public function testAddRepository(): void
     {
         $config = $this->workingDir.'/composer.json';
         copy($this->fixturePath('composer-repositories.json'), $config);
@@ -58,7 +58,7 @@ class JsonConfigSourceTest extends TestCase
         $this->assertFileEquals($this->fixturePath('config/config-with-exampletld-repository.json'), $config);
     }
 
-    public function testAddRepositoryWithOptions()
+    public function testAddRepositoryWithOptions(): void
     {
         $config = $this->workingDir.'/composer.json';
         copy($this->fixturePath('composer-repositories.json'), $config);
@@ -76,7 +76,7 @@ class JsonConfigSourceTest extends TestCase
         $this->assertFileEquals($this->fixturePath('config/config-with-exampletld-repository-and-options.json'), $config);
     }
 
-    public function testRemoveRepository()
+    public function testRemoveRepository(): void
     {
         $config = $this->workingDir.'/composer.json';
         copy($this->fixturePath('config/config-with-exampletld-repository.json'), $config);
@@ -86,7 +86,7 @@ class JsonConfigSourceTest extends TestCase
         $this->assertFileEquals($this->fixturePath('composer-repositories.json'), $config);
     }
 
-    public function testAddPackagistRepositoryWithFalseValue()
+    public function testAddPackagistRepositoryWithFalseValue(): void
     {
         $config = $this->workingDir.'/composer.json';
         copy($this->fixturePath('composer-repositories.json'), $config);
@@ -96,7 +96,7 @@ class JsonConfigSourceTest extends TestCase
         $this->assertFileEquals($this->fixturePath('config/config-with-packagist-false.json'), $config);
     }
 
-    public function testRemovePackagist()
+    public function testRemovePackagist(): void
     {
         $config = $this->workingDir.'/composer.json';
         copy($this->fixturePath('config/config-with-packagist-false.json'), $config);
@@ -117,7 +117,7 @@ class JsonConfigSourceTest extends TestCase
      *
      * @dataProvider provideAddLinkData
      */
-    public function testAddLink($sourceFile, $type, $name, $value, $compareAgainst)
+    public function testAddLink($sourceFile, $type, $name, $value, $compareAgainst): void
     {
         $composerJson = $this->workingDir.'/composer.json';
         copy($sourceFile, $composerJson);
@@ -138,7 +138,7 @@ class JsonConfigSourceTest extends TestCase
      *
      * @dataProvider provideRemoveLinkData
      */
-    public function testRemoveLink($sourceFile, $type, $name, $compareAgainst)
+    public function testRemoveLink($sourceFile, $type, $name, $compareAgainst): void
     {
         $composerJson = $this->workingDir.'/composer.json';
         copy($sourceFile, $composerJson);
@@ -160,7 +160,7 @@ class JsonConfigSourceTest extends TestCase
      *
      * @phpstan-return array{string, string, string, string, string}
      */
-    protected function addLinkDataArguments($type, $name, $value, $fixtureBasename, $before)
+    protected function addLinkDataArguments($type, $name, $value, $fixtureBasename, $before): array
     {
         return array(
             $before,
@@ -174,7 +174,7 @@ class JsonConfigSourceTest extends TestCase
     /**
      * Provide data for testAddLink
      */
-    public function provideAddLinkData()
+    public function provideAddLinkData(): array
     {
         $empty = $this->fixturePath('composer-empty.json');
         $oneOfEverything = $this->fixturePath('composer-one-of-everything.json');
@@ -217,7 +217,7 @@ class JsonConfigSourceTest extends TestCase
      *
      * @phpstan-return array{string, string, string, string}
      */
-    protected function removeLinkDataArguments($type, $name, $fixtureBasename, $after = null)
+    protected function removeLinkDataArguments($type, $name, $fixtureBasename, $after = null): array
     {
         return array(
             $this->fixturePath('removeLink/'.$fixtureBasename.'.json'),
@@ -230,7 +230,7 @@ class JsonConfigSourceTest extends TestCase
     /**
      * Provide data for testRemoveLink
      */
-    public function provideRemoveLinkData()
+    public function provideRemoveLinkData(): array
     {
         $oneOfEverything = $this->fixturePath('composer-one-of-everything.json');
         $twoOfEverything = $this->fixturePath('composer-two-of-everything.json');
diff --git a/tests/Composer/Test/ConfigTest.php b/tests/Composer/Test/ConfigTest.php
index fc829c159..17f4384c2 100644
--- a/tests/Composer/Test/ConfigTest.php
+++ b/tests/Composer/Test/ConfigTest.php
@@ -23,7 +23,7 @@ class ConfigTest extends TestCase
      * @param mixed[] $localConfig
      * @param ?mixed[] $systemConfig
      */
-    public function testAddPackagistRepository($expected, $localConfig, $systemConfig = null)
+    public function testAddPackagistRepository($expected, $localConfig, $systemConfig = null): void
     {
         $config = new Config(false);
         if ($systemConfig) {
@@ -34,7 +34,7 @@ class ConfigTest extends TestCase
         $this->assertEquals($expected, $config->getRepositories());
     }
 
-    public function dataAddPackagistRepository()
+    public function dataAddPackagistRepository(): array
     {
         $data = array();
         $data['local config inherits system defaults'] = array(
@@ -141,7 +141,7 @@ class ConfigTest extends TestCase
         return $data;
     }
 
-    public function testPreferredInstallAsString()
+    public function testPreferredInstallAsString(): void
     {
         $config = new Config(false);
         $config->merge(array('config' => array('preferred-install' => 'source')));
@@ -150,7 +150,7 @@ class ConfigTest extends TestCase
         $this->assertEquals('dist', $config->get('preferred-install'));
     }
 
-    public function testMergePreferredInstall()
+    public function testMergePreferredInstall(): void
     {
         $config = new Config(false);
         $config->merge(array('config' => array('preferred-install' => 'dist')));
@@ -162,7 +162,7 @@ class ConfigTest extends TestCase
         $this->assertEquals(array('foo/*' => 'source', '*' => 'dist'), $config->get('preferred-install'));
     }
 
-    public function testMergeGithubOauth()
+    public function testMergeGithubOauth(): void
     {
         $config = new Config(false);
         $config->merge(array('config' => array('github-oauth' => array('foo' => 'bar'))));
@@ -171,7 +171,7 @@ class ConfigTest extends TestCase
         $this->assertEquals(array('foo' => 'bar', 'bar' => 'baz'), $config->get('github-oauth'));
     }
 
-    public function testVarReplacement()
+    public function testVarReplacement(): void
     {
         $config = new Config(false);
         $config->merge(array('config' => array('a' => 'b', 'c' => '{$a}')));
@@ -183,7 +183,7 @@ class ConfigTest extends TestCase
         $this->assertEquals($home.'/foo', $config->get('cache-dir'));
     }
 
-    public function testRealpathReplacement()
+    public function testRealpathReplacement(): void
     {
         $config = new Config(false, '/foo/bar');
         $config->merge(array('config' => array(
@@ -198,7 +198,7 @@ class ConfigTest extends TestCase
         $this->assertEquals('/baz', $config->get('cache-dir'));
     }
 
-    public function testStreamWrapperDirs()
+    public function testStreamWrapperDirs(): void
     {
         $config = new Config(false, '/foo/bar');
         $config->merge(array('config' => array(
@@ -208,7 +208,7 @@ class ConfigTest extends TestCase
         $this->assertEquals('s3://baz', $config->get('cache-dir'));
     }
 
-    public function testFetchingRelativePaths()
+    public function testFetchingRelativePaths(): void
     {
         $config = new Config(false, '/foo/bar');
         $config->merge(array('config' => array(
@@ -222,7 +222,7 @@ class ConfigTest extends TestCase
         $this->assertEquals('vendor/foo', $config->get('bin-dir', Config::RELATIVE_PATHS));
     }
 
-    public function testOverrideGithubProtocols()
+    public function testOverrideGithubProtocols(): void
     {
         $config = new Config(false);
         $config->merge(array('config' => array('github-protocols' => array('https', 'ssh'))));
@@ -231,7 +231,7 @@ class ConfigTest extends TestCase
         $this->assertEquals(array('https'), $config->get('github-protocols'));
     }
 
-    public function testGitDisabledByDefaultInGithubProtocols()
+    public function testGitDisabledByDefaultInGithubProtocols(): void
     {
         $config = new Config(false);
         $config->merge(array('config' => array('github-protocols' => array('https', 'git'))));
@@ -247,7 +247,7 @@ class ConfigTest extends TestCase
      *
      * @param string $url
      */
-    public function testAllowedUrlsPass($url)
+    public function testAllowedUrlsPass($url): void
     {
         $config = new Config(false);
         $config->prohibitUrlByConfig($url);
@@ -258,7 +258,7 @@ class ConfigTest extends TestCase
      *
      * @param string $url
      */
-    public function testProhibitedUrlsThrowException($url)
+    public function testProhibitedUrlsThrowException($url): void
     {
         self::expectException('Composer\Downloader\TransportException');
         self::expectExceptionMessage('Your configuration does not allow connections to ' . $url);
@@ -269,7 +269,7 @@ class ConfigTest extends TestCase
     /**
      * @return string[][] List of test URLs that should pass strict security
      */
-    public function allowedUrlProvider()
+    public function allowedUrlProvider(): array
     {
         $urls = array(
             'https://packagist.org',
@@ -282,7 +282,7 @@ class ConfigTest extends TestCase
             'ssh://[user@]host.xz[:port]/path/to/repo.git/',
         );
 
-        return array_combine($urls, array_map(function ($e) {
+        return array_combine($urls, array_map(function ($e): array {
             return array($e);
         }, $urls));
     }
@@ -290,7 +290,7 @@ class ConfigTest extends TestCase
     /**
      * @return string[][] List of test URLs that should not pass strict security
      */
-    public function prohibitedUrlProvider()
+    public function prohibitedUrlProvider(): array
     {
         $urls = array(
             'http://packagist.org',
@@ -303,7 +303,7 @@ class ConfigTest extends TestCase
             'git://5.6.7.8/git.git',
         );
 
-        return array_combine($urls, array_map(function ($e) {
+        return array_combine($urls, array_map(function ($e): array {
             return array($e);
         }, $urls));
     }
@@ -311,7 +311,7 @@ class ConfigTest extends TestCase
     /**
      * @group TLS
      */
-    public function testDisableTlsCanBeOverridden()
+    public function testDisableTlsCanBeOverridden(): void
     {
         $config = new Config;
         $config->merge(
@@ -324,7 +324,7 @@ class ConfigTest extends TestCase
         $this->assertTrue($config->get('disable-tls'));
     }
 
-    public function testProcessTimeout()
+    public function testProcessTimeout(): void
     {
         Platform::putEnv('COMPOSER_PROCESS_TIMEOUT', '0');
         $config = new Config(true);
@@ -334,7 +334,7 @@ class ConfigTest extends TestCase
         $this->assertEquals(0, $result);
     }
 
-    public function testHtaccessProtect()
+    public function testHtaccessProtect(): void
     {
         Platform::putEnv('COMPOSER_HTACCESS_PROTECT', '0');
         $config = new Config(true);
@@ -344,7 +344,7 @@ class ConfigTest extends TestCase
         $this->assertEquals(0, $result);
     }
 
-    public function testGetSourceOfValue()
+    public function testGetSourceOfValue(): void
     {
         Platform::clearEnv('COMPOSER_PROCESS_TIMEOUT');
 
@@ -360,7 +360,7 @@ class ConfigTest extends TestCase
         $this->assertSame('phpunit-test', $config->getSourceOfValue('process-timeout'));
     }
 
-    public function testGetSourceOfValueEnvVariables()
+    public function testGetSourceOfValueEnvVariables(): void
     {
         Platform::putEnv('COMPOSER_HTACCESS_PROTECT', '0');
         $config = new Config;
diff --git a/tests/Composer/Test/Console/HtmlOutputFormatterTest.php b/tests/Composer/Test/Console/HtmlOutputFormatterTest.php
index a105eec0d..60865f726 100644
--- a/tests/Composer/Test/Console/HtmlOutputFormatterTest.php
+++ b/tests/Composer/Test/Console/HtmlOutputFormatterTest.php
@@ -18,7 +18,7 @@ use Symfony\Component\Console\Formatter\OutputFormatterStyle;
 
 class HtmlOutputFormatterTest extends TestCase
 {
-    public function testFormatting()
+    public function testFormatting(): void
     {
         $formatter = new HtmlOutputFormatter(array(
             'warning' => new OutputFormatterStyle('black', 'yellow'),
diff --git a/tests/Composer/Test/DefaultConfigTest.php b/tests/Composer/Test/DefaultConfigTest.php
index 2aeeb783e..0b5f694ea 100644
--- a/tests/Composer/Test/DefaultConfigTest.php
+++ b/tests/Composer/Test/DefaultConfigTest.php
@@ -19,7 +19,7 @@ class DefaultConfigTest extends TestCase
     /**
      * @group TLS
      */
-    public function testDefaultValuesAreAsExpected()
+    public function testDefaultValuesAreAsExpected(): void
     {
         $config = new Config;
         $this->assertFalse($config->get('disable-tls'));
diff --git a/tests/Composer/Test/DependencyResolver/DefaultPolicyTest.php b/tests/Composer/Test/DependencyResolver/DefaultPolicyTest.php
index 9703706d5..27569baee 100644
--- a/tests/Composer/Test/DependencyResolver/DefaultPolicyTest.php
+++ b/tests/Composer/Test/DependencyResolver/DefaultPolicyTest.php
@@ -41,7 +41,7 @@ class DefaultPolicyTest extends TestCase
         $this->policy = new DefaultPolicy;
     }
 
-    public function testSelectSingle()
+    public function testSelectSingle(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repositorySet->addRepository($this->repo);
@@ -56,7 +56,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectNewest()
+    public function testSelectNewest(): void
     {
         $this->repo->addPackage($packageA1 = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageA2 = $this->getPackage('A', '2.0'));
@@ -72,7 +72,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectNewestPicksLatest()
+    public function testSelectNewestPicksLatest(): void
     {
         $this->repo->addPackage($packageA1 = $this->getPackage('A', '1.0.0'));
         $this->repo->addPackage($packageA2 = $this->getPackage('A', '1.0.1-alpha'));
@@ -88,7 +88,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectNewestPicksLatestStableWithPreferStable()
+    public function testSelectNewestPicksLatestStableWithPreferStable(): void
     {
         $this->repo->addPackage($packageA1 = $this->getPackage('A', '1.0.0'));
         $this->repo->addPackage($packageA2 = $this->getPackage('A', '1.0.1-alpha'));
@@ -105,7 +105,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectNewestWithDevPicksNonDev()
+    public function testSelectNewestWithDevPicksNonDev(): void
     {
         $this->repo->addPackage($packageA1 = $this->getPackage('A', 'dev-foo'));
         $this->repo->addPackage($packageA2 = $this->getPackage('A', '1.0.0'));
@@ -121,7 +121,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testRepositoryOrderingAffectsPriority()
+    public function testRepositoryOrderingAffectsPriority(): void
     {
         $repo1 = new ArrayRepository;
         $repo2 = new ArrayRepository;
@@ -154,7 +154,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectLocalReposFirst()
+    public function testSelectLocalReposFirst(): void
     {
         $repoImportant = new ArrayRepository;
 
@@ -185,7 +185,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectAllProviders()
+    public function testSelectAllProviders(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '2.0'));
@@ -205,7 +205,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testPreferNonReplacingFromSameRepo()
+    public function testPreferNonReplacingFromSameRepo(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '2.0'));
@@ -224,7 +224,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testPreferReplacingPackageFromSameVendor()
+    public function testPreferReplacingPackageFromSameVendor(): void
     {
         // test with default order
         $this->repo->addPackage($packageB = $this->getPackage('vendor-b/replacer', '1.0'));
@@ -260,7 +260,7 @@ class DefaultPolicyTest extends TestCase
         $this->assertSame($expected, $selected);
     }
 
-    public function testSelectLowest()
+    public function testSelectLowest(): void
     {
         $policy = new DefaultPolicy(false, true);
 
diff --git a/tests/Composer/Test/DependencyResolver/PoolBuilderTest.php b/tests/Composer/Test/DependencyResolver/PoolBuilderTest.php
index a3a911e46..92b9840a8 100644
--- a/tests/Composer/Test/DependencyResolver/PoolBuilderTest.php
+++ b/tests/Composer/Test/DependencyResolver/PoolBuilderTest.php
@@ -44,13 +44,13 @@ class PoolBuilderTest extends TestCase
      * @param mixed[] $packageRepos
      * @param mixed[] $fixed
      */
-    public function testPoolBuilder($file, $message, $expect, $expectOptimized, $root, $requestData, $packageRepos, $fixed)
+    public function testPoolBuilder($file, $message, $expect, $expectOptimized, $root, $requestData, $packageRepos, $fixed): void
     {
         $rootAliases = !empty($root['aliases']) ? $root['aliases'] : array();
         $minimumStability = !empty($root['minimum-stability']) ? $root['minimum-stability'] : 'stable';
         $stabilityFlags = !empty($root['stability-flags']) ? $root['stability-flags'] : array();
         $rootReferences = !empty($root['references']) ? $root['references'] : array();
-        $stabilityFlags = array_map(function ($stability) {
+        $stabilityFlags = array_map(function ($stability): int {
             return BasePackage::$stabilities[$stability];
         }, $stabilityFlags);
 
@@ -62,7 +62,7 @@ class PoolBuilderTest extends TestCase
 
         $loader = new ArrayLoader(null, true);
         $packageIds = array();
-        $loadPackage = function ($data) use ($loader, &$packageIds) {
+        $loadPackage = function ($data) use ($loader, &$packageIds): \Composer\Package\PackageInterface {
             /** @var ?int $id */
             $id = null;
             if (!empty($data['id'])) {
@@ -149,7 +149,7 @@ class PoolBuilderTest extends TestCase
      * @param array<int, BasePackage> $packageIds
      * @return string[]
      */
-    private function getPackageResultSet(Pool $pool, $packageIds)
+    private function getPackageResultSet(Pool $pool, $packageIds): array
     {
         $result = array();
         for ($i = 1, $count = count($pool); $i <= $count; $i++) {
@@ -184,7 +184,7 @@ class PoolBuilderTest extends TestCase
     /**
      * @return array<string, array<string>>
      */
-    public function getIntegrationTests()
+    public function getIntegrationTests(): array
     {
         $fixturesDir = realpath(__DIR__.'/Fixtures/poolbuilder/');
         $tests = array();
@@ -223,7 +223,7 @@ class PoolBuilderTest extends TestCase
      * @param string $fixturesDir
      * @return array<string, string>
      */
-    protected function readTestFile(\SplFileInfo $file, $fixturesDir)
+    protected function readTestFile(\SplFileInfo $file, $fixturesDir): array
     {
         $tokens = Preg::split('#(?:^|\n*)--([A-Z-]+)--\n#', file_get_contents($file->getRealPath()), -1, PREG_SPLIT_DELIM_CAPTURE);
 
diff --git a/tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php b/tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php
index 442b156ef..1f78a4013 100644
--- a/tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php
+++ b/tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php
@@ -34,7 +34,7 @@ class PoolOptimizerTest extends TestCase
      * @param BasePackage[] $expectedPackages
      * @param string $message
      */
-    public function testPoolOptimizer(array $requestData, array $packagesBefore, array $expectedPackages, $message)
+    public function testPoolOptimizer(array $requestData, array $packagesBefore, array $expectedPackages, $message): void
     {
         $lockedRepo = new LockArrayRepository();
 
@@ -71,7 +71,7 @@ class PoolOptimizerTest extends TestCase
         );
     }
 
-    public function provideIntegrationTests()
+    public function provideIntegrationTests(): array
     {
         $fixturesDir = realpath(__DIR__.'/Fixtures/pooloptimizer/');
         $tests = array();
@@ -100,7 +100,7 @@ class PoolOptimizerTest extends TestCase
      * @param  string $fixturesDir
      * @return mixed[]
      */
-    protected function readTestFile(\SplFileInfo $file, $fixturesDir)
+    protected function readTestFile(\SplFileInfo $file, $fixturesDir): array
     {
         $tokens = Preg::split('#(?:^|\n*)--([A-Z-]+)--\n#', file_get_contents($file->getRealPath()), -1, PREG_SPLIT_DELIM_CAPTURE);
 
@@ -154,7 +154,7 @@ class PoolOptimizerTest extends TestCase
      * @param BasePackage[] $packages
      * @return string[]
      */
-    private function reducePackagesInfoForComparison(array $packages)
+    private function reducePackagesInfoForComparison(array $packages): array
     {
         $packagesInfo = array();
 
@@ -171,7 +171,7 @@ class PoolOptimizerTest extends TestCase
      * @param mixed[][] $packagesData
      * @return BasePackage[]
      */
-    private function loadPackages(array $packagesData)
+    private function loadPackages(array $packagesData): array
     {
         $packages = array();
 
@@ -189,7 +189,7 @@ class PoolOptimizerTest extends TestCase
      * @param mixed[] $packageData
      * @return BasePackage
      */
-    private function loadPackage(array $packageData)
+    private function loadPackage(array $packageData): BasePackage
     {
         $loader = new ArrayLoader();
 
diff --git a/tests/Composer/Test/DependencyResolver/PoolTest.php b/tests/Composer/Test/DependencyResolver/PoolTest.php
index 350a5cab8..c4c5185ae 100644
--- a/tests/Composer/Test/DependencyResolver/PoolTest.php
+++ b/tests/Composer/Test/DependencyResolver/PoolTest.php
@@ -17,7 +17,7 @@ use Composer\Test\TestCase;
 
 class PoolTest extends TestCase
 {
-    public function testPool()
+    public function testPool(): void
     {
         $package = $this->getPackage('foo', '1');
 
@@ -27,7 +27,7 @@ class PoolTest extends TestCase
         $this->assertEquals(array($package), $pool->whatProvides('foo'));
     }
 
-    public function testWhatProvidesPackageWithConstraint()
+    public function testWhatProvidesPackageWithConstraint(): void
     {
         $firstPackage = $this->getPackage('foo', '1');
         $secondPackage = $this->getPackage('foo', '2');
@@ -41,7 +41,7 @@ class PoolTest extends TestCase
         $this->assertEquals(array($secondPackage), $pool->whatProvides('foo', $this->getVersionConstraint('==', '2')));
     }
 
-    public function testPackageById()
+    public function testPackageById(): void
     {
         $package = $this->getPackage('foo', '1');
 
@@ -50,7 +50,7 @@ class PoolTest extends TestCase
         $this->assertSame($package, $pool->packageById(1));
     }
 
-    public function testWhatProvidesWhenPackageCannotBeFound()
+    public function testWhatProvidesWhenPackageCannotBeFound(): void
     {
         $pool = $this->createPool();
 
@@ -61,7 +61,7 @@ class PoolTest extends TestCase
      * @param array<\Composer\Package\BasePackage>|null $packages
      * @return \Composer\DependencyResolver\Pool
      */
-    protected function createPool($packages = array())
+    protected function createPool($packages = array()): \Composer\DependencyResolver\Pool
     {
         return new Pool($packages);
     }
diff --git a/tests/Composer/Test/DependencyResolver/RequestTest.php b/tests/Composer/Test/DependencyResolver/RequestTest.php
index a93bc404b..c3fff4a0e 100644
--- a/tests/Composer/Test/DependencyResolver/RequestTest.php
+++ b/tests/Composer/Test/DependencyResolver/RequestTest.php
@@ -19,7 +19,7 @@ use Composer\Test\TestCase;
 
 class RequestTest extends TestCase
 {
-    public function testRequestInstall()
+    public function testRequestInstall(): void
     {
         $repo = new ArrayRepository;
         $foo = $this->getPackage('foo', '1');
@@ -41,7 +41,7 @@ class RequestTest extends TestCase
         );
     }
 
-    public function testRequestInstallSamePackageFromDifferentRepositories()
+    public function testRequestInstallSamePackageFromDifferentRepositories(): void
     {
         $repo1 = new ArrayRepository;
         $repo2 = new ArrayRepository;
diff --git a/tests/Composer/Test/DependencyResolver/RuleSetIteratorTest.php b/tests/Composer/Test/DependencyResolver/RuleSetIteratorTest.php
index 3f2429024..807311882 100644
--- a/tests/Composer/Test/DependencyResolver/RuleSetIteratorTest.php
+++ b/tests/Composer/Test/DependencyResolver/RuleSetIteratorTest.php
@@ -43,7 +43,7 @@ class RuleSetIteratorTest extends TestCase
         );
     }
 
-    public function testForeach()
+    public function testForeach(): void
     {
         $ruleSetIterator = new RuleSetIterator($this->rules);
 
@@ -61,7 +61,7 @@ class RuleSetIteratorTest extends TestCase
         $this->assertEquals($expected, $result);
     }
 
-    public function testKeys()
+    public function testKeys(): void
     {
         $ruleSetIterator = new RuleSetIterator($this->rules);
 
diff --git a/tests/Composer/Test/DependencyResolver/RuleSetTest.php b/tests/Composer/Test/DependencyResolver/RuleSetTest.php
index f25e3df4f..c891cb936 100644
--- a/tests/Composer/Test/DependencyResolver/RuleSetTest.php
+++ b/tests/Composer/Test/DependencyResolver/RuleSetTest.php
@@ -22,7 +22,7 @@ use Composer\Test\TestCase;
 
 class RuleSetTest extends TestCase
 {
-    public function testAdd()
+    public function testAdd(): void
     {
         $rules = array(
             RuleSet::TYPE_PACKAGE => array(),
@@ -44,7 +44,7 @@ class RuleSetTest extends TestCase
         $this->assertEquals($rules, $ruleSet->getRules());
     }
 
-    public function testAddIgnoresDuplicates()
+    public function testAddIgnoresDuplicates(): void
     {
         $rules = array(
             RuleSet::TYPE_REQUEST => array(
@@ -63,7 +63,7 @@ class RuleSetTest extends TestCase
         $this->assertCount(1, $ruleSet->getIteratorFor(array(RuleSet::TYPE_REQUEST)));
     }
 
-    public function testAddWhenTypeIsNotRecognized()
+    public function testAddWhenTypeIsNotRecognized(): void
     {
         $ruleSet = new RuleSet;
 
@@ -72,7 +72,7 @@ class RuleSetTest extends TestCase
         $ruleSet->add(new GenericRule(array(), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint)), 7);
     }
 
-    public function testCount()
+    public function testCount(): void
     {
         $ruleSet = new RuleSet;
 
@@ -82,7 +82,7 @@ class RuleSetTest extends TestCase
         $this->assertEquals(2, $ruleSet->count());
     }
 
-    public function testRuleById()
+    public function testRuleById(): void
     {
         $ruleSet = new RuleSet;
 
@@ -92,7 +92,7 @@ class RuleSetTest extends TestCase
         $this->assertSame($rule, $ruleSet->ruleById[0]);
     }
 
-    public function testGetIterator()
+    public function testGetIterator(): void
     {
         $ruleSet = new RuleSet;
 
@@ -108,7 +108,7 @@ class RuleSetTest extends TestCase
         $this->assertSame($rule2, $iterator->current());
     }
 
-    public function testGetIteratorFor()
+    public function testGetIteratorFor(): void
     {
         $ruleSet = new RuleSet;
         $rule1 = new GenericRule(array(1), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
@@ -122,7 +122,7 @@ class RuleSetTest extends TestCase
         $this->assertSame($rule2, $iterator->current());
     }
 
-    public function testGetIteratorWithout()
+    public function testGetIteratorWithout(): void
     {
         $ruleSet = new RuleSet;
         $rule1 = new GenericRule(array(1), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
@@ -136,7 +136,7 @@ class RuleSetTest extends TestCase
         $this->assertSame($rule2, $iterator->current());
     }
 
-    public function testPrettyString()
+    public function testPrettyString(): void
     {
         $pool = new Pool(array(
             $p = $this->getPackage('foo', '2.1'),
diff --git a/tests/Composer/Test/DependencyResolver/RuleTest.php b/tests/Composer/Test/DependencyResolver/RuleTest.php
index 18f57c685..ed1401936 100644
--- a/tests/Composer/Test/DependencyResolver/RuleTest.php
+++ b/tests/Composer/Test/DependencyResolver/RuleTest.php
@@ -22,7 +22,7 @@ use Composer\Test\TestCase;
 
 class RuleTest extends TestCase
 {
-    public function testGetHash()
+    public function testGetHash(): void
     {
         $rule = new GenericRule(array(123), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
 
@@ -30,7 +30,7 @@ class RuleTest extends TestCase
         $this->assertEquals($hash['hash'], $rule->getHash());
     }
 
-    public function testEqualsForRulesWithDifferentHashes()
+    public function testEqualsForRulesWithDifferentHashes(): void
     {
         $rule = new GenericRule(array(1, 2), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule2 = new GenericRule(array(1, 3), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
@@ -38,7 +38,7 @@ class RuleTest extends TestCase
         $this->assertFalse($rule->equals($rule2));
     }
 
-    public function testEqualsForRulesWithDifferLiteralsQuantity()
+    public function testEqualsForRulesWithDifferLiteralsQuantity(): void
     {
         $rule = new GenericRule(array(1, 12), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule2 = new GenericRule(array(1), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
@@ -46,7 +46,7 @@ class RuleTest extends TestCase
         $this->assertFalse($rule->equals($rule2));
     }
 
-    public function testEqualsForRulesWithSameLiterals()
+    public function testEqualsForRulesWithSameLiterals(): void
     {
         $rule = new GenericRule(array(1, 12), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule2 = new GenericRule(array(1, 12), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
@@ -54,7 +54,7 @@ class RuleTest extends TestCase
         $this->assertTrue($rule->equals($rule2));
     }
 
-    public function testSetAndGetType()
+    public function testSetAndGetType(): void
     {
         $rule = new GenericRule(array(), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule->setType(RuleSet::TYPE_REQUEST);
@@ -62,7 +62,7 @@ class RuleTest extends TestCase
         $this->assertEquals(RuleSet::TYPE_REQUEST, $rule->getType());
     }
 
-    public function testEnable()
+    public function testEnable(): void
     {
         $rule = new GenericRule(array(), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule->disable();
@@ -72,7 +72,7 @@ class RuleTest extends TestCase
         $this->assertFalse($rule->isDisabled());
     }
 
-    public function testDisable()
+    public function testDisable(): void
     {
         $rule = new GenericRule(array(), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule->enable();
@@ -82,7 +82,7 @@ class RuleTest extends TestCase
         $this->assertFalse($rule->isEnabled());
     }
 
-    public function testIsAssertions()
+    public function testIsAssertions(): void
     {
         $rule = new GenericRule(array(1, 12), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
         $rule2 = new GenericRule(array(1), Rule::RULE_ROOT_REQUIRE, array('packageName' => '', 'constraint' => new MatchAllConstraint));
@@ -91,7 +91,7 @@ class RuleTest extends TestCase
         $this->assertTrue($rule2->isAssertion());
     }
 
-    public function testPrettyString()
+    public function testPrettyString(): void
     {
         $pool = new Pool(array(
             $p1 = $this->getPackage('foo', '2.1'),
diff --git a/tests/Composer/Test/DependencyResolver/SolverTest.php b/tests/Composer/Test/DependencyResolver/SolverTest.php
index e6e51c2fb..f63e5167e 100644
--- a/tests/Composer/Test/DependencyResolver/SolverTest.php
+++ b/tests/Composer/Test/DependencyResolver/SolverTest.php
@@ -58,7 +58,7 @@ class SolverTest extends TestCase
         $this->policy = new DefaultPolicy;
     }
 
-    public function testSolverInstallSingle()
+    public function testSolverInstallSingle(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->reposComplete();
@@ -70,7 +70,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverRemoveIfNotRequested()
+    public function testSolverRemoveIfNotRequested(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->reposComplete();
@@ -80,7 +80,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallNonExistingPackageFails()
+    public function testInstallNonExistingPackageFails(): void
     {
         $this->repo->addPackage($this->getPackage('A', '1.0'));
         $this->reposComplete();
@@ -99,7 +99,7 @@ class SolverTest extends TestCase
         }
     }
 
-    public function testSolverInstallSamePackageFromDifferentRepositories()
+    public function testSolverInstallSamePackageFromDifferentRepositories(): void
     {
         $repo1 = new ArrayRepository;
         $repo2 = new ArrayRepository;
@@ -117,7 +117,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverInstallWithDeps()
+    public function testSolverInstallWithDeps(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -135,7 +135,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverInstallHonoursNotEqualOperator()
+    public function testSolverInstallHonoursNotEqualOperator(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -161,7 +161,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverInstallWithDepsInOrder()
+    public function testSolverInstallWithDepsInOrder(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -207,7 +207,7 @@ class SolverTest extends TestCase
      *
      * CAUTION: IF THIS TEST EVER FAILS, SOLVER BEHAVIOR HAS CHANGED AND MAY BREAK DOWNSTREAM USERS
      */
-    public function testSolverMultiPackageNameVersionResolutionDependsOnRequireOrder()
+    public function testSolverMultiPackageNameVersionResolutionDependsOnRequireOrder(): void
     {
         $this->repo->addPackage($php74 = $this->getPackage('ourcustom/PHP', '7.4.23'));
         $this->repo->addPackage($php80 = $this->getPackage('ourcustom/PHP', '8.0.10'));
@@ -256,7 +256,7 @@ class SolverTest extends TestCase
      *
      * CAUTION: IF THIS TEST EVER FAILS, SOLVER BEHAVIOR HAS CHANGED AND MAY BREAK DOWNSTREAM USERS
      */
-    public function testSolverMultiPackageNameVersionResolutionIsIndependentOfRequireOrderIfOrderedDescendingByRequirement()
+    public function testSolverMultiPackageNameVersionResolutionIsIndependentOfRequireOrderIfOrderedDescendingByRequirement(): void
     {
         $this->repo->addPackage($php74 = $this->getPackage('ourcustom/PHP', '7.4'));
         $this->repo->addPackage($php80 = $this->getPackage('ourcustom/PHP', '8.0'));
@@ -297,7 +297,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverFixLocked()
+    public function testSolverFixLocked(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->reposComplete();
@@ -307,7 +307,7 @@ class SolverTest extends TestCase
         $this->checkSolverResult(array());
     }
 
-    public function testSolverFixLockedWithAlternative()
+    public function testSolverFixLockedWithAlternative(): void
     {
         $this->repo->addPackage($this->getPackage('A', '1.0'));
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
@@ -318,7 +318,7 @@ class SolverTest extends TestCase
         $this->checkSolverResult(array());
     }
 
-    public function testSolverUpdateDoesOnlyUpdate()
+    public function testSolverUpdateDoesOnlyUpdate(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repoLocked->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -335,7 +335,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverUpdateSingle()
+    public function testSolverUpdateSingle(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($newPackageA = $this->getPackage('A', '1.1'));
@@ -348,7 +348,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverUpdateAll()
+    public function testSolverUpdateAll(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repoLocked->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -368,7 +368,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverUpdateCurrent()
+    public function testSolverUpdateCurrent(): void
     {
         $this->repoLocked->addPackage($this->getPackage('A', '1.0'));
         $this->repo->addPackage($this->getPackage('A', '1.0'));
@@ -379,7 +379,7 @@ class SolverTest extends TestCase
         $this->checkSolverResult(array());
     }
 
-    public function testSolverUpdateOnlyUpdatesSelectedPackage()
+    public function testSolverUpdateOnlyUpdatesSelectedPackage(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repoLocked->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -396,7 +396,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverUpdateConstrained()
+    public function testSolverUpdateConstrained(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($newPackageA = $this->getPackage('A', '1.2'));
@@ -412,7 +412,7 @@ class SolverTest extends TestCase
         )));
     }
 
-    public function testSolverUpdateFullyConstrained()
+    public function testSolverUpdateFullyConstrained(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($newPackageA = $this->getPackage('A', '1.2'));
@@ -428,7 +428,7 @@ class SolverTest extends TestCase
         )));
     }
 
-    public function testSolverUpdateFullyConstrainedPrunesInstalledPackages()
+    public function testSolverUpdateFullyConstrainedPrunesInstalledPackages(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repoLocked->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -451,7 +451,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverAllJobs()
+    public function testSolverAllJobs(): void
     {
         $this->repoLocked->addPackage($packageD = $this->getPackage('D', '1.0'));
         $this->repoLocked->addPackage($oldPackageC = $this->getPackage('C', '1.0'));
@@ -476,7 +476,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverThreeAlternativeRequireAndConflict()
+    public function testSolverThreeAlternativeRequireAndConflict(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '2.0'));
         $this->repo->addPackage($middlePackageB = $this->getPackage('B', '1.0'));
@@ -495,7 +495,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testSolverObsolete()
+    public function testSolverObsolete(): void
     {
         $this->repoLocked->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -511,7 +511,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallOneOfTwoAlternatives()
+    public function testInstallOneOfTwoAlternatives(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('A', '1.0'));
@@ -525,7 +525,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallProvider()
+    public function testInstallProvider(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageQ = $this->getPackage('Q', '1.0'));
@@ -542,7 +542,7 @@ class SolverTest extends TestCase
         $this->solver->solve($this->request);
     }
 
-    public function testSkipReplacerOfExistingPackage()
+    public function testSkipReplacerOfExistingPackage(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageQ = $this->getPackage('Q', '1.0'));
@@ -560,7 +560,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testNoInstallReplacerOfMissingPackage()
+    public function testNoInstallReplacerOfMissingPackage(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageQ = $this->getPackage('Q', '1.0'));
@@ -576,7 +576,7 @@ class SolverTest extends TestCase
         $this->solver->solve($this->request);
     }
 
-    public function testSkipReplacedPackageIfReplacerIsSelected()
+    public function testSkipReplacedPackageIfReplacerIsSelected(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageQ = $this->getPackage('Q', '1.0'));
@@ -595,7 +595,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testPickOlderIfNewerConflicts()
+    public function testPickOlderIfNewerConflicts(): void
     {
         $this->repo->addPackage($packageX = $this->getPackage('X', '1.0'));
         $packageX->setRequires(array(
@@ -633,7 +633,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallCircularRequire()
+    public function testInstallCircularRequire(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB1 = $this->getPackage('B', '0.9'));
@@ -651,7 +651,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallAlternativeWithCircularRequire()
+    public function testInstallAlternativeWithCircularRequire(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -681,7 +681,7 @@ class SolverTest extends TestCase
      * If a replacer D replaces B and C with C not otherwise available,
      * D must be installed instead of the original B.
      */
-    public function testUseReplacerIfNecessary()
+    public function testUseReplacerIfNecessary(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -714,7 +714,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testIssue265()
+    public function testIssue265(): void
     {
         $this->repo->addPackage($packageA1 = $this->getPackage('A', '2.0.999999-dev'));
         $this->repo->addPackage($packageA2 = $this->getPackage('A', '2.1-dev'));
@@ -749,7 +749,7 @@ class SolverTest extends TestCase
         $this->solver->solve($this->request);
     }
 
-    public function testConflictResultEmpty()
+    public function testConflictResultEmpty(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -782,7 +782,7 @@ class SolverTest extends TestCase
         }
     }
 
-    public function testUnsatisfiableRequires()
+    public function testUnsatisfiableRequires(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -812,7 +812,7 @@ class SolverTest extends TestCase
         }
     }
 
-    public function testRequireMismatchException()
+    public function testRequireMismatchException(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -860,7 +860,7 @@ class SolverTest extends TestCase
         }
     }
 
-    public function testLearnLiteralsWithSortedRuleLiterals()
+    public function testLearnLiteralsWithSortedRuleLiterals(): void
     {
         $this->repo->addPackage($packageTwig2 = $this->getPackage('twig/twig', '2.0'));
         $this->repo->addPackage($packageTwig16 = $this->getPackage('twig/twig', '1.6'));
@@ -887,7 +887,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallRecursiveAliasDependencies()
+    public function testInstallRecursiveAliasDependencies(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '2.0'));
@@ -913,7 +913,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallDevAlias()
+    public function testInstallDevAlias(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '2.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -936,7 +936,7 @@ class SolverTest extends TestCase
         ));
     }
 
-    public function testInstallRootAliasesIfAliasOfIsInstalled()
+    public function testInstallRootAliasesIfAliasOfIsInstalled(): void
     {
         // root aliased, required
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
@@ -975,7 +975,7 @@ class SolverTest extends TestCase
      * In particular in this case the goal is to first have the solver decide X 2.0 should not be installed to later
      * decide to learn that X 2.0 must be installed and revert decisions to retry solving with this new assumption.
      */
-    public function testLearnPositiveLiteral()
+    public function testLearnPositiveLiteral(): void
     {
         $this->repo->addPackage($packageA = $this->getPackage('A', '1.0'));
         $this->repo->addPackage($packageB = $this->getPackage('B', '1.0'));
@@ -1042,7 +1042,7 @@ class SolverTest extends TestCase
     /**
      * @return void
      */
-    protected function reposComplete()
+    protected function reposComplete(): void
     {
         $this->repoSet->addRepository($this->repo);
         $this->repoSet->addRepository($this->repoLocked);
@@ -1051,7 +1051,7 @@ class SolverTest extends TestCase
     /**
      * @return void
      */
-    protected function createSolver()
+    protected function createSolver(): void
     {
         $io = new NullIO();
         $this->pool = $this->repoSet->createPool($this->request, $io);
@@ -1062,7 +1062,7 @@ class SolverTest extends TestCase
      * @param array<array<string, string>> $expected
      * @return void
      */
-    protected function checkSolverResult(array $expected)
+    protected function checkSolverResult(array $expected): void
     {
         $this->createSolver();
         $transaction = $this->solver->solve($this->request);
diff --git a/tests/Composer/Test/DependencyResolver/TransactionTest.php b/tests/Composer/Test/DependencyResolver/TransactionTest.php
index 4c19a6098..703fecf56 100644
--- a/tests/Composer/Test/DependencyResolver/TransactionTest.php
+++ b/tests/Composer/Test/DependencyResolver/TransactionTest.php
@@ -27,7 +27,7 @@ class TransactionTest extends TestCase
     {
     }
 
-    public function testTransactionGenerationAndSorting()
+    public function testTransactionGenerationAndSorting(): void
     {
         $presentPackages = array(
             $packageA = $this->getPackage('a/a', 'dev-master'),
@@ -103,7 +103,7 @@ class TransactionTest extends TestCase
      * @param array<array<string, string>> $expected
      * @return void
      */
-    protected function checkTransactionOperations(Transaction $transaction, array $expected)
+    protected function checkTransactionOperations(Transaction $transaction, array $expected): void
     {
         $result = array();
         foreach ($transaction->getOperations() as $operation) {
diff --git a/tests/Composer/Test/Downloader/ArchiveDownloaderTest.php b/tests/Composer/Test/Downloader/ArchiveDownloaderTest.php
index 9537e462a..00e31a3cb 100644
--- a/tests/Composer/Test/Downloader/ArchiveDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/ArchiveDownloaderTest.php
@@ -19,7 +19,7 @@ class ArchiveDownloaderTest extends TestCase
     /** @var \Composer\Config&\PHPUnit\Framework\MockObject\MockObject */
     protected $config;
 
-    public function testGetFileName()
+    public function testGetFileName(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -41,7 +41,7 @@ class ArchiveDownloaderTest extends TestCase
         $this->assertSame($first, $method->invoke($downloader, $packageMock, '/path'));
     }
 
-    public function testProcessUrl()
+    public function testProcessUrl(): void
     {
         if (!extension_loaded('openssl')) {
             $this->markTestSkipped('Requires openssl');
@@ -57,7 +57,7 @@ class ArchiveDownloaderTest extends TestCase
         $this->assertEquals($expected, $url);
     }
 
-    public function testProcessUrl2()
+    public function testProcessUrl2(): void
     {
         if (!extension_loaded('openssl')) {
             $this->markTestSkipped('Requires openssl');
@@ -73,7 +73,7 @@ class ArchiveDownloaderTest extends TestCase
         $this->assertEquals($expected, $url);
     }
 
-    public function testProcessUrl3()
+    public function testProcessUrl3(): void
     {
         if (!extension_loaded('openssl')) {
             $this->markTestSkipped('Requires openssl');
@@ -93,7 +93,7 @@ class ArchiveDownloaderTest extends TestCase
      * @dataProvider provideUrls
      * @param string $url
      */
-    public function testProcessUrlRewriteDist($url)
+    public function testProcessUrlRewriteDist($url): void
     {
         if (!extension_loaded('openssl')) {
             $this->markTestSkipped('Requires openssl');
@@ -115,7 +115,7 @@ class ArchiveDownloaderTest extends TestCase
         $this->assertEquals($expected, $url);
     }
 
-    public function provideUrls()
+    public function provideUrls(): array
     {
         return array(
             array('https://api.github.com/repos/composer/composer/zipball/master'),
@@ -132,7 +132,7 @@ class ArchiveDownloaderTest extends TestCase
      * @param string $url
      * @param string $extension
      */
-    public function testProcessUrlRewriteBitbucketDist($url, $extension)
+    public function testProcessUrlRewriteBitbucketDist($url, $extension): void
     {
         if (!extension_loaded('openssl')) {
             $this->markTestSkipped('Requires openssl');
@@ -154,7 +154,7 @@ class ArchiveDownloaderTest extends TestCase
         $this->assertEquals($expected, $url);
     }
 
-    public function provideBitbucketUrls()
+    public function provideBitbucketUrls(): array
     {
         return array(
             array('https://bitbucket.org/davereid/drush-virtualhost/get/77ca490c26ac818e024d1138aa8bd3677d1ef21f', 'zip'),
diff --git a/tests/Composer/Test/Downloader/DownloadManagerTest.php b/tests/Composer/Test/Downloader/DownloadManagerTest.php
index 7c84a1c9b..b3eb77c26 100644
--- a/tests/Composer/Test/Downloader/DownloadManagerTest.php
+++ b/tests/Composer/Test/Downloader/DownloadManagerTest.php
@@ -30,7 +30,7 @@ class DownloadManagerTest extends TestCase
         $this->io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
     }
 
-    public function testSetGetDownloader()
+    public function testSetGetDownloader(): void
     {
         $downloader = $this->createDownloaderMock();
         $manager = new DownloadManager($this->io, false, $this->filesystem);
@@ -42,7 +42,7 @@ class DownloadManagerTest extends TestCase
         $manager->getDownloader('unregistered');
     }
 
-    public function testGetDownloaderForIncorrectlyInstalledPackage()
+    public function testGetDownloaderForIncorrectlyInstalledPackage(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -57,7 +57,7 @@ class DownloadManagerTest extends TestCase
         $manager->getDownloaderForPackage($package);
     }
 
-    public function testGetDownloaderForCorrectlyInstalledDistPackage()
+    public function testGetDownloaderForCorrectlyInstalledDistPackage(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -89,7 +89,7 @@ class DownloadManagerTest extends TestCase
         $this->assertSame($downloader, $manager->getDownloaderForPackage($package));
     }
 
-    public function testGetDownloaderForIncorrectlyInstalledDistPackage()
+    public function testGetDownloaderForIncorrectlyInstalledDistPackage(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -123,7 +123,7 @@ class DownloadManagerTest extends TestCase
         $manager->getDownloaderForPackage($package);
     }
 
-    public function testGetDownloaderForCorrectlyInstalledSourcePackage()
+    public function testGetDownloaderForCorrectlyInstalledSourcePackage(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -155,7 +155,7 @@ class DownloadManagerTest extends TestCase
         $this->assertSame($downloader, $manager->getDownloaderForPackage($package));
     }
 
-    public function testGetDownloaderForIncorrectlyInstalledSourcePackage()
+    public function testGetDownloaderForIncorrectlyInstalledSourcePackage(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -189,7 +189,7 @@ class DownloadManagerTest extends TestCase
         $manager->getDownloaderForPackage($package);
     }
 
-    public function testGetDownloaderForMetapackage()
+    public function testGetDownloaderForMetapackage(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -202,7 +202,7 @@ class DownloadManagerTest extends TestCase
         $this->assertNull($manager->getDownloaderForPackage($package));
     }
 
-    public function testFullPackageDownload()
+    public function testFullPackageDownload(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -238,7 +238,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testFullPackageDownloadFailover()
+    public function testFullPackageDownloadFailover(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -291,7 +291,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testBadPackageDownload()
+    public function testBadPackageDownload(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -309,7 +309,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testDistOnlyPackageDownload()
+    public function testDistOnlyPackageDownload(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -345,7 +345,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testSourceOnlyPackageDownload()
+    public function testSourceOnlyPackageDownload(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -381,7 +381,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testMetapackagePackageDownload()
+    public function testMetapackagePackageDownload(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -411,7 +411,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testFullPackageDownloadWithSourcePreferred()
+    public function testFullPackageDownloadWithSourcePreferred(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -448,7 +448,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testDistOnlyPackageDownloadWithSourcePreferred()
+    public function testDistOnlyPackageDownloadWithSourcePreferred(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -485,7 +485,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testSourceOnlyPackageDownloadWithSourcePreferred()
+    public function testSourceOnlyPackageDownloadWithSourcePreferred(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -522,7 +522,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testBadPackageDownloadWithSourcePreferred()
+    public function testBadPackageDownloadWithSourcePreferred(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -541,7 +541,7 @@ class DownloadManagerTest extends TestCase
         $manager->download($package, 'target_dir');
     }
 
-    public function testUpdateDistWithEqualTypes()
+    public function testUpdateDistWithEqualTypes(): void
     {
         $initial = $this->createPackageMock();
         $initial
@@ -579,7 +579,7 @@ class DownloadManagerTest extends TestCase
         $manager->update($initial, $target, 'vendor/bundles/FOS/UserBundle');
     }
 
-    public function testUpdateDistWithNotEqualTypes()
+    public function testUpdateDistWithNotEqualTypes(): void
     {
         $initial = $this->createPackageMock();
         $initial
@@ -636,7 +636,7 @@ class DownloadManagerTest extends TestCase
      * @param bool $targetIsDev
      * @param string[] $expected
      */
-    public function testGetAvailableSourcesUpdateSticksToSameSource($prevPkgSource, $prevPkgIsDev, $targetAvailable, $targetIsDev, $expected)
+    public function testGetAvailableSourcesUpdateSticksToSameSource($prevPkgSource, $prevPkgIsDev, $targetAvailable, $targetIsDev, $expected): void
     {
         $initial = null;
         if ($prevPkgSource) {
@@ -666,7 +666,7 @@ class DownloadManagerTest extends TestCase
         $this->assertEquals($expected, $method->invoke($manager, $target, $initial ?? null));
     }
 
-    public static function updatesProvider()
+    public static function updatesProvider(): array
     {
         return array(
             //    prevPkg source,  prevPkg isDev, pkg available,           pkg isDev,  expected
@@ -689,7 +689,7 @@ class DownloadManagerTest extends TestCase
         );
     }
 
-    public function testUpdateMetapackage()
+    public function testUpdateMetapackage(): void
     {
         $initial = $this->createPackageMock();
         $target = $this->createPackageMock();
@@ -707,7 +707,7 @@ class DownloadManagerTest extends TestCase
         $manager->update($initial, $target, 'vendor/pkg');
     }
 
-    public function testRemove()
+    public function testRemove(): void
     {
         $package = $this->createPackageMock();
 
@@ -730,7 +730,7 @@ class DownloadManagerTest extends TestCase
         $manager->remove($package, 'vendor/bundles/FOS/UserBundle');
     }
 
-    public function testMetapackageRemove()
+    public function testMetapackageRemove(): void
     {
         $package = $this->createPackageMock();
 
@@ -750,7 +750,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithoutPreferenceDev()
+    public function testInstallPreferenceWithoutPreferenceDev(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -793,7 +793,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithoutPreferenceNoDev()
+    public function testInstallPreferenceWithoutPreferenceNoDev(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -836,7 +836,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithoutMatchDev()
+    public function testInstallPreferenceWithoutMatchDev(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -883,7 +883,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithoutMatchNoDev()
+    public function testInstallPreferenceWithoutMatchNoDev(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -930,7 +930,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithMatchAutoDev()
+    public function testInstallPreferenceWithMatchAutoDev(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -977,7 +977,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithMatchAutoNoDev()
+    public function testInstallPreferenceWithMatchAutoNoDev(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -1024,7 +1024,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithMatchSource()
+    public function testInstallPreferenceWithMatchSource(): void
     {
         $package = $this->createPackageMock();
         $package
@@ -1067,7 +1067,7 @@ class DownloadManagerTest extends TestCase
     /**
      * @covers Composer\Downloader\DownloadManager::resolvePackageInstallPreference
      */
-    public function testInstallPreferenceWithMatchDist()
+    public function testInstallPreferenceWithMatchDist(): void
     {
         $package = $this->createPackageMock();
         $package
diff --git a/tests/Composer/Test/Downloader/FileDownloaderTest.php b/tests/Composer/Test/Downloader/FileDownloaderTest.php
index 2ade50ce3..d3f9c0056 100644
--- a/tests/Composer/Test/Downloader/FileDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/FileDownloaderTest.php
@@ -43,7 +43,7 @@ class FileDownloaderTest extends TestCase
      * @param \Composer\Util\Filesystem $filesystem
      * @return \Composer\Downloader\FileDownloader
      */
-    protected function getDownloader($io = null, $config = null, $eventDispatcher = null, $cache = null, $httpDownloader = null, $filesystem = null)
+    protected function getDownloader($io = null, $config = null, $eventDispatcher = null, $cache = null, $httpDownloader = null, $filesystem = null): \Composer\Downloader\FileDownloader
     {
         $io = $io ?: $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $config = $config ?: $this->config;
@@ -57,7 +57,7 @@ class FileDownloaderTest extends TestCase
         return new FileDownloader($io, $this->config, $httpDownloader, $eventDispatcher, $cache, $filesystem);
     }
 
-    public function testDownloadForPackageWithoutDistReference()
+    public function testDownloadForPackageWithoutDistReference(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->once())
@@ -71,7 +71,7 @@ class FileDownloaderTest extends TestCase
         $downloader->download($packageMock, '/path');
     }
 
-    public function testDownloadToExistingFile()
+    public function testDownloadToExistingFile(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -101,7 +101,7 @@ class FileDownloaderTest extends TestCase
         }
     }
 
-    public function testGetFileName()
+    public function testGetFileName(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->once())
@@ -121,7 +121,7 @@ class FileDownloaderTest extends TestCase
         $this->assertMatchesRegularExpression('#/vendor/composer/tmp-[a-z0-9]+\.js#', $method->invoke($downloader, $packageMock, '/path'));
     }
 
-    public function testDownloadButFileIsUnsaved()
+    public function testDownloadButFileIsUnsaved(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -176,7 +176,7 @@ class FileDownloaderTest extends TestCase
         }
     }
 
-    public function testDownloadWithCustomProcessedUrl()
+    public function testDownloadWithCustomProcessedUrl(): void
     {
         $path = $this->getUniqueTmpDirectory();
 
@@ -218,7 +218,7 @@ class FileDownloaderTest extends TestCase
             $this->getMockBuilder('Composer\IO\IOInterface')->getMock(),
             $this->getProcessExecutorMock()
         );
-        $dispatcher->addListener(PluginEvents::PRE_FILE_DOWNLOAD, function (PreFileDownloadEvent $event) use ($expectedUrl) {
+        $dispatcher->addListener(PluginEvents::PRE_FILE_DOWNLOAD, function (PreFileDownloadEvent $event) use ($expectedUrl): void {
             $event->setProcessedUrl($expectedUrl);
         });
 
@@ -228,7 +228,7 @@ class FileDownloaderTest extends TestCase
         $cacheMock
             ->expects($this->any())
             ->method('copyTo')
-            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey) {
+            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey): bool {
                 $this->assertEquals($expectedCacheKey, $cacheKey, 'Failed assertion on $cacheKey argument of Cache::copyTo method:');
 
                 return false;
@@ -236,7 +236,7 @@ class FileDownloaderTest extends TestCase
         $cacheMock
             ->expects($this->any())
             ->method('copyFrom')
-            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey) {
+            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey): bool {
                 $this->assertEquals($expectedCacheKey, $cacheKey, 'Failed assertion on $cacheKey argument of Cache::copyFrom method:');
 
                 return false;
@@ -275,7 +275,7 @@ class FileDownloaderTest extends TestCase
         }
     }
 
-    public function testDownloadWithCustomCacheKey()
+    public function testDownloadWithCustomCacheKey(): void
     {
         $path = $this->getUniqueTmpDirectory();
 
@@ -318,7 +318,7 @@ class FileDownloaderTest extends TestCase
             $this->getMockBuilder('Composer\IO\IOInterface')->getMock(),
             $this->getProcessExecutorMock()
         );
-        $dispatcher->addListener(PluginEvents::PRE_FILE_DOWNLOAD, function (PreFileDownloadEvent $event) use ($customCacheKey) {
+        $dispatcher->addListener(PluginEvents::PRE_FILE_DOWNLOAD, function (PreFileDownloadEvent $event) use ($customCacheKey): void {
             $event->setCustomCacheKey($customCacheKey);
         });
 
@@ -328,7 +328,7 @@ class FileDownloaderTest extends TestCase
         $cacheMock
             ->expects($this->any())
             ->method('copyTo')
-            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey) {
+            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey): bool {
                 $this->assertEquals($expectedCacheKey, $cacheKey, 'Failed assertion on $cacheKey argument of Cache::copyTo method:');
 
                 return false;
@@ -336,7 +336,7 @@ class FileDownloaderTest extends TestCase
         $cacheMock
             ->expects($this->any())
             ->method('copyFrom')
-            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey) {
+            ->will($this->returnCallback(function ($cacheKey) use ($expectedCacheKey): bool {
                 $this->assertEquals($expectedCacheKey, $cacheKey, 'Failed assertion on $cacheKey argument of Cache::copyFrom method:');
 
                 return false;
@@ -375,7 +375,7 @@ class FileDownloaderTest extends TestCase
         }
     }
 
-    public function testCacheGarbageCollectionIsCalled()
+    public function testCacheGarbageCollectionIsCalled(): void
     {
         $expectedTtl = '99999999';
 
@@ -403,7 +403,7 @@ class FileDownloaderTest extends TestCase
         $downloader = $this->getDownloader(null, $this->config, null, $cacheMock, null, null);
     }
 
-    public function testDownloadFileWithInvalidChecksum()
+    public function testDownloadFileWithInvalidChecksum(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -458,7 +458,7 @@ class FileDownloaderTest extends TestCase
         }
     }
 
-    public function testDowngradeShowsAppropriateMessage()
+    public function testDowngradeShowsAppropriateMessage(): void
     {
         $oldPackage = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $oldPackage->expects($this->once())
diff --git a/tests/Composer/Test/Downloader/FossilDownloaderTest.php b/tests/Composer/Test/Downloader/FossilDownloaderTest.php
index 7a5e84e61..2ba0e33f0 100644
--- a/tests/Composer/Test/Downloader/FossilDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/FossilDownloaderTest.php
@@ -42,7 +42,7 @@ class FossilDownloaderTest extends TestCase
      * @param \Composer\Util\Filesystem $filesystem
      * @return FossilDownloader
      */
-    protected function getDownloaderMock($io = null, $config = null, $executor = null, $filesystem = null)
+    protected function getDownloaderMock($io = null, $config = null, $executor = null, $filesystem = null): FossilDownloader
     {
         $io = $io ?: $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $config = $config ?: $this->getMockBuilder('Composer\Config')->getMock();
@@ -52,7 +52,7 @@ class FossilDownloaderTest extends TestCase
         return new FossilDownloader($io, $config, $executor, $filesystem);
     }
 
-    public function testInstallForPackageWithoutSourceReference()
+    public function testInstallForPackageWithoutSourceReference(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->once())
@@ -65,7 +65,7 @@ class FossilDownloaderTest extends TestCase
         $downloader->install($packageMock, '/path');
     }
 
-    public function testInstall()
+    public function testInstall(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -86,7 +86,7 @@ class FossilDownloaderTest extends TestCase
         $downloader->install($packageMock, 'repo');
     }
 
-    public function testUpdateforPackageWithoutSourceReference()
+    public function testUpdateforPackageWithoutSourceReference(): void
     {
         $initialPackageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $sourcePackageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
@@ -102,7 +102,7 @@ class FossilDownloaderTest extends TestCase
         $downloader->cleanup('update', $sourcePackageMock, '/path', $initialPackageMock);
     }
 
-    public function testUpdate()
+    public function testUpdate(): void
     {
         // Ensure file exists
         $file = $this->workingDir . '/.fslckout';
@@ -134,7 +134,7 @@ class FossilDownloaderTest extends TestCase
         $downloader->cleanup('update', $packageMock, $this->workingDir, $packageMock);
     }
 
-    public function testRemove()
+    public function testRemove(): void
     {
         // Ensure file exists
         $file = $this->workingDir . '/.fslckout';
@@ -159,7 +159,7 @@ class FossilDownloaderTest extends TestCase
         $downloader->cleanup('uninstall', $packageMock, $this->workingDir);
     }
 
-    public function testGetInstallationSource()
+    public function testGetInstallationSource(): void
     {
         $downloader = $this->getDownloaderMock(null);
 
diff --git a/tests/Composer/Test/Downloader/GitDownloaderTest.php b/tests/Composer/Test/Downloader/GitDownloaderTest.php
index 9791fd1ee..fce2ba033 100644
--- a/tests/Composer/Test/Downloader/GitDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/GitDownloaderTest.php
@@ -50,7 +50,7 @@ class GitDownloaderTest extends TestCase
      * @param string|bool $version
      * @return void
      */
-    private function initGitVersion($version)
+    private function initGitVersion($version): void
     {
         // reset the static version cache
         $refl = new \ReflectionProperty('Composer\Util\Git', 'version');
@@ -62,7 +62,7 @@ class GitDownloaderTest extends TestCase
      * @param ?\Composer\Config $config
      * @return \Composer\Config
      */
-    protected function setupConfig($config = null)
+    protected function setupConfig($config = null): \Composer\Config
     {
         if (!$config) {
             $config = new Config();
@@ -82,7 +82,7 @@ class GitDownloaderTest extends TestCase
      * @param \Composer\Util\Filesystem $filesystem
      * @return GitDownloader
      */
-    protected function getDownloaderMock($io = null, $config = null, $executor = null, $filesystem = null)
+    protected function getDownloaderMock($io = null, $config = null, $executor = null, $filesystem = null): GitDownloader
     {
         $io = $io ?: $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $executor = $executor ?: $this->getProcessExecutorMock();
@@ -92,7 +92,7 @@ class GitDownloaderTest extends TestCase
         return new GitDownloader($io, $config, $executor, $filesystem);
     }
 
-    public function testDownloadForPackageWithoutSourceReference()
+    public function testDownloadForPackageWithoutSourceReference(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->once())
@@ -108,7 +108,7 @@ class GitDownloaderTest extends TestCase
         $downloader->cleanup('install', $packageMock, '/path');
     }
 
-    public function testDownload()
+    public function testDownload(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -138,7 +138,7 @@ class GitDownloaderTest extends TestCase
         $downloader->cleanup('install', $packageMock, 'composerPath');
     }
 
-    public function testDownloadWithCache()
+    public function testDownloadWithCache(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -165,7 +165,7 @@ class GitDownloaderTest extends TestCase
 
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
-            array('cmd' => $this->winCompat(sprintf("git clone --mirror -- 'https://example.com/composer/composer' '%s'", $cachePath)), 'callback' => function () use ($cachePath) {
+            array('cmd' => $this->winCompat(sprintf("git clone --mirror -- 'https://example.com/composer/composer' '%s'", $cachePath)), 'callback' => function () use ($cachePath): void {
                 @mkdir($cachePath, 0777, true);
             }),
             array('cmd' => 'git rev-parse --git-dir', 'stdout' => '.'),
@@ -183,7 +183,7 @@ class GitDownloaderTest extends TestCase
         @rmdir($cachePath);
     }
 
-    public function testDownloadUsesVariousProtocolsAndSetsPushUrlForGithub()
+    public function testDownloadUsesVariousProtocolsAndSetsPushUrlForGithub(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -220,7 +220,7 @@ class GitDownloaderTest extends TestCase
         $downloader->cleanup('install', $packageMock, 'composerPath');
     }
 
-    public function pushUrlProvider()
+    public function pushUrlProvider(): array
     {
         return array(
             // ssh proto should use git@ all along
@@ -238,7 +238,7 @@ class GitDownloaderTest extends TestCase
      * @param string $url
      * @param string $pushUrl
      */
-    public function testDownloadAndSetPushUrlUseCustomVariousProtocolsForGithub($protocols, $url, $pushUrl)
+    public function testDownloadAndSetPushUrlUseCustomVariousProtocolsForGithub($protocols, $url, $pushUrl): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -272,7 +272,7 @@ class GitDownloaderTest extends TestCase
         $downloader->cleanup('install', $packageMock, 'composerPath');
     }
 
-    public function testDownloadThrowsRuntimeExceptionIfGitCommandFails()
+    public function testDownloadThrowsRuntimeExceptionIfGitCommandFails(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -313,7 +313,7 @@ class GitDownloaderTest extends TestCase
         }
     }
 
-    public function testUpdateforPackageWithoutSourceReference()
+    public function testUpdateforPackageWithoutSourceReference(): void
     {
         $initialPackageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $sourcePackageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
@@ -330,7 +330,7 @@ class GitDownloaderTest extends TestCase
         $downloader->cleanup('update', $sourcePackageMock, '/path', $initialPackageMock);
     }
 
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $expectedGitUpdateCommand = $this->winCompat("(git remote set-url composer -- 'https://github.com/composer/composer' && git rev-parse --quiet --verify 'ref^{commit}' || (git fetch composer && git fetch --tags composer)) && git remote set-url composer -- 'https://github.com/composer/composer'");
 
@@ -367,7 +367,7 @@ class GitDownloaderTest extends TestCase
         $downloader->cleanup('update', $packageMock, $this->workingDir, $packageMock);
     }
 
-    public function testUpdateWithNewRepoUrl()
+    public function testUpdateWithNewRepoUrl(): void
     {
         $expectedGitUpdateCommand = $this->winCompat("(git remote set-url composer -- 'https://github.com/composer/composer' && git rev-parse --quiet --verify 'ref^{commit}' || (git fetch composer && git fetch --tags composer)) && git remote set-url composer -- 'https://github.com/composer/composer'");
 
@@ -419,7 +419,7 @@ composer https://github.com/old/url (push)
     /**
      * @group failing
      */
-    public function testUpdateThrowsRuntimeExceptionIfGitCommandFails()
+    public function testUpdateThrowsRuntimeExceptionIfGitCommandFails(): void
     {
         $expectedGitUpdateCommand = $this->winCompat("(git remote set-url composer -- 'https://github.com/composer/composer' && git rev-parse --quiet --verify 'ref^{commit}' || (git fetch composer && git fetch --tags composer)) && git remote set-url composer -- 'https://github.com/composer/composer'");
         $expectedGitUpdateCommand2 = $this->winCompat("(git remote set-url composer -- 'git@github.com:composer/composer' && git rev-parse --quiet --verify 'ref^{commit}' || (git fetch composer && git fetch --tags composer)) && git remote set-url composer -- 'git@github.com:composer/composer'");
@@ -470,7 +470,7 @@ composer https://github.com/old/url (push)
         }
     }
 
-    public function testUpdateDoesntThrowsRuntimeExceptionIfGitCommandFailsAtFirstButIsAbleToRecover()
+    public function testUpdateDoesntThrowsRuntimeExceptionIfGitCommandFailsAtFirstButIsAbleToRecover(): void
     {
         $expectedFirstGitUpdateCommand = $this->winCompat("(git remote set-url composer -- '".(Platform::isWindows() ? 'C:\\' : '/')."' && git rev-parse --quiet --verify 'ref^{commit}' || (git fetch composer && git fetch --tags composer)) && git remote set-url composer -- '".(Platform::isWindows() ? 'C:\\' : '/')."'");
         $expectedSecondGitUpdateCommand = $this->winCompat("(git remote set-url composer -- 'https://github.com/composer/composer' && git rev-parse --quiet --verify 'ref^{commit}' || (git fetch composer && git fetch --tags composer)) && git remote set-url composer -- 'https://github.com/composer/composer'");
@@ -517,7 +517,7 @@ composer https://github.com/old/url (push)
         $downloader->cleanup('update', $packageMock, $this->workingDir, $packageMock);
     }
 
-    public function testDowngradeShowsAppropriateMessage()
+    public function testDowngradeShowsAppropriateMessage(): void
     {
         $oldPackage = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $oldPackage->expects($this->any())
@@ -567,7 +567,7 @@ composer https://github.com/old/url (push)
         $downloader->cleanup('update', $newPackage, $this->workingDir, $oldPackage);
     }
 
-    public function testNotUsingDowngradingWithReferences()
+    public function testNotUsingDowngradingWithReferences(): void
     {
         $oldPackage = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $oldPackage->expects($this->any())
@@ -611,7 +611,7 @@ composer https://github.com/old/url (push)
         $downloader->cleanup('update', $newPackage, $this->workingDir, $oldPackage);
     }
 
-    public function testRemove()
+    public function testRemove(): void
     {
         $expectedGitResetCommand = $this->winCompat("git status --porcelain --untracked-files=no");
 
@@ -636,7 +636,7 @@ composer https://github.com/old/url (push)
         $downloader->cleanup('uninstall', $packageMock, $this->workingDir);
     }
 
-    public function testGetInstallationSource()
+    public function testGetInstallationSource(): void
     {
         $downloader = $this->getDownloaderMock();
 
@@ -647,7 +647,7 @@ composer https://github.com/old/url (push)
      * @param string $cmd
      * @return string
      */
-    private function winCompat($cmd)
+    private function winCompat($cmd): string
     {
         if (Platform::isWindows()) {
             $cmd = str_replace('cd ', 'cd /D ', $cmd);
diff --git a/tests/Composer/Test/Downloader/HgDownloaderTest.php b/tests/Composer/Test/Downloader/HgDownloaderTest.php
index b2e555bab..81a039a24 100644
--- a/tests/Composer/Test/Downloader/HgDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/HgDownloaderTest.php
@@ -42,7 +42,7 @@ class HgDownloaderTest extends TestCase
      * @param \Composer\Util\Filesystem $filesystem
      * @return HgDownloader
      */
-    protected function getDownloaderMock($io = null, $config = null, $executor = null, $filesystem = null)
+    protected function getDownloaderMock($io = null, $config = null, $executor = null, $filesystem = null): HgDownloader
     {
         $io = $io ?: $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $config = $config ?: $this->getMockBuilder('Composer\Config')->getMock();
@@ -52,7 +52,7 @@ class HgDownloaderTest extends TestCase
         return new HgDownloader($io, $config, $executor, $filesystem);
     }
 
-    public function testDownloadForPackageWithoutSourceReference()
+    public function testDownloadForPackageWithoutSourceReference(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->once())
@@ -65,7 +65,7 @@ class HgDownloaderTest extends TestCase
         $downloader->install($packageMock, '/path');
     }
 
-    public function testDownload()
+    public function testDownload(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
@@ -85,7 +85,7 @@ class HgDownloaderTest extends TestCase
         $downloader->install($packageMock, 'composerPath');
     }
 
-    public function testUpdateforPackageWithoutSourceReference()
+    public function testUpdateforPackageWithoutSourceReference(): void
     {
         $initialPackageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $sourcePackageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
@@ -101,7 +101,7 @@ class HgDownloaderTest extends TestCase
         $downloader->cleanup('update', $sourcePackageMock, '/path', $initialPackageMock);
     }
 
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $fs = new Filesystem;
         $fs->ensureDirectoryExists($this->workingDir.'/.hg');
@@ -128,7 +128,7 @@ class HgDownloaderTest extends TestCase
         $downloader->cleanup('update', $packageMock, $this->workingDir, $packageMock);
     }
 
-    public function testRemove()
+    public function testRemove(): void
     {
         $fs = new Filesystem;
         $fs->ensureDirectoryExists($this->workingDir.'/.hg');
@@ -151,7 +151,7 @@ class HgDownloaderTest extends TestCase
         $downloader->cleanup('uninstall', $packageMock, $this->workingDir);
     }
 
-    public function testGetInstallationSource()
+    public function testGetInstallationSource(): void
     {
         $downloader = $this->getDownloaderMock(null);
 
diff --git a/tests/Composer/Test/Downloader/PerforceDownloaderTest.php b/tests/Composer/Test/Downloader/PerforceDownloaderTest.php
index ebcc0b662..16bd4d128 100644
--- a/tests/Composer/Test/Downloader/PerforceDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/PerforceDownloaderTest.php
@@ -56,7 +56,7 @@ class PerforceDownloaderTest extends TestCase
     /**
      * @return \Composer\Config
      */
-    protected function getConfig()
+    protected function getConfig(): \Composer\Config
     {
         $config = new Config();
         $settings = array('config' => array('home' => $this->testPath));
@@ -87,7 +87,7 @@ class PerforceDownloaderTest extends TestCase
     /**
      * @return string[]
      */
-    protected function getRepoConfig()
+    protected function getRepoConfig(): array
     {
         return array('url' => 'TEST_URL', 'p4user' => 'TEST_USER');
     }
@@ -112,12 +112,12 @@ class PerforceDownloaderTest extends TestCase
     /**
      * @doesNotPerformAssertions
      */
-    public function testInitPerforceInstantiatesANewPerforceObject()
+    public function testInitPerforceInstantiatesANewPerforceObject(): void
     {
         $this->downloader->initPerforce($this->package, $this->testPath, 'SOURCE_REF');
     }
 
-    public function testInitPerforceDoesNothingIfPerforceAlreadySet()
+    public function testInitPerforceDoesNothingIfPerforceAlreadySet(): void
     {
         $perforce = $this->getMockBuilder('Composer\Util\Perforce')->disableOriginalConstructor()->getMock();
         $this->downloader->setPerforce($perforce);
@@ -129,7 +129,7 @@ class PerforceDownloaderTest extends TestCase
      * @depends testInitPerforceInstantiatesANewPerforceObject
      * @depends testInitPerforceDoesNothingIfPerforceAlreadySet
      */
-    public function testDoInstallWithTag()
+    public function testDoInstallWithTag(): void
     {
         //I really don't like this test but the logic of each Perforce method is tested in the Perforce class.  Really I am just enforcing workflow.
         $ref = 'SOURCE_REF@123';
@@ -153,7 +153,7 @@ class PerforceDownloaderTest extends TestCase
      * @depends testInitPerforceInstantiatesANewPerforceObject
      * @depends testInitPerforceDoesNothingIfPerforceAlreadySet
      */
-    public function testDoInstallWithNoTag()
+    public function testDoInstallWithNoTag(): void
     {
         $ref = 'SOURCE_REF';
         $label = null;
diff --git a/tests/Composer/Test/Downloader/XzDownloaderTest.php b/tests/Composer/Test/Downloader/XzDownloaderTest.php
index cae7bf299..1f2501d37 100644
--- a/tests/Composer/Test/Downloader/XzDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/XzDownloaderTest.php
@@ -49,7 +49,7 @@ class XzDownloaderTest extends TestCase
         $this->fs->removeDirectory($this->testDir);
     }
 
-    public function testErrorMessages()
+    public function testErrorMessages(): void
     {
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
diff --git a/tests/Composer/Test/Downloader/ZipDownloaderTest.php b/tests/Composer/Test/Downloader/ZipDownloaderTest.php
index 8ecb2719f..49e9dcda4 100644
--- a/tests/Composer/Test/Downloader/ZipDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/ZipDownloaderTest.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Test\Downloader;
 
+use React\Promise\PromiseInterface;
 use Composer\Downloader\ZipDownloader;
 use Composer\Package\PackageInterface;
 use Composer\Test\TestCase;
@@ -56,7 +57,7 @@ class ZipDownloaderTest extends TestCase
      * @param ?\Composer\Test\Downloader\MockedZipDownloader $obj
      * @return void
      */
-    public function setPrivateProperty($name, $value, $obj = null)
+    public function setPrivateProperty($name, $value, $obj = null): void
     {
         $reflectionClass = new \ReflectionClass('Composer\Downloader\ZipDownloader');
         $reflectedProperty = $reflectionClass->getProperty($name);
@@ -68,7 +69,7 @@ class ZipDownloaderTest extends TestCase
         }
     }
 
-    public function testErrorMessages()
+    public function testErrorMessages(): void
     {
         if (!class_exists('ZipArchive')) {
             $this->markTestSkipped('zip extension missing');
@@ -106,7 +107,7 @@ class ZipDownloaderTest extends TestCase
         }
     }
 
-    public function testZipArchiveOnlyFailed()
+    public function testZipArchiveOnlyFailed(): void
     {
         self::expectException('RuntimeException');
         self::expectExceptionMessage('There was an error extracting the ZIP file');
@@ -129,7 +130,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testZipArchiveExtractOnlyFailed()
+    public function testZipArchiveExtractOnlyFailed(): void
     {
         self::expectException('RuntimeException');
         self::expectExceptionMessage('The archive may contain identical file names with different capitalization (which fails on case insensitive filesystems): Not a directory');
@@ -152,7 +153,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testZipArchiveOnlyGood()
+    public function testZipArchiveOnlyGood(): void
     {
         if (!class_exists('ZipArchive')) {
             $this->markTestSkipped('zip extension missing');
@@ -173,7 +174,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testSystemUnzipOnlyFailed()
+    public function testSystemUnzipOnlyFailed(): void
     {
         self::expectException('Exception');
         self::expectExceptionMessage('Failed to extract : (1) unzip');
@@ -202,7 +203,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testSystemUnzipOnlyGood()
+    public function testSystemUnzipOnlyGood(): void
     {
         $this->setPrivateProperty('isWindows', false);
         $this->setPrivateProperty('hasZipArchive', false);
@@ -229,7 +230,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testNonWindowsFallbackGood()
+    public function testNonWindowsFallbackGood(): void
     {
         if (!class_exists('ZipArchive')) {
             $this->markTestSkipped('zip extension missing');
@@ -268,7 +269,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testNonWindowsFallbackFailed()
+    public function testNonWindowsFallbackFailed(): void
     {
         self::expectException('Exception');
         self::expectExceptionMessage('There was an error extracting the ZIP file');
@@ -313,16 +314,16 @@ class ZipDownloaderTest extends TestCase
      * @param ?\React\Promise\PromiseInterface $promise
      * @return void
      */
-    private function wait($promise)
+    private function wait($promise): void
     {
         if (null === $promise) {
             return;
         }
 
         $e = null;
-        $promise->then(function () {
+        $promise->then(function (): void {
             // noop
-        }, function ($ex) use (&$e) {
+        }, function ($ex) use (&$e): void {
             $e = $ex;
         });
 
@@ -334,17 +335,17 @@ class ZipDownloaderTest extends TestCase
 
 class MockedZipDownloader extends ZipDownloader
 {
-    public function download(PackageInterface $package, $path, PackageInterface $prevPackage = null, $output = true)
+    public function download(PackageInterface $package, $path, PackageInterface $prevPackage = null, $output = true): ?PromiseInterface
     {
         return \React\Promise\resolve();
     }
 
-    public function install(PackageInterface $package, $path, $output = true)
+    public function install(PackageInterface $package, $path, $output = true): PromiseInterface
     {
         return \React\Promise\resolve();
     }
 
-    public function extract(PackageInterface $package, $file, $path)
+    public function extract(PackageInterface $package, $file, $path): ?PromiseInterface
     {
         return parent::extract($package, $file, $path);
     }
diff --git a/tests/Composer/Test/EventDispatcher/EventDispatcherTest.php b/tests/Composer/Test/EventDispatcher/EventDispatcherTest.php
index 3217d8bf4..57b8efb5c 100644
--- a/tests/Composer/Test/EventDispatcher/EventDispatcherTest.php
+++ b/tests/Composer/Test/EventDispatcher/EventDispatcherTest.php
@@ -28,7 +28,7 @@ use Symfony\Component\Console\Output\OutputInterface;
 
 class EventDispatcherTest extends TestCase
 {
-    public function testListenerExceptionsAreCaught()
+    public function testListenerExceptionsAreCaught(): void
     {
         self::expectException('RuntimeException');
 
@@ -56,7 +56,7 @@ class EventDispatcherTest extends TestCase
      *
      * @param string $command
      */
-    public function testDispatcherCanExecuteSingleCommandLineScript($command)
+    public function testDispatcherCanExecuteSingleCommandLineScript($command): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -85,7 +85,7 @@ class EventDispatcherTest extends TestCase
      *
      * @param bool $devMode
      */
-    public function testDispatcherPassDevModeToAutoloadGeneratorForScriptEvents($devMode)
+    public function testDispatcherPassDevModeToAutoloadGeneratorForScriptEvents($devMode): void
     {
         $composer = $this->createComposerInstance();
 
@@ -120,7 +120,7 @@ class EventDispatcherTest extends TestCase
         $dispatcher->hasEventListeners($event);
     }
 
-    public function provideDevModes()
+    public function provideDevModes(): array
     {
         return array(
             array(true),
@@ -177,7 +177,7 @@ class EventDispatcherTest extends TestCase
         return $rm;
     }
 
-    public function testDispatcherRemoveListener()
+    public function testDispatcherRemoveListener(): void
     {
         $composer = $this->createComposerInstance();
 
@@ -220,7 +220,7 @@ class EventDispatcherTest extends TestCase
         $this->assertEquals($expected, $io->getOutput());
     }
 
-    public function testDispatcherCanExecuteCliAndPhpInSameEventScriptStack()
+    public function testDispatcherCanExecuteCliAndPhpInSameEventScriptStack(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -257,7 +257,7 @@ class EventDispatcherTest extends TestCase
         $this->assertEquals($expected, $io->getOutput());
     }
 
-    public function testDispatcherCanPutEnv()
+    public function testDispatcherCanPutEnv(): void
     {
         $dispatcher = $this->getMockBuilder('Composer\EventDispatcher\EventDispatcher')
             ->setConstructorArgs(array(
@@ -286,7 +286,7 @@ class EventDispatcherTest extends TestCase
         $this->assertEquals($expected, $io->getOutput());
     }
 
-    public function testDispatcherAppendsDirBinOnPathForEveryListener()
+    public function testDispatcherAppendsDirBinOnPathForEveryListener(): void
     {
         $currentDirectoryBkp = getcwd();
         $composerBinDirBkp = Platform::getEnv('COMPOSER_BIN_DIR');
@@ -323,7 +323,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return void
      */
-    public static function createsVendorBinFolderChecksEnvDoesNotContainsBin()
+    public static function createsVendorBinFolderChecksEnvDoesNotContainsBin(): void
     {
         mkdir(__DIR__ . '/vendor/bin', 0700, true);
         $val = getenv('PATH');
@@ -338,7 +338,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return void
      */
-    public static function createsVendorBinFolderChecksEnvContainsBin()
+    public static function createsVendorBinFolderChecksEnvContainsBin(): void
     {
         $val = getenv('PATH');
 
@@ -352,7 +352,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return void
      */
-    public static function getTestEnv()
+    public static function getTestEnv(): void
     {
         $val = getenv('ABC');
         if ($val !== '123') {
@@ -360,7 +360,7 @@ class EventDispatcherTest extends TestCase
         }
     }
 
-    public function testDispatcherCanExecuteComposerScriptGroups()
+    public function testDispatcherCanExecuteComposerScriptGroups(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -382,7 +382,7 @@ class EventDispatcherTest extends TestCase
 
         $dispatcher->expects($this->atLeastOnce())
             ->method('getListeners')
-            ->will($this->returnCallback(function (Event $event) {
+            ->will($this->returnCallback(function (Event $event): array {
                 if ($event->getName() === 'root') {
                     return array('@group');
                 }
@@ -407,7 +407,7 @@ class EventDispatcherTest extends TestCase
         $this->assertEquals($expected, $io->getOutput());
     }
 
-    public function testRecursionInScriptsNames()
+    public function testRecursionInScriptsNames(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -427,7 +427,7 @@ class EventDispatcherTest extends TestCase
 
         $dispatcher->expects($this->atLeastOnce())
             ->method('getListeners')
-            ->will($this->returnCallback(function (Event $event) {
+            ->will($this->returnCallback(function (Event $event): array {
                 if ($event->getName() === 'hello') {
                     return array('echo Hello');
                 }
@@ -446,7 +446,7 @@ class EventDispatcherTest extends TestCase
         $this->assertEquals($expected, $io->getOutput());
     }
 
-    public function testDispatcherDetectInfiniteRecursion()
+    public function testDispatcherDetectInfiniteRecursion(): void
     {
         self::expectException('RuntimeException');
 
@@ -463,7 +463,7 @@ class EventDispatcherTest extends TestCase
 
         $dispatcher->expects($this->atLeastOnce())
             ->method('getListeners')
-            ->will($this->returnCallback(function (Event $event) {
+            ->will($this->returnCallback(function (Event $event): array {
                 if ($event->getName() === 'root') {
                     return array('@recurse');
                 }
@@ -500,7 +500,7 @@ class EventDispatcherTest extends TestCase
         return $dispatcher;
     }
 
-    public function provideValidCommands()
+    public function provideValidCommands(): array
     {
         return array(
             array('phpunit'),
@@ -509,7 +509,7 @@ class EventDispatcherTest extends TestCase
         );
     }
 
-    public function testDispatcherOutputsCommand()
+    public function testDispatcherOutputsCommand(): void
     {
         $dispatcher = $this->getMockBuilder('Composer\EventDispatcher\EventDispatcher')
             ->setConstructorArgs(array(
@@ -536,7 +536,7 @@ class EventDispatcherTest extends TestCase
         $dispatcher->dispatchScript(ScriptEvents::POST_INSTALL_CMD, false);
     }
 
-    public function testDispatcherOutputsErrorOnFailedCommand()
+    public function testDispatcherOutputsErrorOnFailedCommand(): void
     {
         $dispatcher = $this->getMockBuilder('Composer\EventDispatcher\EventDispatcher')
             ->setConstructorArgs(array(
@@ -572,7 +572,7 @@ class EventDispatcherTest extends TestCase
         $dispatcher->dispatchScript(ScriptEvents::POST_INSTALL_CMD, false);
     }
 
-    public function testDispatcherInstallerEvents()
+    public function testDispatcherInstallerEvents(): void
     {
         $dispatcher = $this->getMockBuilder('Composer\EventDispatcher\EventDispatcher')
             ->setConstructorArgs(array(
@@ -595,7 +595,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return void
      */
-    public static function call()
+    public static function call(): void
     {
         throw new \RuntimeException();
     }
@@ -603,7 +603,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return true
      */
-    public static function someMethod()
+    public static function someMethod(): bool
     {
         return true;
     }
@@ -611,7 +611,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return true
      */
-    public static function someMethod2()
+    public static function someMethod2(): bool
     {
         return true;
     }
@@ -619,7 +619,7 @@ class EventDispatcherTest extends TestCase
     /**
      * @return Composer
      */
-    private function createComposerInstance()
+    private function createComposerInstance(): Composer
     {
         $composer = new Composer;
         $config = new Config();
diff --git a/tests/Composer/Test/FactoryTest.php b/tests/Composer/Test/FactoryTest.php
index 96b0e95d5..ec41827fb 100644
--- a/tests/Composer/Test/FactoryTest.php
+++ b/tests/Composer/Test/FactoryTest.php
@@ -19,7 +19,7 @@ class FactoryTest extends TestCase
     /**
      * @group TLS
      */
-    public function testDefaultValuesAreAsExpected()
+    public function testDefaultValuesAreAsExpected(): void
     {
         $ioMock = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
 
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilterTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilterTest.php
index 8d97636a9..d7dcd9b33 100644
--- a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilterTest.php
+++ b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreAllPlatformRequirementFilterTest.php
@@ -23,7 +23,7 @@ final class IgnoreAllPlatformRequirementFilterTest extends TestCase
      * @param string $req
      * @param bool $expectIgnored
      */
-    public function testIsIgnored($req, $expectIgnored)
+    public function testIsIgnored($req, $expectIgnored): void
     {
         $platformRequirementFilter = new IgnoreAllPlatformRequirementFilter();
 
@@ -33,7 +33,7 @@ final class IgnoreAllPlatformRequirementFilterTest extends TestCase
     /**
      * @return array<string, mixed[]>
      */
-    public function dataIsIgnored()
+    public function dataIsIgnored(): array
     {
         return array(
             'php is ignored' => array('php', true),
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilterTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilterTest.php
index 3cf7469a4..47af67fd2 100644
--- a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilterTest.php
+++ b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilterTest.php
@@ -24,7 +24,7 @@ final class IgnoreListPlatformRequirementFilterTest extends TestCase
      * @param string $req
      * @param bool $expectIgnored
      */
-    public function testIsIgnored(array $reqList, $req, $expectIgnored)
+    public function testIsIgnored(array $reqList, $req, $expectIgnored): void
     {
         $platformRequirementFilter = new IgnoreListPlatformRequirementFilter($reqList);
 
@@ -34,7 +34,7 @@ final class IgnoreListPlatformRequirementFilterTest extends TestCase
     /**
      * @return array<string, mixed[]>
      */
-    public function dataIsIgnored()
+    public function dataIsIgnored(): array
     {
         return array(
             'ext-json is ignored if listed' => array(array('ext-json', 'monolog/monolog'), 'ext-json', true),
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilterTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilterTest.php
index 4698e94fa..8b5b1f897 100644
--- a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilterTest.php
+++ b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilterTest.php
@@ -22,7 +22,7 @@ final class IgnoreNothingPlatformRequirementFilterTest extends TestCase
      *
      * @param string $req
      */
-    public function testIsIgnored($req)
+    public function testIsIgnored($req): void
     {
         $platformRequirementFilter = new IgnoreNothingPlatformRequirementFilter();
 
@@ -32,7 +32,7 @@ final class IgnoreNothingPlatformRequirementFilterTest extends TestCase
     /**
      * @return array<string, mixed[]>
      */
-    public function dataIsIgnored()
+    public function dataIsIgnored(): array
     {
         return array(
             'php is not ignored' => array('php'),
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactoryTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactoryTest.php
index 44bcfce9e..d443532e3 100644
--- a/tests/Composer/Test/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactoryTest.php
+++ b/tests/Composer/Test/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactoryTest.php
@@ -23,7 +23,7 @@ final class PlatformRequirementFilterFactoryTest extends TestCase
      * @param mixed $boolOrList
      * @param class-string $expectedInstance
      */
-    public function testFromBoolOrList($boolOrList, $expectedInstance)
+    public function testFromBoolOrList($boolOrList, $expectedInstance): void
     {
         $this->assertInstanceOf($expectedInstance, PlatformRequirementFilterFactory::fromBoolOrList($boolOrList));
     }
@@ -31,7 +31,7 @@ final class PlatformRequirementFilterFactoryTest extends TestCase
     /**
      * @return array<string, mixed[]>
      */
-    public function dataFromBoolOrList()
+    public function dataFromBoolOrList(): array
     {
         return array(
             'true creates IgnoreAllFilter' => array(true, 'Composer\Filter\PlatformRequirementFilter\IgnoreAllPlatformRequirementFilter'),
@@ -40,7 +40,7 @@ final class PlatformRequirementFilterFactoryTest extends TestCase
         );
     }
 
-    public function testFromBoolThrowsExceptionIfTypeIsUnknown()
+    public function testFromBoolThrowsExceptionIfTypeIsUnknown(): void
     {
         self::expectException('InvalidArgumentException');
         self::expectExceptionMessage('PlatformRequirementFilter: Unknown $boolOrList parameter NULL. Please report at https://github.com/composer/composer/issues/new.');
@@ -48,14 +48,14 @@ final class PlatformRequirementFilterFactoryTest extends TestCase
         PlatformRequirementFilterFactory::fromBoolOrList(null);
     }
 
-    public function testIgnoreAll()
+    public function testIgnoreAll(): void
     {
         $platformRequirementFilter = PlatformRequirementFilterFactory::ignoreAll();
 
         $this->assertInstanceOf('Composer\Filter\PlatformRequirementFilter\IgnoreAllPlatformRequirementFilter', $platformRequirementFilter);
     }
 
-    public function testIgnoreNothing()
+    public function testIgnoreNothing(): void
     {
         $platformRequirementFilter = PlatformRequirementFilterFactory::ignoreNothing();
 
diff --git a/tests/Composer/Test/IO/BufferIOTest.php b/tests/Composer/Test/IO/BufferIOTest.php
index 52c11562a..8e86f1903 100644
--- a/tests/Composer/Test/IO/BufferIOTest.php
+++ b/tests/Composer/Test/IO/BufferIOTest.php
@@ -18,7 +18,7 @@ use Symfony\Component\Console\Input\StreamableInputInterface;
 
 class BufferIOTest extends TestCase
 {
-    public function testSetUserInputs()
+    public function testSetUserInputs(): void
     {
         $bufferIO = new BufferIO();
 
diff --git a/tests/Composer/Test/IO/ConsoleIOTest.php b/tests/Composer/Test/IO/ConsoleIOTest.php
index e275274cc..65995c884 100644
--- a/tests/Composer/Test/IO/ConsoleIOTest.php
+++ b/tests/Composer/Test/IO/ConsoleIOTest.php
@@ -19,7 +19,7 @@ use Symfony\Component\Console\Output\OutputInterface;
 
 class ConsoleIOTest extends TestCase
 {
-    public function testIsInteractive()
+    public function testIsInteractive(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $inputMock->expects($this->exactly(2))
@@ -38,7 +38,7 @@ class ConsoleIOTest extends TestCase
         $this->assertFalse($consoleIO->isInteractive());
     }
 
-    public function testWrite()
+    public function testWrite(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -54,7 +54,7 @@ class ConsoleIOTest extends TestCase
         $consoleIO->write('some information about something', false);
     }
 
-    public function testWriteError()
+    public function testWriteError(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\ConsoleOutputInterface')->getMock();
@@ -73,7 +73,7 @@ class ConsoleIOTest extends TestCase
         $consoleIO->writeError('some information about something', false);
     }
 
-    public function testWriteWithMultipleLineStringWhenDebugging()
+    public function testWriteWithMultipleLineStringWhenDebugging(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -83,7 +83,7 @@ class ConsoleIOTest extends TestCase
         $outputMock->expects($this->once())
             ->method('write')
             ->with(
-                $this->callback(function ($messages) {
+                $this->callback(function ($messages): bool {
                     $result = Preg::isMatch("[(.*)/(.*) First line]", $messages[0]);
                     $result = $result && Preg::isMatch("[(.*)/(.*) Second line]", $messages[1]);
 
@@ -101,7 +101,7 @@ class ConsoleIOTest extends TestCase
         $consoleIO->write($example, false);
     }
 
-    public function testOverwrite()
+    public function testOverwrite(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -129,7 +129,7 @@ class ConsoleIOTest extends TestCase
         $consoleIO->overwrite('something longer than initial (<info>34</info>)');
     }
 
-    public function testAsk()
+    public function testAsk(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -157,7 +157,7 @@ class ConsoleIOTest extends TestCase
         $consoleIO->ask('Why?', 'default');
     }
 
-    public function testAskConfirmation()
+    public function testAskConfirmation(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -185,7 +185,7 @@ class ConsoleIOTest extends TestCase
         $consoleIO->askConfirmation('Why?', false);
     }
 
-    public function testAskAndValidate()
+    public function testAskAndValidate(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -209,14 +209,14 @@ class ConsoleIOTest extends TestCase
             ->will($this->returnValue($helperMock))
         ;
 
-        $validator = function ($value) {
+        $validator = function ($value): bool {
             return true;
         };
         $consoleIO = new ConsoleIO($inputMock, $outputMock, $setMock);
         $consoleIO->askAndValidate('Why?', $validator, 10, 'default');
     }
 
-    public function testSelect()
+    public function testSelect(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -245,7 +245,7 @@ class ConsoleIOTest extends TestCase
         $this->assertEquals(array('1'), $result);
     }
 
-    public function testSetAndgetAuthentication()
+    public function testSetAndgetAuthentication(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -260,7 +260,7 @@ class ConsoleIOTest extends TestCase
         );
     }
 
-    public function testGetAuthenticationWhenDidNotSet()
+    public function testGetAuthenticationWhenDidNotSet(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
@@ -274,7 +274,7 @@ class ConsoleIOTest extends TestCase
         );
     }
 
-    public function testHasAuthentication()
+    public function testHasAuthentication(): void
     {
         $inputMock = $this->getMockBuilder('Symfony\Component\Console\Input\InputInterface')->getMock();
         $outputMock = $this->getMockBuilder('Symfony\Component\Console\Output\OutputInterface')->getMock();
diff --git a/tests/Composer/Test/IO/NullIOTest.php b/tests/Composer/Test/IO/NullIOTest.php
index eecdc0aec..b2ecc84d8 100644
--- a/tests/Composer/Test/IO/NullIOTest.php
+++ b/tests/Composer/Test/IO/NullIOTest.php
@@ -17,28 +17,28 @@ use Composer\Test\TestCase;
 
 class NullIOTest extends TestCase
 {
-    public function testIsInteractive()
+    public function testIsInteractive(): void
     {
         $io = new NullIO();
 
         $this->assertFalse($io->isInteractive());
     }
 
-    public function testhasAuthentication()
+    public function testhasAuthentication(): void
     {
         $io = new NullIO();
 
         $this->assertFalse($io->hasAuthentication('foo'));
     }
 
-    public function testAskAndHideAnswer()
+    public function testAskAndHideAnswer(): void
     {
         $io = new NullIO();
 
         $this->assertNull($io->askAndHideAnswer('foo'));
     }
 
-    public function testgetAuthentications()
+    public function testgetAuthentications(): void
     {
         $io = new NullIO();
 
@@ -47,30 +47,30 @@ class NullIOTest extends TestCase
         $this->assertEquals(array('username' => null, 'password' => null), $io->getAuthentication('foo'));
     }
 
-    public function testAsk()
+    public function testAsk(): void
     {
         $io = new NullIO();
 
         $this->assertEquals('foo', $io->ask('bar', 'foo'));
     }
 
-    public function testAskConfirmation()
+    public function testAskConfirmation(): void
     {
         $io = new NullIO();
 
         $this->assertEquals(false, $io->askConfirmation('bar', false));
     }
 
-    public function testAskAndValidate()
+    public function testAskAndValidate(): void
     {
         $io = new NullIO();
 
-        $this->assertEquals('foo', $io->askAndValidate('question', function ($x) {
+        $this->assertEquals('foo', $io->askAndValidate('question', function ($x): bool {
             return true;
         }, null, 'foo'));
     }
 
-    public function testSelect()
+    public function testSelect(): void
     {
         $io = new NullIO();
 
diff --git a/tests/Composer/Test/InstalledVersionsTest.php b/tests/Composer/Test/InstalledVersionsTest.php
index 3865beb0e..9cfb96faf 100644
--- a/tests/Composer/Test/InstalledVersionsTest.php
+++ b/tests/Composer/Test/InstalledVersionsTest.php
@@ -52,7 +52,7 @@ class InstalledVersionsTest extends TestCase
         InstalledVersions::reload(require __DIR__.'/Repository/Fixtures/installed.php');
     }
 
-    public function testGetInstalledPackages()
+    public function testGetInstalledPackages(): void
     {
         $names = array(
             '__root__',
@@ -74,12 +74,12 @@ class InstalledVersionsTest extends TestCase
      * @param string $name
      * @param bool $includeDevRequirements
      */
-    public function testIsInstalled($expected, $name, $includeDevRequirements = true)
+    public function testIsInstalled($expected, $name, $includeDevRequirements = true): void
     {
         $this->assertSame($expected, InstalledVersions::isInstalled($name, $includeDevRequirements));
     }
 
-    public static function isInstalledProvider()
+    public static function isInstalledProvider(): array
     {
         return array(
             array(true,  'foo/impl'),
@@ -99,12 +99,12 @@ class InstalledVersionsTest extends TestCase
      * @param string $name
      * @param string $constraint
      */
-    public function testSatisfies($expected, $name, $constraint)
+    public function testSatisfies($expected, $name, $constraint): void
     {
         $this->assertSame($expected, InstalledVersions::satisfies(new VersionParser, $name, $constraint));
     }
 
-    public static function satisfiesProvider()
+    public static function satisfiesProvider(): array
     {
         return array(
             array(true,  'foo/impl', '1.5'),
@@ -139,12 +139,12 @@ class InstalledVersionsTest extends TestCase
      * @param string $expected
      * @param string $name
      */
-    public function testGetVersionRanges($expected, $name)
+    public function testGetVersionRanges($expected, $name): void
     {
         $this->assertSame($expected, InstalledVersions::getVersionRanges($name));
     }
 
-    public static function getVersionRangesProvider()
+    public static function getVersionRangesProvider(): array
     {
         return array(
             array('dev-master || 1.10.x-dev',   '__root__'),
@@ -163,12 +163,12 @@ class InstalledVersionsTest extends TestCase
      * @param ?string $expected
      * @param string $name
      */
-    public function testGetVersion($expected, $name)
+    public function testGetVersion($expected, $name): void
     {
         $this->assertSame($expected, InstalledVersions::getVersion($name));
     }
 
-    public static function getVersionProvider()
+    public static function getVersionProvider(): array
     {
         return array(
             array('dev-master',  '__root__'),
@@ -187,12 +187,12 @@ class InstalledVersionsTest extends TestCase
      * @param ?string $expected
      * @param string $name
      */
-    public function testGetPrettyVersion($expected, $name)
+    public function testGetPrettyVersion($expected, $name): void
     {
         $this->assertSame($expected, InstalledVersions::getPrettyVersion($name));
     }
 
-    public static function getPrettyVersionProvider()
+    public static function getPrettyVersionProvider(): array
     {
         return array(
             array('dev-master',  '__root__'),
@@ -206,13 +206,13 @@ class InstalledVersionsTest extends TestCase
         );
     }
 
-    public function testGetVersionOutOfBounds()
+    public function testGetVersionOutOfBounds(): void
     {
         self::expectException('OutOfBoundsException');
         InstalledVersions::getVersion('not/installed');
     }
 
-    public function testGetRootPackage()
+    public function testGetRootPackage(): void
     {
         $this->assertSame(array(
             'pretty_version' => 'dev-master',
@@ -231,7 +231,7 @@ class InstalledVersionsTest extends TestCase
     /**
      * @group legacy
      */
-    public function testGetRawData()
+    public function testGetRawData(): void
     {
         $dir = $this->root;
         $this->assertSame(require __DIR__.'/Repository/Fixtures/installed.php', InstalledVersions::getRawData());
@@ -242,12 +242,12 @@ class InstalledVersionsTest extends TestCase
      * @param ?string $expected
      * @param string $name
      */
-    public function testGetReference($expected, $name)
+    public function testGetReference($expected, $name): void
     {
         $this->assertSame($expected, InstalledVersions::getReference($name));
     }
 
-    public static function getReferenceProvider()
+    public static function getReferenceProvider(): array
     {
         return array(
             array('sourceref-by-default',  '__root__'),
@@ -261,7 +261,7 @@ class InstalledVersionsTest extends TestCase
         );
     }
 
-    public function testGetInstalledPackagesByType()
+    public function testGetInstalledPackagesByType(): void
     {
         $names = array(
             '__root__',
@@ -274,7 +274,7 @@ class InstalledVersionsTest extends TestCase
         $this->assertSame($names, \Composer\InstalledVersions::getInstalledPackagesByType('library'));
     }
 
-    public function testGetInstallPath()
+    public function testGetInstallPath(): void
     {
         $this->assertSame(realpath($this->root), realpath(\Composer\InstalledVersions::getInstallPath('__root__')));
         $this->assertSame('/foo/bar/vendor/c/c', \Composer\InstalledVersions::getInstallPath('c/c'));
diff --git a/tests/Composer/Test/Installer/BinaryInstallerTest.php b/tests/Composer/Test/Installer/BinaryInstallerTest.php
index 5bb991f90..821bc3ce4 100644
--- a/tests/Composer/Test/Installer/BinaryInstallerTest.php
+++ b/tests/Composer/Test/Installer/BinaryInstallerTest.php
@@ -68,7 +68,7 @@ class BinaryInstallerTest extends TestCase
      * @dataProvider executableBinaryProvider
      * @param string $contents
      */
-    public function testInstallAndExecBinaryWithFullCompat($contents)
+    public function testInstallAndExecBinaryWithFullCompat($contents): void
     {
         $package = $this->createPackageMock();
         $package->expects($this->any())
@@ -87,7 +87,7 @@ class BinaryInstallerTest extends TestCase
         $this->assertEquals('success arg', $output);
     }
 
-    public function executableBinaryProvider()
+    public function executableBinaryProvider(): array
     {
         return array(
             'simple php file' => array(<<<'EOL'
diff --git a/tests/Composer/Test/Installer/InstallationManagerTest.php b/tests/Composer/Test/Installer/InstallationManagerTest.php
index 4447919a3..91057a72a 100644
--- a/tests/Composer/Test/Installer/InstallationManagerTest.php
+++ b/tests/Composer/Test/Installer/InstallationManagerTest.php
@@ -43,14 +43,14 @@ class InstallationManagerTest extends TestCase
         $this->io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
     }
 
-    public function testAddGetInstaller()
+    public function testAddGetInstaller(): void
     {
         $installer = $this->createInstallerMock();
 
         $installer
             ->expects($this->exactly(2))
             ->method('supports')
-            ->will($this->returnCallback(function ($arg) {
+            ->will($this->returnCallback(function ($arg): bool {
                 return $arg === 'vendor';
             }));
 
@@ -63,14 +63,14 @@ class InstallationManagerTest extends TestCase
         $manager->getInstaller('unregistered');
     }
 
-    public function testAddRemoveInstaller()
+    public function testAddRemoveInstaller(): void
     {
         $installer = $this->createInstallerMock();
 
         $installer
             ->expects($this->exactly(2))
             ->method('supports')
-            ->will($this->returnCallback(function ($arg) {
+            ->will($this->returnCallback(function ($arg): bool {
                 return $arg === 'vendor';
             }));
 
@@ -79,7 +79,7 @@ class InstallationManagerTest extends TestCase
         $installer2
             ->expects($this->exactly(1))
             ->method('supports')
-            ->will($this->returnCallback(function ($arg) {
+            ->will($this->returnCallback(function ($arg): bool {
                 return $arg === 'vendor';
             }));
 
@@ -93,7 +93,7 @@ class InstallationManagerTest extends TestCase
         $this->assertSame($installer, $manager->getInstaller('vendor'));
     }
 
-    public function testExecute()
+    public function testExecute(): void
     {
         $manager = $this->getMockBuilder('Composer\Installer\InstallationManager')
             ->setConstructorArgs(array($this->loop, $this->io))
@@ -128,7 +128,7 @@ class InstallationManagerTest extends TestCase
         $manager->execute($this->repository, array($installOperation, $removeOperation, $updateOperation));
     }
 
-    public function testInstall()
+    public function testInstall(): void
     {
         $installer = $this->createInstallerMock();
         $manager = new InstallationManager($this->loop, $this->io);
@@ -156,7 +156,7 @@ class InstallationManagerTest extends TestCase
         $manager->install($this->repository, $operation);
     }
 
-    public function testUpdateWithEqualTypes()
+    public function testUpdateWithEqualTypes(): void
     {
         $installer = $this->createInstallerMock();
         $manager = new InstallationManager($this->loop, $this->io);
@@ -189,7 +189,7 @@ class InstallationManagerTest extends TestCase
         $manager->update($this->repository, $operation);
     }
 
-    public function testUpdateWithNotEqualTypes()
+    public function testUpdateWithNotEqualTypes(): void
     {
         $libInstaller = $this->createInstallerMock();
         $bundleInstaller = $this->createInstallerMock();
@@ -212,7 +212,7 @@ class InstallationManagerTest extends TestCase
         $bundleInstaller
             ->expects($this->exactly(2))
             ->method('supports')
-            ->will($this->returnCallback(function ($arg) {
+            ->will($this->returnCallback(function ($arg): bool {
                 return $arg === 'bundles';
             }));
 
@@ -236,7 +236,7 @@ class InstallationManagerTest extends TestCase
         $manager->update($this->repository, $operation);
     }
 
-    public function testUninstall()
+    public function testUninstall(): void
     {
         $installer = $this->createInstallerMock();
         $manager = new InstallationManager($this->loop, $this->io);
@@ -264,7 +264,7 @@ class InstallationManagerTest extends TestCase
         $manager->uninstall($this->repository, $operation);
     }
 
-    public function testInstallBinary()
+    public function testInstallBinary(): void
     {
         $installer = $this->getMockBuilder('Composer\Installer\LibraryInstaller')
             ->disableOriginalConstructor()
diff --git a/tests/Composer/Test/Installer/InstallerEventTest.php b/tests/Composer/Test/Installer/InstallerEventTest.php
index 6c0689844..d2146f3a7 100644
--- a/tests/Composer/Test/Installer/InstallerEventTest.php
+++ b/tests/Composer/Test/Installer/InstallerEventTest.php
@@ -17,7 +17,7 @@ use Composer\Test\TestCase;
 
 class InstallerEventTest extends TestCase
 {
-    public function testGetter()
+    public function testGetter(): void
     {
         $composer = $this->getMockBuilder('Composer\Composer')->getMock();
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
diff --git a/tests/Composer/Test/Installer/LibraryInstallerTest.php b/tests/Composer/Test/Installer/LibraryInstallerTest.php
index a302c0b90..5f2cc55f8 100644
--- a/tests/Composer/Test/Installer/LibraryInstallerTest.php
+++ b/tests/Composer/Test/Installer/LibraryInstallerTest.php
@@ -13,6 +13,7 @@
 namespace Composer\Test\Installer;
 
 use Composer\Installer\LibraryInstaller;
+use Composer\Repository\InstalledArrayRepository;
 use Composer\Util\Filesystem;
 use Composer\Test\TestCase;
 use Composer\Composer;
@@ -75,10 +76,10 @@ class LibraryInstallerTest extends TestCase
 
         $this->rootDir = $this->getUniqueTmpDirectory();
         $this->vendorDir = $this->rootDir.DIRECTORY_SEPARATOR.'vendor';
-        $this->ensureDirectoryExistsAndClear($this->vendorDir);
+        self::ensureDirectoryExistsAndClear($this->vendorDir);
 
         $this->binDir = $this->rootDir.DIRECTORY_SEPARATOR.'bin';
-        $this->ensureDirectoryExistsAndClear($this->binDir);
+        self::ensureDirectoryExistsAndClear($this->binDir);
 
         $this->config->merge(array(
             'config' => array(
@@ -102,7 +103,7 @@ class LibraryInstallerTest extends TestCase
         $this->fs->removeDirectory($this->rootDir);
     }
 
-    public function testInstallerCreationShouldNotCreateVendorDirectory()
+    public function testInstallerCreationShouldNotCreateVendorDirectory(): void
     {
         $this->fs->removeDirectory($this->vendorDir);
 
@@ -110,7 +111,7 @@ class LibraryInstallerTest extends TestCase
         $this->assertFileDoesNotExist($this->vendorDir);
     }
 
-    public function testInstallerCreationShouldNotCreateBinDirectory()
+    public function testInstallerCreationShouldNotCreateBinDirectory(): void
     {
         $this->fs->removeDirectory($this->binDir);
 
@@ -118,34 +119,34 @@ class LibraryInstallerTest extends TestCase
         $this->assertFileDoesNotExist($this->binDir);
     }
 
-    public function testIsInstalled()
+    public function testIsInstalled(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
-        $package = $this->createPackageMock();
+        $package = $this->getPackage('test/pkg', '1.0.0');
 
-        $this->repository
-            ->expects($this->exactly(2))
-            ->method('hasPackage')
-            ->with($package)
-            ->will($this->onConsecutiveCalls(true, false));
+        $repository = new InstalledArrayRepository();
+        $this->assertFalse($library->isInstalled($repository, $package));
 
-        $this->assertTrue($library->isInstalled($this->repository, $package));
-        $this->assertFalse($library->isInstalled($this->repository, $package));
+        // package being in repo is not enough to be installed
+        $repository->addPackage($package);
+        $this->assertFalse($library->isInstalled($repository, $package));
+
+        // package being in repo and vendor/pkg/foo dir present means it is seen as installed
+        self::ensureDirectoryExistsAndClear($this->vendorDir.'/'.$package->getPrettyName());
+        $this->assertTrue($library->isInstalled($repository, $package));
+
+        $repository->removePackage($package);
+        $this->assertFalse($library->isInstalled($repository, $package));
     }
 
     /**
      * @depends testInstallerCreationShouldNotCreateVendorDirectory
      * @depends testInstallerCreationShouldNotCreateBinDirectory
      */
-    public function testInstall()
+    public function testInstall(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
-        $package = $this->createPackageMock();
-
-        $package
-            ->expects($this->any())
-            ->method('getPrettyName')
-            ->will($this->returnValue('some/package'));
+        $package = $this->getPackage('some/package', '1.0.0');
 
         $this->dm
             ->expects($this->once())
@@ -166,37 +167,20 @@ class LibraryInstallerTest extends TestCase
      * @depends testInstallerCreationShouldNotCreateVendorDirectory
      * @depends testInstallerCreationShouldNotCreateBinDirectory
      */
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $filesystem = $this->getMockBuilder('Composer\Util\Filesystem')
           ->getMock();
         $filesystem
           ->expects($this->once())
           ->method('rename')
-          ->with($this->vendorDir.'/package1/oldtarget', $this->vendorDir.'/package1/newtarget');
+          ->with($this->vendorDir.'/vendor/package1/oldtarget', $this->vendorDir.'/vendor/package1/newtarget');
 
-        $initial = $this->createPackageMock();
-        $target = $this->createPackageMock();
+        $initial = $this->getPackage('vendor/package1', '1.0.0');
+        $target = $this->getPackage('vendor/package1', '2.0.0');
 
-        $initial
-            ->expects($this->any())
-            ->method('getPrettyName')
-            ->will($this->returnValue('package1'));
-
-        $initial
-            ->expects($this->any())
-            ->method('getTargetDir')
-            ->will($this->returnValue('oldtarget'));
-
-        $target
-            ->expects($this->any())
-            ->method('getPrettyName')
-            ->will($this->returnValue('package1'));
-
-        $target
-            ->expects($this->any())
-            ->method('getTargetDir')
-            ->will($this->returnValue('newtarget'));
+        $initial->setTargetDir('oldtarget');
+        $target->setTargetDir('newtarget');
 
         $this->repository
             ->expects($this->exactly(3))
@@ -206,7 +190,7 @@ class LibraryInstallerTest extends TestCase
         $this->dm
             ->expects($this->once())
             ->method('update')
-            ->with($initial, $target, $this->vendorDir.'/package1/newtarget');
+            ->with($initial, $target, $this->vendorDir.'/vendor/package1/newtarget');
 
         $this->repository
             ->expects($this->once())
@@ -228,19 +212,10 @@ class LibraryInstallerTest extends TestCase
         $library->update($this->repository, $initial, $target);
     }
 
-    public function testUninstall()
+    public function testUninstall(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
-        $package = $this->createPackageMock();
-
-        $package
-            ->expects($this->any())
-            ->method('getPrettyName')
-            ->will($this->returnValue('pkg'));
-        $package
-            ->expects($this->any())
-            ->method('getName')
-            ->will($this->returnValue('pkg'));
+        $package = $this->getPackage('vendor/pkg', '1.0.0');
 
         $this->repository
             ->expects($this->exactly(2))
@@ -251,7 +226,7 @@ class LibraryInstallerTest extends TestCase
         $this->dm
             ->expects($this->once())
             ->method('remove')
-            ->with($package, $this->vendorDir.'/pkg');
+            ->with($package, $this->vendorDir.'/vendor/pkg');
 
         $this->repository
             ->expects($this->once())
@@ -265,32 +240,19 @@ class LibraryInstallerTest extends TestCase
         $library->uninstall($this->repository, $package);
     }
 
-    public function testGetInstallPath()
+    public function testGetInstallPathWithoutTargetDir(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
-        $package = $this->createPackageMock();
+        $package = $this->getPackage('Vendor/Pkg', '1.0.0');
 
-        $package
-            ->expects($this->once())
-            ->method('getTargetDir')
-            ->will($this->returnValue(null));
-
-        $this->assertEquals($this->vendorDir.'/'.$package->getName(), $library->getInstallPath($package));
+        $this->assertEquals($this->vendorDir.'/'.$package->getPrettyName(), $library->getInstallPath($package));
     }
 
-    public function testGetInstallPathWithTargetDir()
+    public function testGetInstallPathWithTargetDir(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
-        $package = $this->createPackageMock();
-
-        $package
-            ->expects($this->once())
-            ->method('getTargetDir')
-            ->will($this->returnValue('Some/Namespace'));
-        $package
-            ->expects($this->any())
-            ->method('getPrettyName')
-            ->will($this->returnValue('foo/bar'));
+        $package = $this->getPackage('Foo/Bar', '1.0.0');
+        $package->setTargetDir('Some/Namespace');
 
         $this->assertEquals($this->vendorDir.'/'.$package->getPrettyName().'/Some/Namespace', $library->getInstallPath($package));
     }
@@ -299,14 +261,14 @@ class LibraryInstallerTest extends TestCase
      * @depends testInstallerCreationShouldNotCreateVendorDirectory
      * @depends testInstallerCreationShouldNotCreateBinDirectory
      */
-    public function testEnsureBinariesInstalled()
+    public function testEnsureBinariesInstalled(): void
     {
         $binaryInstallerMock = $this->getMockBuilder('Composer\Installer\BinaryInstaller')
             ->disableOriginalConstructor()
             ->getMock();
 
         $library = new LibraryInstaller($this->io, $this->composer, 'library', null, $binaryInstallerMock);
-        $package = $this->createPackageMock();
+        $package = $this->getPackage('foo/bar', '1.0.0');
 
         $binaryInstallerMock
             ->expects($this->never())
@@ -320,14 +282,4 @@ class LibraryInstallerTest extends TestCase
 
         $library->ensureBinariesPresence($package);
     }
-
-    /**
-     * @return \Composer\Package\PackageInterface&\PHPUnit\Framework\MockObject\MockObject
-     */
-    protected function createPackageMock()
-    {
-        return $this->getMockBuilder('Composer\Package\Package')
-            ->setConstructorArgs(array(md5((string) mt_rand()), '1.0.0.0', '1.0.0'))
-            ->getMock();
-    }
 }
diff --git a/tests/Composer/Test/Installer/MetapackageInstallerTest.php b/tests/Composer/Test/Installer/MetapackageInstallerTest.php
index e66bf28d3..0b0b2035a 100644
--- a/tests/Composer/Test/Installer/MetapackageInstallerTest.php
+++ b/tests/Composer/Test/Installer/MetapackageInstallerTest.php
@@ -39,7 +39,7 @@ class MetapackageInstallerTest extends TestCase
         $this->installer = new MetapackageInstaller($this->io);
     }
 
-    public function testInstall()
+    public function testInstall(): void
     {
         $package = $this->createPackageMock();
 
@@ -51,7 +51,7 @@ class MetapackageInstallerTest extends TestCase
         $this->installer->install($this->repository, $package);
     }
 
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $initial = $this->createPackageMock();
         $initial->expects($this->once())
@@ -85,7 +85,7 @@ class MetapackageInstallerTest extends TestCase
         $this->installer->update($this->repository, $initial, $target);
     }
 
-    public function testUninstall()
+    public function testUninstall(): void
     {
         $package = $this->createPackageMock();
 
diff --git a/tests/Composer/Test/Installer/SuggestedPackagesReporterTest.php b/tests/Composer/Test/Installer/SuggestedPackagesReporterTest.php
index 47152ff90..5cd693d4f 100644
--- a/tests/Composer/Test/Installer/SuggestedPackagesReporterTest.php
+++ b/tests/Composer/Test/Installer/SuggestedPackagesReporterTest.php
@@ -42,7 +42,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::__construct
      */
-    public function testConstructor()
+    public function testConstructor(): void
     {
         $this->io->expects($this->once())
             ->method('write');
@@ -54,7 +54,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::getPackages
      */
-    public function testGetPackagesEmptyByDefault()
+    public function testGetPackagesEmptyByDefault(): void
     {
         $this->assertEmpty($this->suggestedPackagesReporter->getPackages());
     }
@@ -63,7 +63,7 @@ class SuggestedPackagesReporterTest extends TestCase
      * @covers ::getPackages
      * @covers ::addPackage
      */
-    public function testGetPackages()
+    public function testGetPackages(): void
     {
         $suggestedPackage = $this->getSuggestedPackageArray();
         $this->suggestedPackagesReporter->addPackage(
@@ -83,7 +83,7 @@ class SuggestedPackagesReporterTest extends TestCase
      *
      * @covers ::addPackage
      */
-    public function testAddPackageAppends()
+    public function testAddPackageAppends(): void
     {
         $suggestedPackageA = $this->getSuggestedPackageArray();
         $suggestedPackageB = $this->getSuggestedPackageArray();
@@ -108,7 +108,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::addSuggestionsFromPackage
      */
-    public function testAddSuggestionsFromPackage()
+    public function testAddSuggestionsFromPackage(): void
     {
         $package = $this->createPackageMock();
         $package->expects($this->once())
@@ -139,7 +139,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::output
      */
-    public function testOutput()
+    public function testOutput(): void
     {
         $this->suggestedPackagesReporter->addPackage('a', 'b', 'c');
 
@@ -157,7 +157,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::output
      */
-    public function testOutputWithNoSuggestionReason()
+    public function testOutputWithNoSuggestionReason(): void
     {
         $this->suggestedPackagesReporter->addPackage('a', 'b', '');
 
@@ -175,7 +175,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::output
      */
-    public function testOutputIgnoresFormatting()
+    public function testOutputIgnoresFormatting(): void
     {
         $this->suggestedPackagesReporter->addPackage('source', 'target1', "\x1b[1;37;42m Like us\r\non Facebook \x1b[0m");
         $this->suggestedPackagesReporter->addPackage('source', 'target2', "<bg=green>Like us on Facebook</>");
@@ -199,7 +199,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::output
      */
-    public function testOutputMultiplePackages()
+    public function testOutputMultiplePackages(): void
     {
         $this->suggestedPackagesReporter->addPackage('a', 'b', 'c');
         $this->suggestedPackagesReporter->addPackage('source package', 'target', 'because reasons');
@@ -221,7 +221,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::output
      */
-    public function testOutputSkipInstalledPackages()
+    public function testOutputSkipInstalledPackages(): void
     {
         $repository = $this->getMockBuilder('Composer\Repository\InstalledRepository')->disableOriginalConstructor()->getMock();
         $package1 = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
@@ -259,7 +259,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @covers ::output
      */
-    public function testOutputNotGettingInstalledPackagesWhenNoSuggestions()
+    public function testOutputNotGettingInstalledPackagesWhenNoSuggestions(): void
     {
         $repository = $this->getMockBuilder('Composer\Repository\InstalledRepository')->disableOriginalConstructor()->getMock();
         $repository->expects($this->exactly(0))
@@ -271,7 +271,7 @@ class SuggestedPackagesReporterTest extends TestCase
     /**
      * @return array<string, string>
      */
-    private function getSuggestedPackageArray()
+    private function getSuggestedPackageArray(): array
     {
         return array(
             'source' => 'a',
diff --git a/tests/Composer/Test/InstallerTest.php b/tests/Composer/Test/InstallerTest.php
index c0ea9b1af..a38d02424 100644
--- a/tests/Composer/Test/InstallerTest.php
+++ b/tests/Composer/Test/InstallerTest.php
@@ -74,7 +74,7 @@ class InstallerTest extends TestCase
      * @param RepositoryInterface[] $repositories
      * @param mixed[] $options
      */
-    public function testInstaller(RootPackageInterface $rootPackage, $repositories, array $options)
+    public function testInstaller(RootPackageInterface $rootPackage, $repositories, array $options): void
     {
         $io = new BufferIO('', OutputInterface::VERBOSITY_NORMAL, new OutputFormatter(false));
 
@@ -122,12 +122,12 @@ class InstallerTest extends TestCase
             }));
         $lockJsonMock->expects($this->any())
             ->method('exists')
-            ->will($this->returnCallback(function () use (&$lockData) {
+            ->will($this->returnCallback(function () use (&$lockData): bool {
                 return $lockData !== null;
             }));
         $lockJsonMock->expects($this->any())
             ->method('write')
-            ->will($this->returnCallback(function ($value, $options = 0) use (&$lockData) {
+            ->will($this->returnCallback(function ($value, $options = 0) use (&$lockData): void {
                 $lockData = json_encode($value, JSON_PRETTY_PRINT);
             }));
 
@@ -160,7 +160,7 @@ class InstallerTest extends TestCase
      * @param  PackageInterface[] $packages
      * @return mixed[]
      */
-    protected function makePackagesComparable($packages)
+    protected function makePackagesComparable($packages): array
     {
         $dumper = new ArrayDumper();
 
@@ -172,7 +172,7 @@ class InstallerTest extends TestCase
         return $comparable;
     }
 
-    public function provideInstaller()
+    public function provideInstaller(): array
     {
         $cases = array();
 
@@ -237,7 +237,7 @@ class InstallerTest extends TestCase
      * @param string $expect
      * @param int|class-string<\Throwable> $expectResult
      */
-    public function testSlowIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult)
+    public function testSlowIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult): void
     {
         Platform::putEnv('COMPOSER_POOL_OPTIMIZER', '0');
 
@@ -260,7 +260,7 @@ class InstallerTest extends TestCase
      * @param string $expect
      * @param int|class-string<\Throwable> $expectResult
      */
-    public function testIntegrationWithPoolOptimizer($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult)
+    public function testIntegrationWithPoolOptimizer($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult): void
     {
         Platform::putEnv('COMPOSER_POOL_OPTIMIZER', '1');
 
@@ -283,7 +283,7 @@ class InstallerTest extends TestCase
      * @param string $expect
      * @param int|class-string<\Throwable> $expectResult
      */
-    public function testIntegrationWithRawPool($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult)
+    public function testIntegrationWithRawPool($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expectOutputOptimized, $expect, $expectResult): void
     {
         Platform::putEnv('COMPOSER_POOL_OPTIMIZER', '0');
 
@@ -305,11 +305,11 @@ class InstallerTest extends TestCase
      * @param int|class-string<\Throwable> $expectResult
      * @return void
      */
-    private function doTestIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult)
+    private function doTestIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectInstalled, $expectOutput, $expect, $expectResult): void
     {
         if ($condition) {
             eval('$res = '.$condition.';');
-            if (!$res) {
+            if (!$res) { // @phpstan-ignore-line
                 $this->markTestSkipped($condition);
             }
         }
@@ -348,12 +348,12 @@ class InstallerTest extends TestCase
             }));
         $lockJsonMock->expects($this->any())
             ->method('exists')
-            ->will($this->returnCallback(function () use (&$lockData) {
+            ->will($this->returnCallback(function () use (&$lockData): bool {
                 return $lockData !== null;
             }));
         $lockJsonMock->expects($this->any())
             ->method('write')
-            ->will($this->returnCallback(function ($value, $options = 0) use (&$lockData) {
+            ->will($this->returnCallback(function ($value, $options = 0) use (&$lockData): void {
                 $lockData = json_encode($value, JSON_PRETTY_PRINT);
             }));
 
@@ -361,7 +361,7 @@ class InstallerTest extends TestCase
             $actualLock = array();
             $lockJsonMock->expects($this->atLeastOnce())
                 ->method('write')
-                ->will($this->returnCallback(function ($hash, $options) use (&$actualLock) {
+                ->will($this->returnCallback(function ($hash, $options) use (&$actualLock): void {
                     // need to do assertion outside of mock for nice phpunit output
                     // so store value temporarily in reference for later assetion
                     $actualLock = $hash;
@@ -390,7 +390,7 @@ class InstallerTest extends TestCase
         $install->addOption('ignore-platform-req', null, InputOption::VALUE_REQUIRED | InputOption::VALUE_IS_ARRAY);
         $install->addOption('no-dev', null, InputOption::VALUE_NONE);
         $install->addOption('dry-run', null, InputOption::VALUE_NONE);
-        $install->setCode(function ($input, $output) use ($installer) {
+        $install->setCode(function ($input, $output) use ($installer): int {
             $ignorePlatformReqs = $input->getOption('ignore-platform-reqs') ?: ($input->getOption('ignore-platform-req') ?: false);
 
             $installer
@@ -414,9 +414,9 @@ class InstallerTest extends TestCase
         $update->addOption('prefer-stable', null, InputOption::VALUE_NONE);
         $update->addOption('prefer-lowest', null, InputOption::VALUE_NONE);
         $update->addArgument('packages', InputArgument::IS_ARRAY | InputArgument::OPTIONAL);
-        $update->setCode(function ($input, $output) use ($installer) {
+        $update->setCode(function ($input, $output) use ($installer): int {
             $packages = $input->getArgument('packages');
-            $filteredPackages = array_filter($packages, function ($package) {
+            $filteredPackages = array_filter($packages, function ($package): bool {
                 return !in_array($package, array('lock', 'nothing', 'mirrors'), true);
             });
             $updateMirrors = $input->getOption('lock') || count($filteredPackages) != count($packages);
@@ -483,7 +483,7 @@ class InstallerTest extends TestCase
                 $actualInstalled[] = $package;
             }
 
-            usort($actualInstalled, function ($a, $b) {
+            usort($actualInstalled, function ($a, $b): int {
                 return strcmp($a['name'], $b['name']);
             });
 
@@ -502,12 +502,12 @@ class InstallerTest extends TestCase
         }
     }
 
-    public function provideSlowIntegrationTests()
+    public function provideSlowIntegrationTests(): array
     {
         return $this->loadIntegrationTests('installer-slow/');
     }
 
-    public function provideIntegrationTests()
+    public function provideIntegrationTests(): array
     {
         return $this->loadIntegrationTests('installer/');
     }
@@ -516,7 +516,7 @@ class InstallerTest extends TestCase
      * @param  string $path
      * @return mixed[]
      */
-    public function loadIntegrationTests($path)
+    public function loadIntegrationTests($path): array
     {
         $fixturesDir = realpath(__DIR__.'/Fixtures/'.$path);
         $tests = array();
@@ -602,7 +602,7 @@ class InstallerTest extends TestCase
      * @param  string $fixturesDir
      * @return mixed[]
      */
-    protected function readTestFile(\SplFileInfo $file, $fixturesDir)
+    protected function readTestFile(\SplFileInfo $file, $fixturesDir): array
     {
         $tokens = Preg::split('#(?:^|\n*)--([A-Z-]+)--\n#', file_get_contents($file->getRealPath()), -1, PREG_SPLIT_DELIM_CAPTURE);
 
diff --git a/tests/Composer/Test/Json/ComposerSchemaTest.php b/tests/Composer/Test/Json/ComposerSchemaTest.php
index e45da5df3..474523b46 100644
--- a/tests/Composer/Test/Json/ComposerSchemaTest.php
+++ b/tests/Composer/Test/Json/ComposerSchemaTest.php
@@ -20,7 +20,7 @@ use Composer\Test\TestCase;
  */
 class ComposerSchemaTest extends TestCase
 {
-    public function testNamePattern()
+    public function testNamePattern(): void
     {
         $expectedError = array(
             array(
@@ -37,13 +37,13 @@ class ComposerSchemaTest extends TestCase
         $this->assertEquals($expectedError, $this->check($json));
     }
 
-    public function testOptionalAbandonedProperty()
+    public function testOptionalAbandonedProperty(): void
     {
         $json = '{"name": "vendor/package", "description": "description", "abandoned": true}';
         $this->assertTrue($this->check($json));
     }
 
-    public function testRequireTypes()
+    public function testRequireTypes(): void
     {
         $json = '{"name": "vendor/package", "description": "description", "require": {"a": ["b"]} }';
         $this->assertEquals(array(
@@ -51,7 +51,7 @@ class ComposerSchemaTest extends TestCase
         ), $this->check($json));
     }
 
-    public function testMinimumStabilityValues()
+    public function testMinimumStabilityValues(): void
     {
         $expectedError = array(
             array(
diff --git a/tests/Composer/Test/Json/JsonFileTest.php b/tests/Composer/Test/Json/JsonFileTest.php
index b0242622f..131730c9d 100644
--- a/tests/Composer/Test/Json/JsonFileTest.php
+++ b/tests/Composer/Test/Json/JsonFileTest.php
@@ -19,7 +19,7 @@ use Composer\Test\TestCase;
 
 class JsonFileTest extends TestCase
 {
-    public function testParseErrorDetectExtraComma()
+    public function testParseErrorDetectExtraComma(): void
     {
         $json = '{
         "foo": "bar",
@@ -27,7 +27,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 2', $json);
     }
 
-    public function testParseErrorDetectExtraCommaInArray()
+    public function testParseErrorDetectExtraCommaInArray(): void
     {
         $json = '{
         "foo": [
@@ -37,7 +37,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 3', $json);
     }
 
-    public function testParseErrorDetectUnescapedBackslash()
+    public function testParseErrorDetectUnescapedBackslash(): void
     {
         $json = '{
         "fo\o": "bar"
@@ -45,7 +45,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 1', $json);
     }
 
-    public function testParseErrorSkipsEscapedBackslash()
+    public function testParseErrorSkipsEscapedBackslash(): void
     {
         $json = '{
         "fo\\\\o": "bar"
@@ -54,7 +54,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 2', $json);
     }
 
-    public function testParseErrorDetectSingleQuotes()
+    public function testParseErrorDetectSingleQuotes(): void
     {
         if (defined('JSON_PARSER_NOTSTRICT') && version_compare(phpversion('json'), '1.3.9', '<')) {
             $this->markTestSkipped('jsonc issue, see https://github.com/remicollet/pecl-json-c/issues/23');
@@ -65,7 +65,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 1', $json);
     }
 
-    public function testParseErrorDetectMissingQuotes()
+    public function testParseErrorDetectMissingQuotes(): void
     {
         $json = '{
         foo: "bar"
@@ -73,7 +73,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 1', $json);
     }
 
-    public function testParseErrorDetectArrayAsHash()
+    public function testParseErrorDetectArrayAsHash(): void
     {
         $json = '{
         "foo": ["bar": "baz"]
@@ -81,7 +81,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 2', $json);
     }
 
-    public function testParseErrorDetectMissingComma()
+    public function testParseErrorDetectMissingComma(): void
     {
         $json = '{
         "foo": "bar"
@@ -90,14 +90,14 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 2', $json);
     }
 
-    public function testSchemaValidation()
+    public function testSchemaValidation(): void
     {
         $json = new JsonFile(__DIR__.'/Fixtures/composer.json');
         $this->assertTrue($json->validateSchema());
         $this->assertTrue($json->validateSchema(JsonFile::LAX_SCHEMA));
     }
 
-    public function testSchemaValidationError()
+    public function testSchemaValidationError(): void
     {
         $file = $this->createTempFile();
         file_put_contents($file, '{ "name": null }');
@@ -121,7 +121,7 @@ class JsonFileTest extends TestCase
         unlink($file);
     }
 
-    public function testSchemaValidationLaxAdditionalProperties()
+    public function testSchemaValidationLaxAdditionalProperties(): void
     {
         $file = $this->createTempFile();
         file_put_contents($file, '{ "name": "vendor/package", "description": "generic description", "foo": "bar" }');
@@ -137,7 +137,7 @@ class JsonFileTest extends TestCase
         unlink($file);
     }
 
-    public function testSchemaValidationLaxRequired()
+    public function testSchemaValidationLaxRequired(): void
     {
         $file = $this->createTempFile();
         $json = new JsonFile($file);
@@ -207,7 +207,7 @@ class JsonFileTest extends TestCase
         unlink($file);
     }
 
-    public function testCustomSchemaValidationLax()
+    public function testCustomSchemaValidationLax(): void
     {
         $file = $this->createTempFile();
         file_put_contents($file, '{ "custom": "property", "another custom": "property" }');
@@ -223,7 +223,7 @@ class JsonFileTest extends TestCase
         unlink($schema);
     }
 
-    public function testCustomSchemaValidationStrict()
+    public function testCustomSchemaValidationStrict(): void
     {
         $file = $this->createTempFile();
         file_put_contents($file, '{ "custom": "property" }');
@@ -239,7 +239,7 @@ class JsonFileTest extends TestCase
         unlink($schema);
     }
 
-    public function testParseErrorDetectMissingCommaMultiline()
+    public function testParseErrorDetectMissingCommaMultiline(): void
     {
         $json = '{
         "foo": "barbar"
@@ -249,7 +249,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 2', $json);
     }
 
-    public function testParseErrorDetectMissingColon()
+    public function testParseErrorDetectMissingColon(): void
     {
         $json = '{
         "foo": "bar",
@@ -258,7 +258,7 @@ class JsonFileTest extends TestCase
         $this->expectParseException('Parse error on line 3', $json);
     }
 
-    public function testSimpleJsonString()
+    public function testSimpleJsonString(): void
     {
         $data = array('name' => 'composer/composer');
         $json = '{
@@ -267,7 +267,7 @@ class JsonFileTest extends TestCase
         $this->assertJsonFormat($json, $data);
     }
 
-    public function testTrailingBackslash()
+    public function testTrailingBackslash(): void
     {
         $data = array('Metadata\\' => 'src/');
         $json = '{
@@ -276,7 +276,7 @@ class JsonFileTest extends TestCase
         $this->assertJsonFormat($json, $data);
     }
 
-    public function testFormatEmptyArray()
+    public function testFormatEmptyArray(): void
     {
         $data = array('test' => array(), 'test2' => new \stdClass);
         $json = '{
@@ -286,7 +286,7 @@ class JsonFileTest extends TestCase
         $this->assertJsonFormat($json, $data);
     }
 
-    public function testEscape()
+    public function testEscape(): void
     {
         $data = array("Metadata\\\"" => 'src/');
         $json = '{
@@ -296,7 +296,7 @@ class JsonFileTest extends TestCase
         $this->assertJsonFormat($json, $data);
     }
 
-    public function testUnicode()
+    public function testUnicode(): void
     {
         $data = array("Žluťoučký \" kůň" => "úpěl ďábelské ódy za €");
         $json = '{
@@ -306,35 +306,35 @@ class JsonFileTest extends TestCase
         $this->assertJsonFormat($json, $data);
     }
 
-    public function testOnlyUnicode()
+    public function testOnlyUnicode(): void
     {
         $data = "\\/ƌ";
 
         $this->assertJsonFormat('"\\\\\\/ƌ"', $data, JSON_UNESCAPED_UNICODE);
     }
 
-    public function testEscapedSlashes()
+    public function testEscapedSlashes(): void
     {
         $data = "\\/foo";
 
         $this->assertJsonFormat('"\\\\\\/foo"', $data, 0);
     }
 
-    public function testEscapedBackslashes()
+    public function testEscapedBackslashes(): void
     {
         $data = "a\\b";
 
         $this->assertJsonFormat('"a\\\\b"', $data, 0);
     }
 
-    public function testEscapedUnicode()
+    public function testEscapedUnicode(): void
     {
         $data = "ƌ";
 
         $this->assertJsonFormat('"\\u018c"', $data, 0);
     }
 
-    public function testDoubleEscapedUnicode()
+    public function testDoubleEscapedUnicode(): void
     {
         $jsonFile = new JsonFile('composer.json');
         $data = array("Zdjęcia","hjkjhl\\u0119kkjk");
@@ -351,7 +351,7 @@ class JsonFileTest extends TestCase
      * @param string $json
      * @return void
      */
-    private function expectParseException($text, $json)
+    private function expectParseException($text, $json): void
     {
         try {
             $result = JsonFile::parseJson($json);
@@ -367,7 +367,7 @@ class JsonFileTest extends TestCase
      * @param int|null $options
      * @return void
      */
-    private function assertJsonFormat($json, $data, $options = null)
+    private function assertJsonFormat($json, $data, $options = null): void
     {
         $file = new JsonFile('composer.json');
 
diff --git a/tests/Composer/Test/Json/JsonFormatterTest.php b/tests/Composer/Test/Json/JsonFormatterTest.php
index d2bd8e0d6..cd95c6fc9 100644
--- a/tests/Composer/Test/Json/JsonFormatterTest.php
+++ b/tests/Composer/Test/Json/JsonFormatterTest.php
@@ -21,7 +21,7 @@ class JsonFormatterTest extends TestCase
      * Test if \u0119 will get correctly formatted (unescaped)
      * https://github.com/composer/composer/issues/2613
      */
-    public function testUnicodeWithPrependedSlash()
+    public function testUnicodeWithPrependedSlash(): void
     {
         if (!extension_loaded('mbstring')) {
             $this->markTestSkipped('Test requires the mbstring extension');
@@ -37,7 +37,7 @@ class JsonFormatterTest extends TestCase
      * Surrogate pairs are intentionally skipped and not unescaped
      * https://github.com/composer/composer/issues/7510
      */
-    public function testUtf16SurrogatePair()
+    public function testUtf16SurrogatePair(): void
     {
         if (!extension_loaded('mbstring')) {
             $this->markTestSkipped('Test requires the mbstring extension');
diff --git a/tests/Composer/Test/Json/JsonManipulatorTest.php b/tests/Composer/Test/Json/JsonManipulatorTest.php
index 00fd74bae..6d0900b91 100644
--- a/tests/Composer/Test/Json/JsonManipulatorTest.php
+++ b/tests/Composer/Test/Json/JsonManipulatorTest.php
@@ -25,14 +25,14 @@ class JsonManipulatorTest extends TestCase
      * @param string $constraint
      * @param string $expected
      */
-    public function testAddLink($json, $type, $package, $constraint, $expected)
+    public function testAddLink($json, $type, $package, $constraint, $expected): void
     {
         $manipulator = new JsonManipulator($json);
         $this->assertTrue($manipulator->addLink($type, $package, $constraint));
         $this->assertEquals($expected, $manipulator->getContents());
     }
 
-    public function linkProvider()
+    public function linkProvider(): array
     {
         return array(
             array(
@@ -1301,14 +1301,14 @@ class JsonManipulatorTest extends TestCase
      * @param bool $sortPackages
      * @param string $expected
      */
-    public function testAddLinkAndSortPackages($json, $type, $package, $constraint, $sortPackages, $expected)
+    public function testAddLinkAndSortPackages($json, $type, $package, $constraint, $sortPackages, $expected): void
     {
         $manipulator = new JsonManipulator($json);
         $this->assertTrue($manipulator->addLink($type, $package, $constraint, $sortPackages));
         $this->assertEquals($expected, $manipulator->getContents());
     }
 
-    public function providerAddLinkAndSortPackages()
+    public function providerAddLinkAndSortPackages(): array
     {
         return array(
             array(
@@ -1385,7 +1385,7 @@ class JsonManipulatorTest extends TestCase
      * @param string $expected
      * @param ?string $expectedContent
      */
-    public function testRemoveSubNode($json, $name, $expected, $expectedContent = null)
+    public function testRemoveSubNode($json, $name, $expected, $expectedContent = null): void
     {
         $manipulator = new JsonManipulator($json);
 
@@ -1395,7 +1395,7 @@ class JsonManipulatorTest extends TestCase
         }
     }
 
-    public function removeSubNodeProvider()
+    public function removeSubNodeProvider(): array
     {
         return array(
             'works on simple ones first' => array(
@@ -1686,7 +1686,7 @@ class JsonManipulatorTest extends TestCase
         );
     }
 
-    public function testRemoveSubNodeFromRequire()
+    public function testRemoveSubNodeFromRequire(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": [
@@ -1736,7 +1736,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddSubNodeInRequire()
+    public function testAddSubNodeInRequire(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": [
@@ -1788,7 +1788,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddExtraWithPackage()
+    public function testAddExtraWithPackage(): void
     {
         //$this->markTestSkipped();
         $manipulator = new JsonManipulator('{
@@ -1829,7 +1829,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigWithPackage()
+    public function testAddConfigWithPackage(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": [
@@ -1875,7 +1875,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddSuggestWithPackage()
+    public function testAddSuggestWithPackage(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": [
@@ -1915,7 +1915,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddRepositoryCanInitializeEmptyRepositories()
+    public function testAddRepositoryCanInitializeEmptyRepositories(): void
     {
         $manipulator = new JsonManipulator('{
   "repositories": {
@@ -1933,7 +1933,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddRepositoryCanInitializeFromScratch()
+    public function testAddRepositoryCanInitializeFromScratch(): void
     {
         $manipulator = new JsonManipulator("{
 \t\"a\": \"b\"
@@ -1951,7 +1951,7 @@ class JsonManipulatorTest extends TestCase
 ", $manipulator->getContents());
     }
 
-    public function testAddRepositoryCanAppend()
+    public function testAddRepositoryCanAppend(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": {
@@ -1977,7 +1977,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddRepositoryCanPrepend()
+    public function testAddRepositoryCanPrepend(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": {
@@ -2003,7 +2003,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddRepositoryCanOverrideDeepRepos()
+    public function testAddRepositoryCanOverrideDeepRepos(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": {
@@ -2025,7 +2025,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingEscapes()
+    public function testAddConfigSettingEscapes(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2043,7 +2043,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingWorksFromScratch()
+    public function testAddConfigSettingWorksFromScratch(): void
     {
         $manipulator = new JsonManipulator('{
 }');
@@ -2059,7 +2059,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanAdd()
+    public function testAddConfigSettingCanAdd(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2077,7 +2077,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanOverwrite()
+    public function testAddConfigSettingCanOverwrite(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2096,7 +2096,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanOverwriteNumbers()
+    public function testAddConfigSettingCanOverwriteNumbers(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2113,7 +2113,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanOverwriteArrays()
+    public function testAddConfigSettingCanOverwriteArrays(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2148,7 +2148,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanAddSubKeyInEmptyConfig()
+    public function testAddConfigSettingCanAddSubKeyInEmptyConfig(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2166,7 +2166,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanAddSubKeyInEmptyVal()
+    public function testAddConfigSettingCanAddSubKeyInEmptyVal(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2195,7 +2195,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddConfigSettingCanAddSubKeyInHash()
+    public function testAddConfigSettingCanAddSubKeyInHash(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2217,7 +2217,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddRootSettingDoesNotBreakDots()
+    public function testAddRootSettingDoesNotBreakDots(): void
     {
         $manipulator = new JsonManipulator('{
     "github-oauth": {
@@ -2235,7 +2235,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testRemoveConfigSettingCanRemoveSubKeyInHash()
+    public function testRemoveConfigSettingCanRemoveSubKeyInHash(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2257,7 +2257,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testRemoveConfigSettingCanRemoveSubKeyInHashWithSiblings()
+    public function testRemoveConfigSettingCanRemoveSubKeyInHashWithSiblings(): void
     {
         $manipulator = new JsonManipulator('{
     "config": {
@@ -2281,7 +2281,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddMainKey()
+    public function testAddMainKey(): void
     {
         $manipulator = new JsonManipulator('{
     "foo": "bar"
@@ -2295,7 +2295,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddMainKeyWithContentHavingDollarSignFollowedByDigit()
+    public function testAddMainKeyWithContentHavingDollarSignFollowedByDigit(): void
     {
         $manipulator = new JsonManipulator('{
     "foo": "bar"
@@ -2309,7 +2309,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testAddMainKeyWithContentHavingDollarSignFollowedByDigit2()
+    public function testAddMainKeyWithContentHavingDollarSignFollowedByDigit2(): void
     {
         $manipulator = new JsonManipulator('{}');
 
@@ -2320,7 +2320,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testUpdateMainKey()
+    public function testUpdateMainKey(): void
     {
         $manipulator = new JsonManipulator('{
     "foo": "bar"
@@ -2333,7 +2333,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testUpdateMainKey2()
+    public function testUpdateMainKey2(): void
     {
         $manipulator = new JsonManipulator('{
     "a": {
@@ -2357,7 +2357,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testUpdateMainKey3()
+    public function testUpdateMainKey3(): void
     {
         $manipulator = new JsonManipulator('{
     "require": {
@@ -2380,7 +2380,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testUpdateMainKeyWithContentHavingDollarSignFollowedByDigit()
+    public function testUpdateMainKeyWithContentHavingDollarSignFollowedByDigit(): void
     {
         $manipulator = new JsonManipulator('{
     "foo": "bar"
@@ -2393,7 +2393,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testRemoveMainKey()
+    public function testRemoveMainKey(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": [
@@ -2453,7 +2453,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testRemoveMainKeyIfEmpty()
+    public function testRemoveMainKeyIfEmpty(): void
     {
         $manipulator = new JsonManipulator('{
     "repositories": [
@@ -2495,7 +2495,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testRemoveMainKeyRemovesKeyWhereValueIsNull()
+    public function testRemoveMainKeyRemovesKeyWhereValueIsNull(): void
     {
         $manipulator = new JsonManipulator(json_encode(array(
             'foo' => 9000,
@@ -2511,7 +2511,7 @@ class JsonManipulatorTest extends TestCase
         $this->assertJsonStringEqualsJsonString($expected, $manipulator->getContents());
     }
 
-    public function testIndentDetection()
+    public function testIndentDetection(): void
     {
         $manipulator = new JsonManipulator('{
 
@@ -2533,7 +2533,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testRemoveMainKeyAtEndOfFile()
+    public function testRemoveMainKeyAtEndOfFile(): void
     {
         $manipulator = new JsonManipulator('{
     "require": {
@@ -2562,7 +2562,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testEscapedUnicodeDoesNotCauseBacktrackLimitErrorGithubIssue8131()
+    public function testEscapedUnicodeDoesNotCauseBacktrackLimitErrorGithubIssue8131(): void
     {
         $manipulator = new JsonManipulator('{
   "description": "Some U\u00F1icode",
@@ -2582,7 +2582,7 @@ class JsonManipulatorTest extends TestCase
 ', $manipulator->getContents());
     }
 
-    public function testLargeFileDoesNotCauseBacktrackLimitErrorGithubIssue9595()
+    public function testLargeFileDoesNotCauseBacktrackLimitErrorGithubIssue9595(): void
     {
         $manipulator = new JsonManipulator('{
     "name": "leoloso/pop",
diff --git a/tests/Composer/Test/Json/JsonValidationExceptionTest.php b/tests/Composer/Test/Json/JsonValidationExceptionTest.php
index 2212b26c2..79c436833 100644
--- a/tests/Composer/Test/Json/JsonValidationExceptionTest.php
+++ b/tests/Composer/Test/Json/JsonValidationExceptionTest.php
@@ -19,27 +19,27 @@ class JsonValidationExceptionTest extends TestCase
 {
     /**
      * @dataProvider errorProvider
-     * @param string|null $message
-     * @param string[]|null $errors
+     * @param string[] $errors
+     * @param string[] $expectedErrors
      */
-    public function testGetErrors($message, $errors)
+    public function testGetErrors(?string $message, array $errors, string $expectedMessage, array $expectedErrors): void
     {
         $object = new JsonValidationException($message, $errors);
-        $this->assertEquals($message, $object->getMessage());
-        $this->assertEquals($errors, $object->getErrors());
+        $this->assertSame($expectedMessage, $object->getMessage());
+        $this->assertSame($expectedErrors, $object->getErrors());
     }
 
-    public function testGetErrorsWhenNoErrorsProvided()
+    public function testGetErrorsWhenNoErrorsProvided(): void
     {
         $object = new JsonValidationException('test message');
         $this->assertEquals(array(), $object->getErrors());
     }
 
-    public function errorProvider()
+    public function errorProvider(): array
     {
         return array(
-            array('test message', array()),
-            array(null, null),
+            array('test message', array(), 'test message', []),
+            array(null, ['foo'], '', ['foo']),
         );
     }
 }
diff --git a/tests/Composer/Test/Mock/FactoryMock.php b/tests/Composer/Test/Mock/FactoryMock.php
index 79b50e19d..8db24fe11 100644
--- a/tests/Composer/Test/Mock/FactoryMock.php
+++ b/tests/Composer/Test/Mock/FactoryMock.php
@@ -12,9 +12,12 @@
 
 namespace Composer\Test\Mock;
 
+use Composer\Installer\InstallationManager;
+use Composer\Package\Loader\RootPackageLoader;
 use Composer\Composer;
 use Composer\Config;
 use Composer\Factory;
+use Composer\PartialComposer;
 use Composer\Repository\RepositoryManager;
 use Composer\Package\Version\VersionGuesser;
 use Composer\Package\Version\VersionParser;
@@ -30,7 +33,7 @@ use Composer\Util\ProcessExecutor;
 
 class FactoryMock extends Factory
 {
-    public static function createConfig(IOInterface $io = null, $cwd = null)
+    public static function createConfig(IOInterface $io = null, ?string $cwd = null): Config
     {
         $config = new Config(true, $cwd);
 
@@ -42,26 +45,26 @@ class FactoryMock extends Factory
         return $config;
     }
 
-    protected function loadRootPackage(RepositoryManager $rm, Config $config, VersionParser $parser, VersionGuesser $guesser, IOInterface $io)
+    protected function loadRootPackage(RepositoryManager $rm, Config $config, VersionParser $parser, VersionGuesser $guesser, IOInterface $io): RootPackageLoader
     {
         return new \Composer\Package\Loader\RootPackageLoader($rm, $config, $parser, new VersionGuesserMock(), $io);
     }
 
-    protected function addLocalRepository(IOInterface $io, RepositoryManager $rm, $vendorDir, RootPackageInterface $rootPackage, ProcessExecutor $process = null)
+    protected function addLocalRepository(IOInterface $io, RepositoryManager $rm, $vendorDir, RootPackageInterface $rootPackage, ProcessExecutor $process = null): void
     {
         $rm->setLocalRepository(new InstalledArrayRepository);
     }
 
-    public function createInstallationManager(Loop $loop, IOInterface $io, EventDispatcher $dispatcher = null)
+    public function createInstallationManager(Loop $loop, IOInterface $io, EventDispatcher $dispatcher = null): InstallationManager
     {
         return new InstallationManagerMock();
     }
 
-    protected function createDefaultInstallers(Installer\InstallationManager $im, Composer $composer, IOInterface $io, ProcessExecutor $process = null)
+    protected function createDefaultInstallers(InstallationManager $im, PartialComposer $composer, IOInterface $io, ProcessExecutor $process = null): void
     {
     }
 
-    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im)
+    protected function purgePackages(InstalledRepositoryInterface $repo, InstallationManager $im): void
     {
     }
 }
diff --git a/tests/Composer/Test/Mock/HttpDownloaderMock.php b/tests/Composer/Test/Mock/HttpDownloaderMock.php
index a317d8b45..b919b294b 100644
--- a/tests/Composer/Test/Mock/HttpDownloaderMock.php
+++ b/tests/Composer/Test/Mock/HttpDownloaderMock.php
@@ -91,7 +91,7 @@ class HttpDownloaderMock extends HttpDownloader
         }
 
         if (count($this->expectations) > 0) {
-            $expectations = array_map(function ($expect) {
+            $expectations = array_map(function ($expect): string {
                 return $expect['url'];
             }, $this->expectations);
             throw new AssertionFailedError(
diff --git a/tests/Composer/Test/Mock/InstallationManagerMock.php b/tests/Composer/Test/Mock/InstallationManagerMock.php
index ed4b33996..ed4eaa873 100644
--- a/tests/Composer/Test/Mock/InstallationManagerMock.php
+++ b/tests/Composer/Test/Mock/InstallationManagerMock.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Test\Mock;
 
+use React\Promise\PromiseInterface;
 use Composer\Installer\InstallationManager;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
@@ -45,7 +46,7 @@ class InstallationManagerMock extends InstallationManager
     {
     }
 
-    public function execute(InstalledRepositoryInterface $repo, array $operations, $devMode = true, $runScripts = true)
+    public function execute(InstalledRepositoryInterface $repo, array $operations, $devMode = true, $runScripts = true): void
     {
         foreach ($operations as $operation) {
             $method = $operation->getOperationType();
@@ -54,12 +55,12 @@ class InstallationManagerMock extends InstallationManager
         }
     }
 
-    public function getInstallPath(PackageInterface $package)
+    public function getInstallPath(PackageInterface $package): string
     {
         return '';
     }
 
-    public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
+    public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageInterface $package): bool
     {
         return $repo->hasPackage($package);
     }
@@ -67,7 +68,7 @@ class InstallationManagerMock extends InstallationManager
     /**
      * @inheritDoc
      */
-    public function install(InstalledRepositoryInterface $repo, InstallOperation $operation)
+    public function install(InstalledRepositoryInterface $repo, InstallOperation $operation): ?PromiseInterface
     {
         $this->installed[] = $operation->getPackage();
         $this->trace[] = strip_tags((string) $operation);
@@ -79,7 +80,7 @@ class InstallationManagerMock extends InstallationManager
     /**
      * @inheritDoc
      */
-    public function update(InstalledRepositoryInterface $repo, UpdateOperation $operation)
+    public function update(InstalledRepositoryInterface $repo, UpdateOperation $operation): ?PromiseInterface
     {
         $this->updated[] = array($operation->getInitialPackage(), $operation->getTargetPackage());
         $this->trace[] = strip_tags((string) $operation);
@@ -94,7 +95,7 @@ class InstallationManagerMock extends InstallationManager
     /**
      * @inheritDoc
      */
-    public function uninstall(InstalledRepositoryInterface $repo, UninstallOperation $operation)
+    public function uninstall(InstalledRepositoryInterface $repo, UninstallOperation $operation): ?PromiseInterface
     {
         $this->uninstalled[] = $operation->getPackage();
         $this->trace[] = strip_tags((string) $operation);
@@ -103,7 +104,7 @@ class InstallationManagerMock extends InstallationManager
         return null;
     }
 
-    public function markAliasInstalled(InstalledRepositoryInterface $repo, MarkAliasInstalledOperation $operation)
+    public function markAliasInstalled(InstalledRepositoryInterface $repo, MarkAliasInstalledOperation $operation): void
     {
         $package = $operation->getPackage();
 
@@ -113,7 +114,7 @@ class InstallationManagerMock extends InstallationManager
         parent::markAliasInstalled($repo, $operation);
     }
 
-    public function markAliasUninstalled(InstalledRepositoryInterface $repo, MarkAliasUninstalledOperation $operation)
+    public function markAliasUninstalled(InstalledRepositoryInterface $repo, MarkAliasUninstalledOperation $operation): void
     {
         $this->uninstalled[] = $operation->getPackage();
         $this->trace[] = strip_tags((string) $operation);
@@ -122,36 +123,36 @@ class InstallationManagerMock extends InstallationManager
     }
 
     /** @return string[] */
-    public function getTrace()
+    public function getTrace(): array
     {
         return $this->trace;
     }
 
     /** @return PackageInterface[] */
-    public function getInstalledPackages()
+    public function getInstalledPackages(): array
     {
         return $this->installed;
     }
 
     /** @return PackageInterface[][] */
-    public function getUpdatedPackages()
+    public function getUpdatedPackages(): array
     {
         return $this->updated;
     }
 
     /** @return PackageInterface[] */
-    public function getUninstalledPackages()
+    public function getUninstalledPackages(): array
     {
         return $this->uninstalled;
     }
 
-    public function notifyInstalls(IOInterface $io)
+    public function notifyInstalls(IOInterface $io): void
     {
         // noop
     }
 
     /** @return PackageInterface[] */
-    public function getInstalledPackagesByType()
+    public function getInstalledPackagesByType(): array
     {
         return $this->installed;
     }
diff --git a/tests/Composer/Test/Mock/InstalledFilesystemRepositoryMock.php b/tests/Composer/Test/Mock/InstalledFilesystemRepositoryMock.php
index 574cfbd83..101005cda 100644
--- a/tests/Composer/Test/Mock/InstalledFilesystemRepositoryMock.php
+++ b/tests/Composer/Test/Mock/InstalledFilesystemRepositoryMock.php
@@ -17,11 +17,11 @@ use Composer\Installer\InstallationManager;
 
 class InstalledFilesystemRepositoryMock extends InstalledFilesystemRepository
 {
-    public function reload()
+    public function reload(): void
     {
     }
 
-    public function write($devMode, InstallationManager $installationManager)
+    public function write($devMode, InstallationManager $installationManager): void
     {
     }
 }
diff --git a/tests/Composer/Test/Mock/ProcessExecutorMock.php b/tests/Composer/Test/Mock/ProcessExecutorMock.php
index 9228c1310..277ead749 100644
--- a/tests/Composer/Test/Mock/ProcessExecutorMock.php
+++ b/tests/Composer/Test/Mock/ProcessExecutorMock.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Test\Mock;
 
+use React\Promise\PromiseInterface;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\Platform;
 use PHPUnit\Framework\Assert;
@@ -48,11 +49,11 @@ class ProcessExecutorMock extends ProcessExecutor
      *
      * @return void
      */
-    public function expects(array $expectations, $strict = false, array $defaultHandler = array('return' => 0, 'stdout' => '', 'stderr' => ''))
+    public function expects(array $expectations, $strict = false, array $defaultHandler = array('return' => 0, 'stdout' => '', 'stderr' => '')): void
     {
         /** @var array{cmd: string|list<string>, return?: int, stdout?: string, stderr?: string, callback?: callable} $default */
         $default = array('cmd' => '', 'return' => 0, 'stdout' => '', 'stderr' => '', 'callback' => null);
-        $this->expectations = array_map(function ($expect) use ($default) {
+        $this->expectations = array_map(function ($expect) use ($default): array {
             if (is_string($expect)) {
                 $command = $expect;
                 $expect = $default;
@@ -88,7 +89,7 @@ class ProcessExecutorMock extends ProcessExecutor
         }
 
         if (count($this->expectations) > 0) {
-            $expectations = array_map(function ($expect) {
+            $expectations = array_map(function ($expect): string {
                 return is_array($expect['cmd']) ? implode(' ', $expect['cmd']) : $expect['cmd'];
             }, $this->expectations);
             throw new AssertionFailedError(
@@ -102,7 +103,7 @@ class ProcessExecutorMock extends ProcessExecutor
         Assert::assertTrue(true); // @phpstan-ignore-line
     }
 
-    public function execute($command, &$output = null, $cwd = null)
+    public function execute($command, &$output = null, $cwd = null): int
     {
         if (func_num_args() > 1) {
             return $this->doExecute($command, $cwd, false, $output);
@@ -111,7 +112,7 @@ class ProcessExecutorMock extends ProcessExecutor
         return $this->doExecute($command, $cwd, false);
     }
 
-    public function executeTty($command, $cwd = null)
+    public function executeTty($command, $cwd = null): int
     {
         if (Platform::isTty()) {
             return $this->doExecute($command, $cwd, true);
@@ -173,21 +174,21 @@ class ProcessExecutorMock extends ProcessExecutor
         return $return;
     }
 
-    public function executeAsync($command, $cwd = null)
+    public function executeAsync($command, $cwd = null): PromiseInterface
     {
-        $resolver = function ($resolve, $reject) {
+        $resolver = function ($resolve, $reject): void {
             // TODO strictly speaking this should resolve with a mock Process instance here
             $resolve();
         };
 
-        $canceler = function () {
+        $canceler = function (): void {
             throw new \RuntimeException('Aborted process');
         };
 
         return new Promise($resolver, $canceler);
     }
 
-    public function getErrorOutput()
+    public function getErrorOutput(): string
     {
         return $this->errorOutput;
     }
diff --git a/tests/Composer/Test/Mock/VersionGuesserMock.php b/tests/Composer/Test/Mock/VersionGuesserMock.php
index d2fe1236c..b634b0297 100644
--- a/tests/Composer/Test/Mock/VersionGuesserMock.php
+++ b/tests/Composer/Test/Mock/VersionGuesserMock.php
@@ -20,7 +20,7 @@ class VersionGuesserMock extends VersionGuesser
     {
     }
 
-    public function guessVersion(array $packageConfig, $path)
+    public function guessVersion(array $packageConfig, $path): ?array
     {
         return null;
     }
diff --git a/tests/Composer/Test/Package/Archiver/ArchivableFilesFinderTest.php b/tests/Composer/Test/Package/Archiver/ArchivableFilesFinderTest.php
index 883dd4278..ad9bce604 100644
--- a/tests/Composer/Test/Package/Archiver/ArchivableFilesFinderTest.php
+++ b/tests/Composer/Test/Package/Archiver/ArchivableFilesFinderTest.php
@@ -102,7 +102,7 @@ class ArchivableFilesFinderTest extends TestCase
         $fs->removeDirectory($this->sources);
     }
 
-    public function testManualExcludes()
+    public function testManualExcludes(): void
     {
         $excludes = array(
             'prefixB.foo',
@@ -154,7 +154,7 @@ class ArchivableFilesFinderTest extends TestCase
         ));
     }
 
-    public function testGitExcludes()
+    public function testGitExcludes(): void
     {
         $this->skipIfNotExecutable('git');
 
@@ -202,7 +202,7 @@ class ArchivableFilesFinderTest extends TestCase
         ));
     }
 
-    public function testSkipExcludes()
+    public function testSkipExcludes(): void
     {
         $excludes = array(
             'prefixB.foo',
@@ -260,7 +260,7 @@ class ArchivableFilesFinderTest extends TestCase
     /**
      * @return string[]
      */
-    protected function getArchivableFiles()
+    protected function getArchivableFiles(): array
     {
         $files = array();
         foreach ($this->finder as $file) {
@@ -279,7 +279,7 @@ class ArchivableFilesFinderTest extends TestCase
      *
      * @return string[]
      */
-    protected function getArchivedFiles($command)
+    protected function getArchivedFiles($command): array
     {
         $process = Process::fromShellCommandline($command, $this->sources);
         $process->run();
@@ -303,7 +303,7 @@ class ArchivableFilesFinderTest extends TestCase
      *
      * @return void
      */
-    protected function assertArchivableFiles($expectedFiles)
+    protected function assertArchivableFiles($expectedFiles): void
     {
         $actualFiles = $this->getArchivableFiles();
 
diff --git a/tests/Composer/Test/Package/Archiver/ArchiveManagerTest.php b/tests/Composer/Test/Package/Archiver/ArchiveManagerTest.php
index 9d1aff56e..8d620fbc9 100644
--- a/tests/Composer/Test/Package/Archiver/ArchiveManagerTest.php
+++ b/tests/Composer/Test/Package/Archiver/ArchiveManagerTest.php
@@ -48,7 +48,7 @@ class ArchiveManagerTest extends ArchiverTest
         $this->targetDir = $this->testDir.'/composer_archiver_tests';
     }
 
-    public function testUnknownFormat()
+    public function testUnknownFormat(): void
     {
         self::expectException('RuntimeException');
 
@@ -57,7 +57,7 @@ class ArchiveManagerTest extends ArchiverTest
         $this->manager->archive($package, '__unknown_format__', $this->targetDir);
     }
 
-    public function testArchiveTar()
+    public function testArchiveTar(): void
     {
         $this->skipIfNotExecutable('git');
 
@@ -76,7 +76,7 @@ class ArchiveManagerTest extends ArchiverTest
         unlink($target);
     }
 
-    public function testArchiveCustomFileName()
+    public function testArchiveCustomFileName(): void
     {
         $this->skipIfNotExecutable('git');
 
@@ -104,7 +104,7 @@ class ArchiveManagerTest extends ArchiverTest
      *
      * @return string
      */
-    protected function getTargetName(CompletePackage $package, $format, $fileName = null)
+    protected function getTargetName(CompletePackage $package, $format, $fileName = null): string
     {
         if (null === $fileName) {
             $packageName = $this->manager->getPackageFilename($package);
@@ -120,7 +120,7 @@ class ArchiveManagerTest extends ArchiverTest
      *
      * @return void
      */
-    protected function setupGitRepo()
+    protected function setupGitRepo(): void
     {
         $currentWorkDir = getcwd();
         chdir($this->testDir);
diff --git a/tests/Composer/Test/Package/Archiver/ArchiverTest.php b/tests/Composer/Test/Package/Archiver/ArchiverTest.php
index 9601fa1c6..5ac901d1b 100644
--- a/tests/Composer/Test/Package/Archiver/ArchiverTest.php
+++ b/tests/Composer/Test/Package/Archiver/ArchiverTest.php
@@ -52,7 +52,7 @@ abstract class ArchiverTest extends TestCase
      *
      * @return CompletePackage
      */
-    protected function setupPackage()
+    protected function setupPackage(): CompletePackage
     {
         $package = new CompletePackage('archivertest/archivertest', 'master', 'master');
         $package->setSourceUrl(realpath($this->testDir));
diff --git a/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php b/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php
index e8cae5853..2c8f96a2e 100644
--- a/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php
+++ b/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php
@@ -23,14 +23,14 @@ class GitExcludeFilterTest extends TestCase
      * @param string  $ignore
      * @param mixed[] $expected
      */
-    public function testPatternEscape($ignore, $expected)
+    public function testPatternEscape($ignore, $expected): void
     {
         $filter = new GitExcludeFilter('/');
 
         $this->assertEquals($expected, $filter->parseGitAttributesLine($ignore));
     }
 
-    public function providePatterns()
+    public function providePatterns(): array
     {
         return array(
             array('app/config/parameters.yml export-ignore', array('{(?=[^\.])app/(?=[^\.])config/(?=[^\.])parameters\.yml(?=$|/)}', false, false)),
diff --git a/tests/Composer/Test/Package/Archiver/PharArchiverTest.php b/tests/Composer/Test/Package/Archiver/PharArchiverTest.php
index 82ed226f3..5c571316a 100644
--- a/tests/Composer/Test/Package/Archiver/PharArchiverTest.php
+++ b/tests/Composer/Test/Package/Archiver/PharArchiverTest.php
@@ -16,7 +16,7 @@ use Composer\Package\Archiver\PharArchiver;
 
 class PharArchiverTest extends ArchiverTest
 {
-    public function testTarArchive()
+    public function testTarArchive(): void
     {
         // Set up repository
         $this->setupDummyRepo();
@@ -31,7 +31,7 @@ class PharArchiverTest extends ArchiverTest
         $this->filesystem->removeDirectory(dirname($target));
     }
 
-    public function testZipArchive()
+    public function testZipArchive(): void
     {
         // Set up repository
         $this->setupDummyRepo();
@@ -51,7 +51,7 @@ class PharArchiverTest extends ArchiverTest
      *
      * @return void
      */
-    protected function setupDummyRepo()
+    protected function setupDummyRepo(): void
     {
         $currentWorkDir = getcwd();
         chdir($this->testDir);
@@ -72,7 +72,7 @@ class PharArchiverTest extends ArchiverTest
      *
      * @return void
      */
-    protected function writeFile($path, $content, $currentWorkDir)
+    protected function writeFile($path, $content, $currentWorkDir): void
     {
         if (!file_exists(dirname($path))) {
             mkdir(dirname($path), 0777, true);
diff --git a/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php b/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php
index 616768c01..e88711a4c 100644
--- a/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php
+++ b/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php
@@ -23,7 +23,7 @@ class ZipArchiverTest extends ArchiverTest
      *
      * @dataProvider provideGitignoreExcludeNegationTestCases
      */
-    public function testGitignoreExcludeNegation($include)
+    public function testGitignoreExcludeNegation($include): void
     {
         $this->testZipArchive(array(
             'docs/README.md' => '# The doc',
@@ -31,7 +31,7 @@ class ZipArchiverTest extends ArchiverTest
         ));
     }
 
-    public function provideGitignoreExcludeNegationTestCases()
+    public function provideGitignoreExcludeNegationTestCases(): array
     {
         return array(
             array('!/docs'),
@@ -42,7 +42,7 @@ class ZipArchiverTest extends ArchiverTest
     /**
      * @param array<string, string> $files
      */
-    public function testZipArchive(array $files = array())
+    public function testZipArchive(array $files = array()): void
     {
         if (!class_exists('ZipArchive')) {
             $this->markTestSkipped('Cannot run ZipArchiverTest, missing class "ZipArchive".');
@@ -87,7 +87,7 @@ class ZipArchiverTest extends ArchiverTest
      *
      * @return void
      */
-    protected function setupDummyRepo(array &$files)
+    protected function setupDummyRepo(array &$files): void
     {
         $currentWorkDir = getcwd();
         chdir($this->testDir);
@@ -108,7 +108,7 @@ class ZipArchiverTest extends ArchiverTest
      *
      * @return void
      */
-    protected function writeFile($path, $content, $currentWorkDir)
+    protected function writeFile($path, $content, $currentWorkDir): void
     {
         if (!file_exists(dirname($path))) {
             mkdir(dirname($path), 0777, true);
diff --git a/tests/Composer/Test/Package/BasePackageTest.php b/tests/Composer/Test/Package/BasePackageTest.php
index 627bbfb62..3b92316b2 100644
--- a/tests/Composer/Test/Package/BasePackageTest.php
+++ b/tests/Composer/Test/Package/BasePackageTest.php
@@ -20,7 +20,7 @@ class BasePackageTest extends TestCase
     /**
      * @doesNotPerformAssertions
      */
-    public function testSetSameRepository()
+    public function testSetSameRepository(): void
     {
         $package = $this->getMockForAbstractClass('Composer\Package\BasePackage', array('foo'));
         $repository = $this->getMockBuilder('Composer\Repository\RepositoryInterface')->getMock();
@@ -33,7 +33,7 @@ class BasePackageTest extends TestCase
         }
     }
 
-    public function testSetAnotherRepository()
+    public function testSetAnotherRepository(): void
     {
         self::expectException('LogicException');
 
@@ -49,12 +49,12 @@ class BasePackageTest extends TestCase
      * @param bool   $truncate
      * @param string $expected
      */
-    public function testFormatVersionForDevPackage(BasePackage $package, $truncate, $expected)
+    public function testFormatVersionForDevPackage(BasePackage $package, $truncate, $expected): void
     {
         $this->assertSame($expected, $package->getFullPrettyVersion($truncate));
     }
 
-    public function provideFormattedVersions()
+    public function provideFormattedVersions(): array
     {
         $data = array(
             array(
@@ -79,7 +79,7 @@ class BasePackageTest extends TestCase
             ),
         );
 
-        $createPackage = function ($arr) {
+        $createPackage = function ($arr): array {
             $package = $this->getMockForAbstractClass('\Composer\Package\BasePackage', array(), '', false);
             $package->expects($this->once())->method('isDev')->will($this->returnValue(true));
             $package->expects($this->any())->method('getSourceType')->will($this->returnValue('git'));
@@ -99,7 +99,7 @@ class BasePackageTest extends TestCase
      *
      * @dataProvider dataPackageNamesToRegexp
      */
-    public function testPackageNamesToRegexp(array $packageNames, $wrap, $expectedRegexp)
+    public function testPackageNamesToRegexp(array $packageNames, $wrap, $expectedRegexp): void
     {
         $regexp = BasePackage::packageNamesToRegexp($packageNames, $wrap);
 
@@ -109,7 +109,7 @@ class BasePackageTest extends TestCase
     /**
      * @return mixed[][]
      */
-    public function dataPackageNamesToRegexp()
+    public function dataPackageNamesToRegexp(): array
     {
         return array(
             array(
diff --git a/tests/Composer/Test/Package/CompletePackageTest.php b/tests/Composer/Test/Package/CompletePackageTest.php
index 848315a1d..2f8de80d1 100644
--- a/tests/Composer/Test/Package/CompletePackageTest.php
+++ b/tests/Composer/Test/Package/CompletePackageTest.php
@@ -23,7 +23,7 @@ class CompletePackageTest extends TestCase
      *
      * demonstrates several versioning schemes
      */
-    public function providerVersioningSchemes()
+    public function providerVersioningSchemes(): array
     {
         $provider[] = array('foo',              '1-beta');
         $provider[] = array('node',             '0.5.6');
@@ -40,7 +40,7 @@ class CompletePackageTest extends TestCase
      * @param string $name
      * @param string $version
      */
-    public function testPackageHasExpectedNamingSemantics($name, $version)
+    public function testPackageHasExpectedNamingSemantics($name, $version): void
     {
         $versionParser = new VersionParser();
         $normVersion = $versionParser->normalize($version);
@@ -54,7 +54,7 @@ class CompletePackageTest extends TestCase
      * @param string $name
      * @param string $version
      */
-    public function testPackageHasExpectedVersioningSemantics($name, $version)
+    public function testPackageHasExpectedVersioningSemantics($name, $version): void
     {
         $versionParser = new VersionParser();
         $normVersion = $versionParser->normalize($version);
@@ -69,7 +69,7 @@ class CompletePackageTest extends TestCase
      * @param string $name
      * @param string $version
      */
-    public function testPackageHasExpectedMarshallingSemantics($name, $version)
+    public function testPackageHasExpectedMarshallingSemantics($name, $version): void
     {
         $versionParser = new VersionParser();
         $normVersion = $versionParser->normalize($version);
@@ -77,7 +77,7 @@ class CompletePackageTest extends TestCase
         $this->assertEquals(strtolower($name).'-'.$normVersion, (string) $package);
     }
 
-    public function testGetTargetDir()
+    public function testGetTargetDir(): void
     {
         $package = new Package('a', '1.0.0.0', '1.0');
 
diff --git a/tests/Composer/Test/Package/Dumper/ArrayDumperTest.php b/tests/Composer/Test/Package/Dumper/ArrayDumperTest.php
index f3ab86c3c..b5ee6d073 100644
--- a/tests/Composer/Test/Package/Dumper/ArrayDumperTest.php
+++ b/tests/Composer/Test/Package/Dumper/ArrayDumperTest.php
@@ -35,7 +35,7 @@ class ArrayDumperTest extends TestCase
         $this->packageExpects('getTransportOptions', array());
     }
 
-    public function testRequiredInformation()
+    public function testRequiredInformation(): void
     {
         $this
             ->packageExpects('getPrettyName', 'foo')
@@ -54,7 +54,7 @@ class ArrayDumperTest extends TestCase
         );
     }
 
-    public function testRootPackage()
+    public function testRootPackage(): void
     {
         $this->package = $this->getMockBuilder('Composer\Package\RootPackageInterface')->getMock();
 
@@ -67,7 +67,7 @@ class ArrayDumperTest extends TestCase
         $this->assertSame('dev', $config['minimum-stability']);
     }
 
-    public function testDumpAbandoned()
+    public function testDumpAbandoned(): void
     {
         $this->packageExpects('isAbandoned', true);
         $this->packageExpects('getReplacementPackage', true);
@@ -77,7 +77,7 @@ class ArrayDumperTest extends TestCase
         $this->assertTrue($config['abandoned']);
     }
 
-    public function testDumpAbandonedReplacement()
+    public function testDumpAbandonedReplacement(): void
     {
         $this->packageExpects('isAbandoned', true);
         $this->packageExpects('getReplacementPackage', 'foo/bar');
@@ -95,7 +95,7 @@ class ArrayDumperTest extends TestCase
      * @param string $method
      * @param mixed  $expectedValue
      */
-    public function testKeys($key, $value, $method = null, $expectedValue = null)
+    public function testKeys($key, $value, $method = null, $expectedValue = null): void
     {
         $this->package = $this->getMockBuilder('Composer\Package\RootPackageInterface')->getMock();
 
@@ -111,7 +111,7 @@ class ArrayDumperTest extends TestCase
         $this->assertSame($expectedValue ?: $value, $config[$key]);
     }
 
-    public function provideKeys()
+    public function provideKeys(): array
     {
         return array(
             array(
@@ -247,10 +247,8 @@ class ArrayDumperTest extends TestCase
     /**
      * @param string $method
      * @param mixed  $value
-     *
-     * @return self
      */
-    private function packageExpects($method, $value)
+    private function packageExpects($method, $value): \Composer\Test\Package\Dumper\ArrayDumperTest
     {
         $this->package
             ->expects($this->any())
diff --git a/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php b/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
index 86a016e55..cc961575e 100644
--- a/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
@@ -29,7 +29,7 @@ class ArrayLoaderTest extends TestCase
         $this->loader = new ArrayLoader(null);
     }
 
-    public function testSelfVersion()
+    public function testSelfVersion(): void
     {
         $config = array(
             'name' => 'A',
@@ -44,7 +44,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertEquals('== 1.2.3.4', (string) $replaces['foo']->getConstraint());
     }
 
-    public function testTypeDefault()
+    public function testTypeDefault(): void
     {
         $config = array(
             'name' => 'A',
@@ -64,7 +64,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertEquals('foo', $package->getType());
     }
 
-    public function testNormalizedVersionOptimization()
+    public function testNormalizedVersionOptimization(): void
     {
         $config = array(
             'name' => 'A',
@@ -84,7 +84,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertEquals('1.2.3.4', $package->getVersion());
     }
 
-    public function parseDumpProvider()
+    public function parseDumpProvider(): array
     {
         $validConfig = array(
             'name' => 'A/B',
@@ -139,7 +139,7 @@ class ArrayLoaderTest extends TestCase
      *
      * @return array<string, mixed>
      */
-    protected function fixConfigWhenLoadConfigIsFalse($config)
+    protected function fixConfigWhenLoadConfigIsFalse($config): array
     {
         $expectedConfig = $config;
         unset($expectedConfig['transport-options']);
@@ -155,7 +155,7 @@ class ArrayLoaderTest extends TestCase
      *
      * @param array<string, mixed> $config
      */
-    public function testParseDumpDefaultLoadConfig($config)
+    public function testParseDumpDefaultLoadConfig($config): void
     {
         $package = $this->loader->load($config);
         $dumper = new ArrayDumper;
@@ -168,7 +168,7 @@ class ArrayLoaderTest extends TestCase
      *
      * @param array<string, mixed> $config
      */
-    public function testParseDumpTrueLoadConfig($config)
+    public function testParseDumpTrueLoadConfig($config): void
     {
         $loader = new ArrayLoader(null, true);
         $package = $loader->load($config);
@@ -182,7 +182,7 @@ class ArrayLoaderTest extends TestCase
      *
      * @param array<string, mixed> $config
      */
-    public function testParseDumpFalseLoadConfig($config)
+    public function testParseDumpFalseLoadConfig($config): void
     {
         $loader = new ArrayLoader(null, false);
         $package = $loader->load($config);
@@ -191,7 +191,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertEquals($expectedConfig, $dumper->dump($package));
     }
 
-    public function testPackageWithBranchAlias()
+    public function testPackageWithBranchAlias(): void
     {
         $config = array(
             'name' => 'A',
@@ -249,7 +249,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertEquals('4.x-dev', $package->getPrettyVersion());
     }
 
-    public function testAbandoned()
+    public function testAbandoned(): void
     {
         $config = array(
             'name' => 'A',
@@ -262,7 +262,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertEquals('foo/bar', $package->getReplacementPackage());
     }
 
-    public function testNotAbandoned()
+    public function testNotAbandoned(): void
     {
         $config = array(
             'name' => 'A',
@@ -273,7 +273,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertFalse($package->isAbandoned());
     }
 
-    public function providePluginApiVersions()
+    public function providePluginApiVersions(): array
     {
         return array(
             array('1.0'),
@@ -299,7 +299,7 @@ class ArrayLoaderTest extends TestCase
      *
      * @param string $apiVersion
      */
-    public function testPluginApiVersionAreKeptAsDeclared($apiVersion)
+    public function testPluginApiVersionAreKeptAsDeclared($apiVersion): void
     {
         $links = $this->loader->parseLinks('Plugin', '9.9.9', Link::TYPE_REQUIRE, array('composer-plugin-api' => $apiVersion));
 
@@ -307,7 +307,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertSame($apiVersion, $links['composer-plugin-api']->getConstraint()->getPrettyString());
     }
 
-    public function testPluginApiVersionDoesSupportSelfVersion()
+    public function testPluginApiVersionDoesSupportSelfVersion(): void
     {
         $links = $this->loader->parseLinks('Plugin', '6.6.6', Link::TYPE_REQUIRE, array('composer-plugin-api' => 'self.version'));
 
@@ -315,7 +315,7 @@ class ArrayLoaderTest extends TestCase
         $this->assertSame('6.6.6', $links['composer-plugin-api']->getConstraint()->getPrettyString());
     }
 
-    public function testNoneStringVersion()
+    public function testNoneStringVersion(): void
     {
         $config = array(
             'name' => 'acme/package',
diff --git a/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php b/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
index 252fb4840..a2fa3730a 100644
--- a/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
@@ -28,7 +28,7 @@ class RootPackageLoaderTest extends TestCase
      *
      * @return RootPackage|RootAliasPackage
      */
-    protected function loadPackage($data)
+    protected function loadPackage($data): \Composer\Package\PackageInterface
     {
         $manager = $this->getMockBuilder('Composer\\Repository\\RepositoryManager')
             ->disableOriginalConstructor()
@@ -42,7 +42,7 @@ class RootPackageLoaderTest extends TestCase
         return $loader->load($data);
     }
 
-    public function testStabilityFlagsParsing()
+    public function testStabilityFlagsParsing(): void
     {
         $package = $this->loadPackage(array(
             'require' => array(
@@ -70,7 +70,7 @@ class RootPackageLoaderTest extends TestCase
         ), $package->getStabilityFlags());
     }
 
-    public function testNoVersionIsVisibleInPrettyVersion()
+    public function testNoVersionIsVisibleInPrettyVersion(): void
     {
         $manager = $this->getMockBuilder('Composer\\Repository\\RepositoryManager')
             ->disableOriginalConstructor()
@@ -88,7 +88,7 @@ class RootPackageLoaderTest extends TestCase
         $this->assertEquals(RootPackage::DEFAULT_PRETTY_VERSION, $package->getPrettyVersion());
     }
 
-    public function testPrettyVersionForRootPackageInVersionBranch()
+    public function testPrettyVersionForRootPackageInVersionBranch(): void
     {
         // see #6845
         $manager = $this->getMockBuilder('Composer\\Repository\\RepositoryManager')->disableOriginalConstructor()->getMock();
@@ -109,7 +109,7 @@ class RootPackageLoaderTest extends TestCase
         $this->assertEquals('3.0-dev', $package->getPrettyVersion());
     }
 
-    public function testFeatureBranchPrettyVersion()
+    public function testFeatureBranchPrettyVersion(): void
     {
         if (!function_exists('proc_open')) {
             $this->markTestSkipped('proc_open() is not available');
@@ -137,7 +137,7 @@ class RootPackageLoaderTest extends TestCase
         $this->assertEquals("dev-master", $package->getPrettyVersion());
     }
 
-    public function testNonFeatureBranchPrettyVersion()
+    public function testNonFeatureBranchPrettyVersion(): void
     {
         if (!function_exists('proc_open')) {
             $this->markTestSkipped('proc_open() is not available');
diff --git a/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php b/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php
index 68768ca69..0b214cb56 100644
--- a/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/ValidatingArrayLoaderTest.php
@@ -23,7 +23,7 @@ class ValidatingArrayLoaderTest extends TestCase
      *
      * @param array<string, mixed> $config
      */
-    public function testLoadSuccess($config)
+    public function testLoadSuccess($config): void
     {
         $internalLoader = $this->getMockBuilder('Composer\Package\Loader\LoaderInterface')->getMock();
         $internalLoader
@@ -35,7 +35,7 @@ class ValidatingArrayLoaderTest extends TestCase
         $loader->load($config);
     }
 
-    public function successProvider()
+    public function successProvider(): array
     {
         return array(
             array( // minimal
@@ -212,7 +212,7 @@ class ValidatingArrayLoaderTest extends TestCase
      * @param array<string, mixed> $config
      * @param string[]             $expectedErrors
      */
-    public function testLoadFailureThrowsException($config, $expectedErrors)
+    public function testLoadFailureThrowsException($config, $expectedErrors): void
     {
         $internalLoader = $this->getMockBuilder('Composer\Package\Loader\LoaderInterface')->getMock();
         $loader = new ValidatingArrayLoader($internalLoader, true, null, ValidatingArrayLoader::CHECK_ALL);
@@ -233,7 +233,7 @@ class ValidatingArrayLoaderTest extends TestCase
      * @param array<string, mixed> $config
      * @param string[]             $expectedWarnings
      */
-    public function testLoadWarnings($config, $expectedWarnings)
+    public function testLoadWarnings($config, $expectedWarnings): void
     {
         $internalLoader = $this->getMockBuilder('Composer\Package\Loader\LoaderInterface')->getMock();
         $loader = new ValidatingArrayLoader($internalLoader, true, null, ValidatingArrayLoader::CHECK_ALL);
@@ -252,7 +252,7 @@ class ValidatingArrayLoaderTest extends TestCase
      * @param string[]             $expectedWarnings
      * @param bool                 $mustCheck
      */
-    public function testLoadSkipsWarningDataWhenIgnoringErrors($config, $expectedWarnings, $mustCheck = true)
+    public function testLoadSkipsWarningDataWhenIgnoringErrors($config, $expectedWarnings, $mustCheck = true): void
     {
         if (!$mustCheck) {
             $this->assertTrue(true);
@@ -270,7 +270,7 @@ class ValidatingArrayLoaderTest extends TestCase
         $loader->load($config);
     }
 
-    public function errorProvider()
+    public function errorProvider(): array
     {
         $invalidNames = array(
             'foo',
@@ -420,7 +420,7 @@ class ValidatingArrayLoaderTest extends TestCase
         ));
     }
 
-    public function warningProvider()
+    public function warningProvider(): array
     {
         return array(
             array(
diff --git a/tests/Composer/Test/Package/LockerTest.php b/tests/Composer/Test/Package/LockerTest.php
index 3541de44f..17168f62a 100644
--- a/tests/Composer/Test/Package/LockerTest.php
+++ b/tests/Composer/Test/Package/LockerTest.php
@@ -20,7 +20,7 @@ use Composer\Test\TestCase;
 
 class LockerTest extends TestCase
 {
-    public function testIsLocked()
+    public function testIsLocked(): void
     {
         $json = $this->createJsonFileMock();
         $locker = new Locker(
@@ -42,7 +42,7 @@ class LockerTest extends TestCase
         $this->assertTrue($locker->isLocked());
     }
 
-    public function testGetNotLockedPackages()
+    public function testGetNotLockedPackages(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -59,7 +59,7 @@ class LockerTest extends TestCase
         $locker->getLockedRepository();
     }
 
-    public function testGetLockedPackages()
+    public function testGetLockedPackages(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -85,7 +85,7 @@ class LockerTest extends TestCase
         $this->assertNotNull($repo->findPackage('pkg2', '0.1.10'));
     }
 
-    public function testSetLockData()
+    public function testSetLockData(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -158,7 +158,7 @@ class LockerTest extends TestCase
         $locker->setLockData(array($package1, $package2), array(), array(), array(), array(), 'dev', array(), false, false, array('foo/bar' => '1.0'));
     }
 
-    public function testLockBadPackages()
+    public function testLockBadPackages(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -176,7 +176,7 @@ class LockerTest extends TestCase
         $locker->setLockData(array($package1), array(), array(), array(), array(), 'dev', array(), false, false, array());
     }
 
-    public function testIsFresh()
+    public function testIsFresh(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -192,7 +192,7 @@ class LockerTest extends TestCase
         $this->assertTrue($locker->isFresh());
     }
 
-    public function testIsFreshFalse()
+    public function testIsFreshFalse(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -207,7 +207,7 @@ class LockerTest extends TestCase
         $this->assertFalse($locker->isFresh());
     }
 
-    public function testIsFreshWithContentHash()
+    public function testIsFreshWithContentHash(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -223,7 +223,7 @@ class LockerTest extends TestCase
         $this->assertTrue($locker->isFresh());
     }
 
-    public function testIsFreshWithContentHashAndNoHash()
+    public function testIsFreshWithContentHashAndNoHash(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
@@ -239,7 +239,7 @@ class LockerTest extends TestCase
         $this->assertTrue($locker->isFresh());
     }
 
-    public function testIsFreshFalseWithContentHash()
+    public function testIsFreshFalseWithContentHash(): void
     {
         $json = $this->createJsonFileMock();
         $inst = $this->createInstallationManagerMock();
diff --git a/tests/Composer/Test/Package/RootAliasPackageTest.php b/tests/Composer/Test/Package/RootAliasPackageTest.php
index 72ffbebcb..0a42fd31d 100644
--- a/tests/Composer/Test/Package/RootAliasPackageTest.php
+++ b/tests/Composer/Test/Package/RootAliasPackageTest.php
@@ -21,7 +21,7 @@ use PHPUnit\Framework\MockObject\MockObject;
 
 class RootAliasPackageTest extends TestCase
 {
-    public function testUpdateRequires()
+    public function testUpdateRequires(): void
     {
         $links = array(new Link('a', 'b', new MatchAllConstraint(), Link::TYPE_REQUIRE, 'self.version'));
 
@@ -36,7 +36,7 @@ class RootAliasPackageTest extends TestCase
         $this->assertNotEmpty($alias->getRequires());
     }
 
-    public function testUpdateDevRequires()
+    public function testUpdateDevRequires(): void
     {
         $links = array(new Link('a', 'b', new MatchAllConstraint(), Link::TYPE_DEV_REQUIRE, 'self.version'));
 
@@ -51,7 +51,7 @@ class RootAliasPackageTest extends TestCase
         $this->assertNotEmpty($alias->getDevRequires());
     }
 
-    public function testUpdateConflicts()
+    public function testUpdateConflicts(): void
     {
         $links = array(new Link('a', 'b', new MatchAllConstraint(), Link::TYPE_CONFLICT, 'self.version'));
 
@@ -66,7 +66,7 @@ class RootAliasPackageTest extends TestCase
         $this->assertNotEmpty($alias->getConflicts());
     }
 
-    public function testUpdateProvides()
+    public function testUpdateProvides(): void
     {
         $links = array(new Link('a', 'b', new MatchAllConstraint(), Link::TYPE_PROVIDE, 'self.version'));
 
@@ -81,7 +81,7 @@ class RootAliasPackageTest extends TestCase
         $this->assertNotEmpty($alias->getProvides());
     }
 
-    public function testUpdateReplaces()
+    public function testUpdateReplaces(): void
     {
         $links = array(new Link('a', 'b', new MatchAllConstraint(), Link::TYPE_REPLACE, 'self.version'));
 
diff --git a/tests/Composer/Test/Package/Version/VersionGuesserTest.php b/tests/Composer/Test/Package/Version/VersionGuesserTest.php
index 5bb666180..4ec717681 100644
--- a/tests/Composer/Test/Package/Version/VersionGuesserTest.php
+++ b/tests/Composer/Test/Package/Version/VersionGuesserTest.php
@@ -28,7 +28,7 @@ class VersionGuesserTest extends TestCase
         }
     }
 
-    public function testHgGuessVersionReturnsData()
+    public function testHgGuessVersionReturnsData(): void
     {
         $branch = 'default';
 
@@ -54,7 +54,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEmpty($versionArray['commit']);
     }
 
-    public function testGuessVersionReturnsData()
+    public function testGuessVersionReturnsData(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
         $anotherCommitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
@@ -79,7 +79,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals($commitHash, $versionArray['commit']);
     }
 
-    public function testGuessVersionDoesNotSeeCustomDefaultBranchAsNonFeatureBranch()
+    public function testGuessVersionDoesNotSeeCustomDefaultBranchAsNonFeatureBranch(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
         $anotherCommitHash = '13a15d220da53c52eddd5f32ffca64a7b3801bea';
@@ -102,7 +102,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals($anotherCommitHash, $versionArray['commit']);
     }
 
-    public function testGuessVersionReadsAndRespectsNonFeatureBranchesConfigurationForArbitraryNaming()
+    public function testGuessVersionReadsAndRespectsNonFeatureBranchesConfigurationForArbitraryNaming(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
         $anotherCommitHash = '13a15d220da53c52eddd5f32ffca64a7b3801bea';
@@ -130,7 +130,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("dev-feature", $versionArray['feature_pretty_version']);
     }
 
-    public function testGuessVersionReadsAndRespectsNonFeatureBranchesConfigurationForArbitraryNamingRegex()
+    public function testGuessVersionReadsAndRespectsNonFeatureBranchesConfigurationForArbitraryNamingRegex(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
         $anotherCommitHash = '13a15d220da53c52eddd5f32ffca64a7b3801bea';
@@ -158,7 +158,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("dev-feature", $versionArray['feature_pretty_version']);
     }
 
-    public function testGuessVersionReadsAndRespectsNonFeatureBranchesConfigurationForArbitraryNamingWhenOnNonFeatureBranch()
+    public function testGuessVersionReadsAndRespectsNonFeatureBranchesConfigurationForArbitraryNamingWhenOnNonFeatureBranch(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
         $anotherCommitHash = '13a15d220da53c52eddd5f32ffca64a7b3801bea';
@@ -182,7 +182,7 @@ class VersionGuesserTest extends TestCase
         $this->assertArrayNotHasKey('feature_pretty_version', $versionArray);
     }
 
-    public function testDetachedHeadBecomesDevHash()
+    public function testDetachedHeadBecomesDevHash(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
 
@@ -203,7 +203,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("dev-$commitHash", $versionData['version']);
     }
 
-    public function testDetachedFetchHeadBecomesDevHashGit2()
+    public function testDetachedFetchHeadBecomesDevHashGit2(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
 
@@ -224,7 +224,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("dev-$commitHash", $versionData['version']);
     }
 
-    public function testDetachedCommitHeadBecomesDevHashGit2()
+    public function testDetachedCommitHeadBecomesDevHashGit2(): void
     {
         $commitHash = '03a15d220da53c52eddd5f32ffca64a7b3801bea';
 
@@ -245,7 +245,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("dev-$commitHash", $versionData['version']);
     }
 
-    public function testTagBecomesVersion()
+    public function testTagBecomesVersion(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -267,7 +267,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("2.0.5.0-alpha2", $versionData['version']);
     }
 
-    public function testTagBecomesPrettyVersion()
+    public function testTagBecomesPrettyVersion(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -290,7 +290,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals('1.0.0', $versionData['pretty_version']);
     }
 
-    public function testInvalidTagBecomesVersion()
+    public function testInvalidTagBecomesVersion(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -308,7 +308,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("dev-foo", $versionData['version']);
     }
 
-    public function testNumericBranchesShowNicely()
+    public function testNumericBranchesShowNicely(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
@@ -327,7 +327,7 @@ class VersionGuesserTest extends TestCase
         $this->assertEquals("1.5.9999999.9999999-dev", $versionData['version']);
     }
 
-    public function testRemoteBranchesAreSelected()
+    public function testRemoteBranchesAreSelected(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(
diff --git a/tests/Composer/Test/Package/Version/VersionParserTest.php b/tests/Composer/Test/Package/Version/VersionParserTest.php
index 8277a0c0e..e467ba25e 100644
--- a/tests/Composer/Test/Package/Version/VersionParserTest.php
+++ b/tests/Composer/Test/Package/Version/VersionParserTest.php
@@ -23,14 +23,14 @@ class VersionParserTest extends TestCase
      * @param string[]                     $pairs
      * @param array<array<string, string>> $result
      */
-    public function testParseNameVersionPairs($pairs, $result)
+    public function testParseNameVersionPairs($pairs, $result): void
     {
         $versionParser = new VersionParser();
 
         $this->assertSame($result, $versionParser->parseNameVersionPairs($pairs));
     }
 
-    public function provideParseNameVersionPairsData()
+    public function provideParseNameVersionPairsData(): array
     {
         return array(
             array(array('php:^7.0'), array(array('name' => 'php', 'version' => '^7.0'))),
@@ -48,12 +48,12 @@ class VersionParserTest extends TestCase
      * @param string $to
      * @param bool   $expected
      */
-    public function testIsUpgrade($from, $to, $expected)
+    public function testIsUpgrade($from, $to, $expected): void
     {
         $this->assertSame($expected, VersionParser::isUpgrade($from, $to));
     }
 
-    public function provideIsUpgradeTests()
+    public function provideIsUpgradeTests(): array
     {
         return array(
             array('0.9.0.0', '1.0.0.0', true),
diff --git a/tests/Composer/Test/Package/Version/VersionSelectorTest.php b/tests/Composer/Test/Package/Version/VersionSelectorTest.php
index 027534c6e..7a8041031 100644
--- a/tests/Composer/Test/Package/Version/VersionSelectorTest.php
+++ b/tests/Composer/Test/Package/Version/VersionSelectorTest.php
@@ -27,7 +27,7 @@ class VersionSelectorTest extends TestCase
     // B) targetPackageVersion will pass to repo set
     // C) No results, throw exception
 
-    public function testLatestVersionIsReturned()
+    public function testLatestVersionIsReturned(): void
     {
         $packageName = 'foobar';
 
@@ -49,7 +49,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be 1.2.2');
     }
 
-    public function testLatestVersionIsReturnedThatMatchesPhpRequirements()
+    public function testLatestVersionIsReturnedThatMatchesPhpRequirements(): void
     {
         $packageName = 'foobar';
 
@@ -75,7 +75,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned when ignoring platform reqs (2.0.0)');
     }
 
-    public function testLatestVersionIsReturnedThatMatchesExtRequirements()
+    public function testLatestVersionIsReturnedThatMatchesExtRequirements(): void
     {
         $packageName = 'foobar';
 
@@ -101,7 +101,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned when ignoring platform reqs (2.0.0)');
     }
 
-    public function testLatestVersionIsReturnedThatMatchesPlatformExt()
+    public function testLatestVersionIsReturnedThatMatchesPlatformExt(): void
     {
         $packageName = 'foobar';
 
@@ -126,7 +126,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned when ignoring platform reqs (2.0.0)');
     }
 
-    public function testLatestVersionIsReturnedThatMatchesComposerRequirements()
+    public function testLatestVersionIsReturnedThatMatchesComposerRequirements(): void
     {
         $packageName = 'foobar';
 
@@ -152,7 +152,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned when ignoring platform reqs (1.1.0)');
     }
 
-    public function testMostStableVersionIsReturned()
+    public function testMostStableVersionIsReturned(): void
     {
         $packageName = 'foobar';
 
@@ -172,7 +172,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package1, $best, 'Latest most stable version should be returned (1.0.0)');
     }
 
-    public function testMostStableVersionIsReturnedRegardlessOfOrder()
+    public function testMostStableVersionIsReturnedRegardlessOfOrder(): void
     {
         $packageName = 'foobar';
 
@@ -197,7 +197,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Expecting 2.0.0-beta3, cause beta is more stable than dev');
     }
 
-    public function testHighestVersionIsReturned()
+    public function testHighestVersionIsReturned(): void
     {
         $packageName = 'foobar';
 
@@ -217,7 +217,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned (1.1.0-beta)');
     }
 
-    public function testHighestVersionMatchingStabilityIsReturned()
+    public function testHighestVersionMatchingStabilityIsReturned(): void
     {
         $packageName = 'foobar';
 
@@ -238,7 +238,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned (1.1.0-beta)');
     }
 
-    public function testMostStableUnstableVersionIsReturned()
+    public function testMostStableUnstableVersionIsReturned(): void
     {
         $packageName = 'foobar';
 
@@ -258,7 +258,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned (1.1.0-beta)');
     }
 
-    public function testDefaultBranchAliasIsNeverReturned()
+    public function testDefaultBranchAliasIsNeverReturned(): void
     {
         $packageName = 'foobar';
 
@@ -279,7 +279,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($package2, $best, 'Latest version should be returned (dev-main)');
     }
 
-    public function testFalseReturnedOnNoPackages()
+    public function testFalseReturnedOnNoPackages(): void
     {
         $repositorySet = $this->createMockRepositorySet();
         $repositorySet->expects($this->once())
@@ -299,7 +299,7 @@ class VersionSelectorTest extends TestCase
      * @param string|null $branchAlias
      * @param string      $packageName
      */
-    public function testFindRecommendedRequireVersion($prettyVersion, $expectedVersion, $branchAlias = null, $packageName = 'foo/bar')
+    public function testFindRecommendedRequireVersion($prettyVersion, $expectedVersion, $branchAlias = null, $packageName = 'foo/bar'): void
     {
         $repositorySet = $this->createMockRepositorySet();
         $versionSelector = new VersionSelector($repositorySet);
@@ -317,7 +317,7 @@ class VersionSelectorTest extends TestCase
         $this->assertSame($expectedVersion, $recommended);
     }
 
-    public function provideRecommendedRequireVersionPackages()
+    public function provideRecommendedRequireVersionPackages(): array
     {
         return array(
             // real version, expected recommendation, [branch-alias], [pkg name]
@@ -362,7 +362,7 @@ class VersionSelectorTest extends TestCase
      *
      * @return Package
      */
-    private function createPackage($version)
+    private function createPackage($version): Package
     {
         $parser = new VersionParser();
 
diff --git a/tests/Composer/Test/Platform/HhvmDetectorTest.php b/tests/Composer/Test/Platform/HhvmDetectorTest.php
index 736a8ae43..eaf93c982 100644
--- a/tests/Composer/Test/Platform/HhvmDetectorTest.php
+++ b/tests/Composer/Test/Platform/HhvmDetectorTest.php
@@ -31,7 +31,7 @@ class HhvmDetectorTest extends TestCase
         $this->hhvmDetector->reset();
     }
 
-    public function testHHVMVersionWhenExecutingInHHVM()
+    public function testHHVMVersionWhenExecutingInHHVM(): void
     {
         if (!defined('HHVM_VERSION_ID')) {
             self::markTestSkipped('Not running with HHVM');
@@ -40,7 +40,7 @@ class HhvmDetectorTest extends TestCase
         self::assertSame(self::versionIdToVersion(), $version);
     }
 
-    public function testHHVMVersionWhenExecutingInPHP()
+    public function testHHVMVersionWhenExecutingInPHP(): void
     {
         if (defined('HHVM_VERSION_ID')) {
             self::markTestSkipped('Running with HHVM');
@@ -71,7 +71,7 @@ class HhvmDetectorTest extends TestCase
     /**
      * @return ?string
      */
-    private static function versionIdToVersion()
+    private static function versionIdToVersion(): ?string
     {
         if (!defined('HHVM_VERSION_ID')) {
             return null;
diff --git a/tests/Composer/Test/Platform/VersionTest.php b/tests/Composer/Test/Platform/VersionTest.php
index 3ebb03724..f2cf6f8f2 100644
--- a/tests/Composer/Test/Platform/VersionTest.php
+++ b/tests/Composer/Test/Platform/VersionTest.php
@@ -27,7 +27,7 @@ class VersionTest extends TestCase
      * 2) git log --pretty=%h --all -- crypto/opensslv.h include/openssl/opensslv.h | while read hash ; do (git show $hash:crypto/opensslv.h; git show $hash:include/openssl/opensslv.h)  | grep "define OPENSSL_VERSION_TEXT"  ; done > versions.txt
      * 3) cat versions.txt | awk -F "OpenSSL " '{print $2}'  | awk -F " " '{print $1}' | sed -e "s:\([0-9]*\.[0-9]*\.[0-9]*\):1.2.3:g" -e "s:1\.2\.3[a-z]\(-.*\)\{0,1\}$:1.2.3a\1:g"  -e "s:1\.2\.3[a-z]\{2\}\(-.*\)\{0,1\}$:1.2.3zh\1:g"  -e "s:beta[0-9]:beta3:g"  -e "s:pre[0-9]*:pre2:g" | sort | uniq
      */
-    public static function provideOpenSslVersions()
+    public static function provideOpenSslVersions(): array
     {
         return array(
             // Generated
@@ -79,7 +79,7 @@ class VersionTest extends TestCase
      * @param bool        $fipsExpected
      * @param string|null $normalizedVersion
      */
-    public function testParseOpensslVersions($input, $parsedVersion, $fipsExpected = false, $normalizedVersion = null)
+    public function testParseOpensslVersions($input, $parsedVersion, $fipsExpected = false, $normalizedVersion = null): void
     {
         self::assertSame($parsedVersion, Version::parseOpenssl($input, $isFips));
         self::assertSame($fipsExpected, $isFips);
@@ -88,7 +88,7 @@ class VersionTest extends TestCase
         self::assertSame($normalizedVersion, $this->getVersionParser()->normalize($parsedVersion));
     }
 
-    public function provideLibJpegVersions()
+    public function provideLibJpegVersions(): array
     {
         return array(
             array('9', '9.0'),
@@ -104,12 +104,12 @@ class VersionTest extends TestCase
      * @param string $input
      * @param string $parsedVersion
      */
-    public function testParseLibjpegVersion($input, $parsedVersion)
+    public function testParseLibjpegVersion($input, $parsedVersion): void
     {
         self::assertSame($parsedVersion, Version::parseLibjpeg($input));
     }
 
-    public function provideZoneinfoVersions()
+    public function provideZoneinfoVersions(): array
     {
         return array(
             array('2019c', '2019.3'),
@@ -124,7 +124,7 @@ class VersionTest extends TestCase
      * @param string $input
      * @param string $parsedVersion
      */
-    public function testParseZoneinfoVersion($input, $parsedVersion)
+    public function testParseZoneinfoVersion($input, $parsedVersion): void
     {
         self::assertSame($parsedVersion, Version::parseZoneinfoVersion($input));
     }
diff --git a/tests/Composer/Test/Plugin/PluginInstallerTest.php b/tests/Composer/Test/Plugin/PluginInstallerTest.php
index 43022a20d..325f26671 100644
--- a/tests/Composer/Test/Plugin/PluginInstallerTest.php
+++ b/tests/Composer/Test/Plugin/PluginInstallerTest.php
@@ -96,7 +96,7 @@ class PluginInstallerTest extends TestCase
         $im = $this->getMockBuilder('Composer\Installer\InstallationManager')->disableOriginalConstructor()->getMock();
         $im->expects($this->any())
             ->method('getInstallPath')
-            ->will($this->returnCallback(function ($package) {
+            ->will($this->returnCallback(function ($package): string {
                 return __DIR__.'/Fixtures/'.$package->getPrettyName();
             }));
 
@@ -135,7 +135,7 @@ class PluginInstallerTest extends TestCase
         $filesystem->removeDirectory($this->directory);
     }
 
-    public function testInstallNewPlugin()
+    public function testInstallNewPlugin(): void
     {
         $this->repository
             ->expects($this->any())
@@ -154,7 +154,7 @@ class PluginInstallerTest extends TestCase
         );
     }
 
-    public function testInstallPluginWithRootPackageHavingFilesAutoload()
+    public function testInstallPluginWithRootPackageHavingFilesAutoload(): void
     {
         $this->repository
             ->expects($this->any())
@@ -176,7 +176,7 @@ class PluginInstallerTest extends TestCase
         $this->assertEquals('installer-v1', $plugins[0]->version);  // @phpstan-ignore-line
     }
 
-    public function testInstallMultiplePlugins()
+    public function testInstallMultiplePlugins(): void
     {
         $this->repository
             ->expects($this->any())
@@ -195,7 +195,7 @@ class PluginInstallerTest extends TestCase
         $this->assertEquals('activate v4-plugin1'.PHP_EOL.'activate v4-plugin2'.PHP_EOL, $this->io->getOutput());
     }
 
-    public function testUpgradeWithNewClassName()
+    public function testUpgradeWithNewClassName(): void
     {
         $this->repository
             ->expects($this->any())
@@ -216,7 +216,7 @@ class PluginInstallerTest extends TestCase
         $this->assertEquals('activate v1'.PHP_EOL.'deactivate v1'.PHP_EOL.'activate v2'.PHP_EOL, $this->io->getOutput());
     }
 
-    public function testUninstall()
+    public function testUninstall(): void
     {
         $this->repository
             ->expects($this->any())
@@ -236,7 +236,7 @@ class PluginInstallerTest extends TestCase
         $this->assertEquals('activate v1'.PHP_EOL.'deactivate v1'.PHP_EOL.'uninstall v1'.PHP_EOL, $this->io->getOutput());
     }
 
-    public function testUpgradeWithSameClassName()
+    public function testUpgradeWithSameClassName(): void
     {
         $this->repository
             ->expects($this->any())
@@ -256,7 +256,7 @@ class PluginInstallerTest extends TestCase
         $this->assertEquals('activate v2'.PHP_EOL.'deactivate v2'.PHP_EOL.'activate v3'.PHP_EOL, $this->io->getOutput());
     }
 
-    public function testRegisterPluginOnlyOneTime()
+    public function testRegisterPluginOnlyOneTime(): void
     {
         $this->repository
             ->expects($this->any())
@@ -280,7 +280,7 @@ class PluginInstallerTest extends TestCase
      *
      * @return void
      */
-    private function setPluginApiVersionWithPlugins($newPluginApiVersion, array $plugins = array())
+    private function setPluginApiVersionWithPlugins($newPluginApiVersion, array $plugins = array()): void
     {
         // reset the plugin manager's installed plugins
         $this->pm = $this->getMockBuilder('Composer\Plugin\PluginManager')
@@ -303,14 +303,14 @@ class PluginInstallerTest extends TestCase
         $this->repository
             ->expects($this->any())
             ->method('getPackages')
-            ->will($this->returnCallback(function () use ($plugApiInternalPackage, $plugins) {
+            ->will($this->returnCallback(function () use ($plugApiInternalPackage, $plugins): array {
                 return array_merge(array($plugApiInternalPackage), $plugins);
             }));
 
         $this->pm->loadInstalledPlugins();
     }
 
-    public function testStarPluginVersionWorksWithAnyAPIVersion()
+    public function testStarPluginVersionWorksWithAnyAPIVersion(): void
     {
         $starVersionPlugin = array($this->packages[4]);
 
@@ -327,7 +327,7 @@ class PluginInstallerTest extends TestCase
         $this->assertCount(1, $this->pm->getPlugins());
     }
 
-    public function testPluginConstraintWorksOnlyWithCertainAPIVersion()
+    public function testPluginConstraintWorksOnlyWithCertainAPIVersion(): void
     {
         $pluginWithApiConstraint = array($this->packages[5]);
 
@@ -344,7 +344,7 @@ class PluginInstallerTest extends TestCase
         $this->assertCount(1, $this->pm->getPlugins());
     }
 
-    public function testPluginRangeConstraintsWorkOnlyWithCertainAPIVersion()
+    public function testPluginRangeConstraintsWorkOnlyWithCertainAPIVersion(): void
     {
         $pluginWithApiConstraint = array($this->packages[6]);
 
@@ -358,7 +358,7 @@ class PluginInstallerTest extends TestCase
         $this->assertCount(0, $this->pm->getPlugins());
     }
 
-    public function testCommandProviderCapability()
+    public function testCommandProviderCapability(): void
     {
         $this->repository
             ->expects($this->any())
@@ -377,14 +377,14 @@ class PluginInstallerTest extends TestCase
         $this->assertInstanceOf('Composer\Command\BaseCommand', $commands[0]);
     }
 
-    public function testIncapablePluginIsCorrectlyDetected()
+    public function testIncapablePluginIsCorrectlyDetected(): void
     {
         $plugin = $this->getMockBuilder('Composer\Plugin\PluginInterface')
                        ->getMock();
         $this->assertNull($this->pm->getPluginCapability($plugin, 'Fake\Ability'));
     }
 
-    public function testCapabilityImplementsComposerPluginApiClassAndIsConstructedWithArgs()
+    public function testCapabilityImplementsComposerPluginApiClassAndIsConstructedWithArgs(): void
     {
         $capabilityApi = 'Composer\Plugin\Capability\Capability';
         $capabilityImplementation = 'Composer\Test\Plugin\Mock\Capability';
@@ -394,7 +394,7 @@ class PluginInstallerTest extends TestCase
 
         $plugin->expects($this->once())
                ->method('getCapabilities')
-               ->will($this->returnCallback(function () use ($capabilityImplementation, $capabilityApi) {
+               ->will($this->returnCallback(function () use ($capabilityImplementation, $capabilityApi): array {
                    return array($capabilityApi => $capabilityImplementation);
                }));
 
@@ -407,7 +407,7 @@ class PluginInstallerTest extends TestCase
     }
 
     /** @return mixed[] */
-    public function invalidImplementationClassNames()
+    public function invalidImplementationClassNames(): array
     {
         return array(
             array(null),
@@ -422,7 +422,7 @@ class PluginInstallerTest extends TestCase
     }
 
     /** @return mixed[] */
-    public function nonExistingOrInvalidImplementationClassTypes()
+    public function nonExistingOrInvalidImplementationClassTypes(): array
     {
         return array(
             array('\stdClass'),
@@ -437,7 +437,7 @@ class PluginInstallerTest extends TestCase
      *
      * @return void
      */
-    public function testQueryingWithInvalidCapabilityClassNameThrows($invalidImplementationClassNames, $expect = 'UnexpectedValueException')
+    public function testQueryingWithInvalidCapabilityClassNameThrows($invalidImplementationClassNames, $expect = 'UnexpectedValueException'): void
     {
         self::expectException($expect);
 
@@ -448,7 +448,7 @@ class PluginInstallerTest extends TestCase
 
         $plugin->expects($this->once())
                ->method('getCapabilities')
-               ->will($this->returnCallback(function () use ($invalidImplementationClassNames, $capabilityApi) {
+               ->will($this->returnCallback(function () use ($invalidImplementationClassNames, $capabilityApi): array {
                    return array($capabilityApi => $invalidImplementationClassNames);
                }));
 
@@ -456,7 +456,7 @@ class PluginInstallerTest extends TestCase
     }
 
     /** @return void */
-    public function testQueryingNonProvidedCapabilityReturnsNullSafely()
+    public function testQueryingNonProvidedCapabilityReturnsNullSafely(): void
     {
         $capabilityApi = 'Composer\Plugin\Capability\MadeUpCapability';
 
@@ -465,7 +465,7 @@ class PluginInstallerTest extends TestCase
 
         $plugin->expects($this->once())
                ->method('getCapabilities')
-               ->will($this->returnCallback(function () {
+               ->will($this->returnCallback(function (): array {
                    return array();
                }));
 
@@ -478,7 +478,7 @@ class PluginInstallerTest extends TestCase
      *
      * @return void
      */
-    public function testQueryingWithNonExistingOrWrongCapabilityClassTypesThrows($wrongImplementationClassTypes)
+    public function testQueryingWithNonExistingOrWrongCapabilityClassTypesThrows($wrongImplementationClassTypes): void
     {
         $this->testQueryingWithInvalidCapabilityClassNameThrows($wrongImplementationClassTypes, 'RuntimeException');
     }
diff --git a/tests/Composer/Test/Question/StrictConfirmationQuestionTest.php b/tests/Composer/Test/Question/StrictConfirmationQuestionTest.php
index 925f278bb..7d9c94a75 100644
--- a/tests/Composer/Test/Question/StrictConfirmationQuestionTest.php
+++ b/tests/Composer/Test/Question/StrictConfirmationQuestionTest.php
@@ -30,7 +30,7 @@ class StrictConfirmationQuestionTest extends TestCase
      *
      * @phpstan-return list<array{non-empty-string}>
      */
-    public function getAskConfirmationBadData()
+    public function getAskConfirmationBadData(): array
     {
         return array(
             array('not correct'),
@@ -45,7 +45,7 @@ class StrictConfirmationQuestionTest extends TestCase
      *
      * @param string $answer
      */
-    public function testAskConfirmationBadAnswer($answer)
+    public function testAskConfirmationBadAnswer($answer): void
     {
         list($input, $dialog) = $this->createInput($answer."\n");
 
@@ -64,7 +64,7 @@ class StrictConfirmationQuestionTest extends TestCase
      * @param bool   $expected
      * @param bool   $default
      */
-    public function testAskConfirmation($question, $expected, $default = true)
+    public function testAskConfirmation($question, $expected, $default = true): void
     {
         list($input, $dialog) = $this->createInput($question."\n");
 
@@ -77,7 +77,7 @@ class StrictConfirmationQuestionTest extends TestCase
      *
      * @phpstan-return list<array{string, bool}>|list<array{string, bool, bool}>
      */
-    public function getAskConfirmationData()
+    public function getAskConfirmationData(): array
     {
         return array(
             array('', true),
@@ -89,7 +89,7 @@ class StrictConfirmationQuestionTest extends TestCase
         );
     }
 
-    public function testAskConfirmationWithCustomTrueAndFalseAnswer()
+    public function testAskConfirmationWithCustomTrueAndFalseAnswer(): void
     {
         $question = new StrictConfirmationQuestion('Do you like French fries?', false, '/^ja$/i', '/^nein$/i');
 
@@ -119,7 +119,7 @@ class StrictConfirmationQuestionTest extends TestCase
     /**
      * @return StreamOutput
      */
-    protected function createOutputInterface()
+    protected function createOutputInterface(): StreamOutput
     {
         return new StreamOutput(fopen('php://memory', 'r+', false));
     }
@@ -131,7 +131,7 @@ class StrictConfirmationQuestionTest extends TestCase
      *
      * @phpstan-return array{ArrayInput, QuestionHelper}
      */
-    protected function createInput($entry)
+    protected function createInput($entry): array
     {
         $input = new ArrayInput(array('--no-interaction'));
         $input->setStream($this->getInputStream($entry));
diff --git a/tests/Composer/Test/Repository/ArrayRepositoryTest.php b/tests/Composer/Test/Repository/ArrayRepositoryTest.php
index fe50a29b2..f2261eac2 100644
--- a/tests/Composer/Test/Repository/ArrayRepositoryTest.php
+++ b/tests/Composer/Test/Repository/ArrayRepositoryTest.php
@@ -18,7 +18,7 @@ use Composer\Test\TestCase;
 
 class ArrayRepositoryTest extends TestCase
 {
-    public function testAddPackage()
+    public function testAddPackage(): void
     {
         $repo = new ArrayRepository;
         $repo->addPackage($this->getPackage('foo', '1'));
@@ -26,7 +26,7 @@ class ArrayRepositoryTest extends TestCase
         $this->assertCount(1, $repo);
     }
 
-    public function testRemovePackage()
+    public function testRemovePackage(): void
     {
         $package = $this->getPackage('bar', '2');
 
@@ -42,7 +42,7 @@ class ArrayRepositoryTest extends TestCase
         $this->assertEquals(array($package), $repo->getPackages());
     }
 
-    public function testHasPackage()
+    public function testHasPackage(): void
     {
         $repo = new ArrayRepository;
         $repo->addPackage($this->getPackage('foo', '1'));
@@ -52,7 +52,7 @@ class ArrayRepositoryTest extends TestCase
         $this->assertFalse($repo->hasPackage($this->getPackage('bar', '1')));
     }
 
-    public function testFindPackages()
+    public function testFindPackages(): void
     {
         $repo = new ArrayRepository();
         $repo->addPackage($this->getPackage('foo', '1'));
@@ -68,7 +68,7 @@ class ArrayRepositoryTest extends TestCase
         $this->assertEquals('bar', $bar[0]->getName());
     }
 
-    public function testAutomaticallyAddAliasedPackageButNotRemove()
+    public function testAutomaticallyAddAliasedPackageButNotRemove(): void
     {
         $repo = new ArrayRepository();
 
@@ -86,7 +86,7 @@ class ArrayRepositoryTest extends TestCase
         $this->assertCount(1, $repo);
     }
 
-    public function testSearch()
+    public function testSearch(): void
     {
         $repo = new ArrayRepository();
 
@@ -108,7 +108,7 @@ class ArrayRepositoryTest extends TestCase
         );
     }
 
-    public function testSearchWithPackageType()
+    public function testSearchWithPackageType(): void
     {
         $repo = new ArrayRepository();
 
@@ -132,7 +132,7 @@ class ArrayRepositoryTest extends TestCase
         );
     }
 
-    public function testSearchWithAbandonedPackages()
+    public function testSearchWithAbandonedPackages(): void
     {
         $repo = new ArrayRepository();
 
diff --git a/tests/Composer/Test/Repository/ArtifactRepositoryTest.php b/tests/Composer/Test/Repository/ArtifactRepositoryTest.php
index 49c37a61a..ee44153a3 100644
--- a/tests/Composer/Test/Repository/ArtifactRepositoryTest.php
+++ b/tests/Composer/Test/Repository/ArtifactRepositoryTest.php
@@ -27,7 +27,7 @@ class ArtifactRepositoryTest extends TestCase
         }
     }
 
-    public function testExtractsConfigsFromZipArchives()
+    public function testExtractsConfigsFromZipArchives(): void
     {
         $expectedPackages = array(
             'vendor0/package0-0.0.1',
@@ -53,7 +53,7 @@ class ArtifactRepositoryTest extends TestCase
 
         $this->assertSame($expectedPackages, $foundPackages);
 
-        $tarPackage = array_filter($repo->getPackages(), function (BasePackage $package) {
+        $tarPackage = array_filter($repo->getPackages(), function (BasePackage $package): bool {
             return $package->getPrettyName() === 'test/jsonInRootTarFile';
         });
         $this->assertCount(1, $tarPackage);
@@ -61,7 +61,7 @@ class ArtifactRepositoryTest extends TestCase
         $this->assertSame('tar', $tarPackage->getDistType());
     }
 
-    public function testAbsoluteRepoUrlCreatesAbsoluteUrlPackages()
+    public function testAbsoluteRepoUrlCreatesAbsoluteUrlPackages(): void
     {
         $absolutePath = __DIR__ . '/Fixtures/artifacts';
         $coordinates = array('type' => 'artifact', 'url' => $absolutePath);
@@ -72,7 +72,7 @@ class ArtifactRepositoryTest extends TestCase
         }
     }
 
-    public function testRelativeRepoUrlCreatesRelativeUrlPackages()
+    public function testRelativeRepoUrlCreatesRelativeUrlPackages(): void
     {
         $relativePath = 'tests/Composer/Test/Repository/Fixtures/artifacts';
         $coordinates = array('type' => 'artifact', 'url' => $relativePath);
diff --git a/tests/Composer/Test/Repository/ComposerRepositoryTest.php b/tests/Composer/Test/Repository/ComposerRepositoryTest.php
index 21b9f56be..f6076b3f5 100644
--- a/tests/Composer/Test/Repository/ComposerRepositoryTest.php
+++ b/tests/Composer/Test/Repository/ComposerRepositoryTest.php
@@ -28,7 +28,7 @@ class ComposerRepositoryTest extends TestCase
      * @param mixed[]              $expected
      * @param array<string, mixed> $repoPackages
      */
-    public function testLoadData(array $expected, array $repoPackages)
+    public function testLoadData(array $expected, array $repoPackages): void
     {
         $repoConfig = array(
             'url' => 'http://example.org',
@@ -61,7 +61,7 @@ class ComposerRepositoryTest extends TestCase
         }
     }
 
-    public function loadDataProvider()
+    public function loadDataProvider(): array
     {
         return array(
             // Old repository format
@@ -92,7 +92,7 @@ class ComposerRepositoryTest extends TestCase
         );
     }
 
-    public function testWhatProvides()
+    public function testWhatProvides(): void
     {
         $repo = $this->getMockBuilder('Composer\Repository\ComposerRepository')
             ->setConstructorArgs(array(
@@ -156,7 +156,7 @@ class ComposerRepositoryTest extends TestCase
         $this->assertSame($packages['2'], $packages['2-alias']->getAliasOf());
     }
 
-    public function testSearchWithType()
+    public function testSearchWithType(): void
     {
         $repoConfig = array(
             'url' => 'http://example.org',
@@ -198,7 +198,7 @@ class ComposerRepositoryTest extends TestCase
         );
     }
 
-    public function testSearchWithSpecialChars()
+    public function testSearchWithSpecialChars(): void
     {
         $repoConfig = array(
             'url' => 'http://example.org',
@@ -225,7 +225,7 @@ class ComposerRepositoryTest extends TestCase
         );
     }
 
-    public function testSearchWithAbandonedPackages()
+    public function testSearchWithAbandonedPackages(): void
     {
         $repoConfig = array(
             'url' => 'http://example.org',
@@ -279,7 +279,7 @@ class ComposerRepositoryTest extends TestCase
      * @param string $url
      * @param string $repositoryUrl
      */
-    public function testCanonicalizeUrl($expected, $url, $repositoryUrl)
+    public function testCanonicalizeUrl($expected, $url, $repositoryUrl): void
     {
         $repository = new ComposerRepository(
             array('url' => $repositoryUrl),
@@ -303,7 +303,7 @@ class ComposerRepositoryTest extends TestCase
         $this->assertSame($expected, $method->invoke($repository, $url));
     }
 
-    public function provideCanonicalizeUrlTestCases()
+    public function provideCanonicalizeUrlTestCases(): array
     {
         return array(
             array(
@@ -341,7 +341,7 @@ class ComposerRepositoryTest extends TestCase
         );
     }
 
-    public function testGetProviderNamesWillReturnPartialPackageNames()
+    public function testGetProviderNamesWillReturnPartialPackageNames(): void
     {
         $httpDownloader = $this->getHttpDownloaderMock();
         $httpDownloader->expects(
diff --git a/tests/Composer/Test/Repository/CompositeRepositoryTest.php b/tests/Composer/Test/Repository/CompositeRepositoryTest.php
index 5e49d725a..1cf8cbe84 100644
--- a/tests/Composer/Test/Repository/CompositeRepositoryTest.php
+++ b/tests/Composer/Test/Repository/CompositeRepositoryTest.php
@@ -18,7 +18,7 @@ use Composer\Test\TestCase;
 
 class CompositeRepositoryTest extends TestCase
 {
-    public function testHasPackage()
+    public function testHasPackage(): void
     {
         $arrayRepoOne = new ArrayRepository;
         $arrayRepoOne->addPackage($this->getPackage('foo', '1'));
@@ -35,7 +35,7 @@ class CompositeRepositoryTest extends TestCase
         $this->assertFalse($repo->hasPackage($this->getPackage('bar', '2')), "Should not have package 'bar/2'");
     }
 
-    public function testFindPackage()
+    public function testFindPackage(): void
     {
         $arrayRepoOne = new ArrayRepository;
         $arrayRepoOne->addPackage($this->getPackage('foo', '1'));
@@ -52,7 +52,7 @@ class CompositeRepositoryTest extends TestCase
         $this->assertNull($repo->findPackage('foo', '2'), "Should not find package 'foo/2'");
     }
 
-    public function testFindPackages()
+    public function testFindPackages(): void
     {
         $arrayRepoOne = new ArrayRepository;
         $arrayRepoOne->addPackage($this->getPackage('foo', '1'));
@@ -79,7 +79,7 @@ class CompositeRepositoryTest extends TestCase
         $this->assertEquals('foo', $foos[0]->getName(), "Should find packages named 'foo'");
     }
 
-    public function testGetPackages()
+    public function testGetPackages(): void
     {
         $arrayRepoOne = new ArrayRepository;
         $arrayRepoOne->addPackage($this->getPackage('foo', '1'));
@@ -97,7 +97,7 @@ class CompositeRepositoryTest extends TestCase
         $this->assertEquals("1", $packages[1]->getPrettyVersion(), "Second package should have pretty version of '1'");
     }
 
-    public function testAddRepository()
+    public function testAddRepository(): void
     {
         $arrayRepoOne = new ArrayRepository;
         $arrayRepoOne->addPackage($this->getPackage('foo', '1'));
@@ -113,7 +113,7 @@ class CompositeRepositoryTest extends TestCase
         $this->assertCount(4, $repo, "Composite repository should have four packages after addRepository() is called");
     }
 
-    public function testCount()
+    public function testCount(): void
     {
         $arrayRepoOne = new ArrayRepository;
         $arrayRepoOne->addPackage($this->getPackage('foo', '1'));
@@ -132,13 +132,13 @@ class CompositeRepositoryTest extends TestCase
      * @param string $method
      * @param mixed[] $args
      */
-    public function testNoRepositories($method, $args)
+    public function testNoRepositories($method, $args): void
     {
         $repo = new CompositeRepository(array());
         $this->assertEquals(array(), call_user_func_array(array($repo, $method), $args));
     }
 
-    public function provideMethodCalls()
+    public function provideMethodCalls(): array
     {
         return array(
             array('findPackages', array('foo')),
diff --git a/tests/Composer/Test/Repository/FilesystemRepositoryTest.php b/tests/Composer/Test/Repository/FilesystemRepositoryTest.php
index ac1d78e4d..ad0466191 100644
--- a/tests/Composer/Test/Repository/FilesystemRepositoryTest.php
+++ b/tests/Composer/Test/Repository/FilesystemRepositoryTest.php
@@ -21,7 +21,7 @@ use Composer\Util\Filesystem;
 
 class FilesystemRepositoryTest extends TestCase
 {
-    public function testRepositoryRead()
+    public function testRepositoryRead(): void
     {
         $json = $this->createJsonFileMock();
 
@@ -46,7 +46,7 @@ class FilesystemRepositoryTest extends TestCase
         $this->assertSame('vendor', $packages[0]->getType());
     }
 
-    public function testCorruptedRepositoryFile()
+    public function testCorruptedRepositoryFile(): void
     {
         self::expectException('Composer\Repository\InvalidRepositoryException');
         $json = $this->createJsonFileMock();
@@ -65,7 +65,7 @@ class FilesystemRepositoryTest extends TestCase
         $repository->getPackages();
     }
 
-    public function testUnexistentRepositoryFile()
+    public function testUnexistentRepositoryFile(): void
     {
         $json = $this->createJsonFileMock();
 
@@ -79,7 +79,7 @@ class FilesystemRepositoryTest extends TestCase
         $this->assertEquals(array(), $repository->getPackages());
     }
 
-    public function testRepositoryWrite()
+    public function testRepositoryWrite(): void
     {
         $json = $this->createJsonFileMock();
 
@@ -125,7 +125,7 @@ class FilesystemRepositoryTest extends TestCase
         $repository->write(true, $im);
     }
 
-    public function testRepositoryWritesInstalledPhp()
+    public function testRepositoryWritesInstalledPhp(): void
     {
         $dir = $this->getUniqueTmpDirectory();
         chdir($dir);
@@ -171,7 +171,7 @@ class FilesystemRepositoryTest extends TestCase
             ->getMock();
         $im->expects($this->any())
             ->method('getInstallPath')
-            ->will($this->returnCallback(function ($package) use ($dir) {
+            ->will($this->returnCallback(function ($package) use ($dir): string {
                 // check for empty paths handling
                 if ($package->getType() === 'metapackage') {
                     return '';
diff --git a/tests/Composer/Test/Repository/FilterRepositoryTest.php b/tests/Composer/Test/Repository/FilterRepositoryTest.php
index d18e8a13e..3a1c34e11 100644
--- a/tests/Composer/Test/Repository/FilterRepositoryTest.php
+++ b/tests/Composer/Test/Repository/FilterRepositoryTest.php
@@ -40,17 +40,17 @@ class FilterRepositoryTest extends TestCase
      * @param string[]                                                               $expected
      * @param array{only?: array<string>, exclude?: array<string>, canonical?: bool} $config
      */
-    public function testRepoMatching($expected, $config)
+    public function testRepoMatching($expected, $config): void
     {
         $repo = new FilterRepository($this->arrayRepo, $config);
         $packages = $repo->getPackages();
 
-        $this->assertSame($expected, array_map(function ($p) {
+        $this->assertSame($expected, array_map(function ($p): string {
             return $p->getName();
         }, $packages));
     }
 
-    public static function provideRepoMatchingTestCases()
+    public static function provideRepoMatchingTestCases(): array
     {
         return array(
             array(array('foo/aaa', 'foo/bbb'), array('only' => array('foo/*'))),
@@ -62,7 +62,7 @@ class FilterRepositoryTest extends TestCase
         );
     }
 
-    public function testCanonicalDefaultTrue()
+    public function testCanonicalDefaultTrue(): void
     {
         $repo = new FilterRepository($this->arrayRepo, array());
         $result = $repo->loadPackages(array('foo/aaa' => new MatchAllConstraint), BasePackage::$stabilities, array());
@@ -70,7 +70,7 @@ class FilterRepositoryTest extends TestCase
         $this->assertCount(1, $result['namesFound']);
     }
 
-    public function testNonCanonical()
+    public function testNonCanonical(): void
     {
         $repo = new FilterRepository($this->arrayRepo, array('canonical' => false));
         $result = $repo->loadPackages(array('foo/aaa' => new MatchAllConstraint), BasePackage::$stabilities, array());
diff --git a/tests/Composer/Test/Repository/InstalledRepositoryTest.php b/tests/Composer/Test/Repository/InstalledRepositoryTest.php
index 136c19c7b..c4f32a1a5 100644
--- a/tests/Composer/Test/Repository/InstalledRepositoryTest.php
+++ b/tests/Composer/Test/Repository/InstalledRepositoryTest.php
@@ -21,7 +21,7 @@ use Composer\Test\TestCase;
 
 class InstalledRepositoryTest extends TestCase
 {
-    public function testFindPackagesWithReplacersAndProviders()
+    public function testFindPackagesWithReplacersAndProviders(): void
     {
         $arrayRepoOne = new InstalledArrayRepository;
         $arrayRepoOne->addPackage($foo = $this->getPackage('foo', '1'));
@@ -41,7 +41,7 @@ class InstalledRepositoryTest extends TestCase
         $this->assertEquals(array($foo, $bar2), $repo->findPackagesWithReplacersAndProviders('provided'));
     }
 
-    public function testAddRepository()
+    public function testAddRepository(): void
     {
         $arrayRepoOne = new ArrayRepository;
 
diff --git a/tests/Composer/Test/Repository/PathRepositoryTest.php b/tests/Composer/Test/Repository/PathRepositoryTest.php
index bff6fd6b2..9717a4457 100644
--- a/tests/Composer/Test/Repository/PathRepositoryTest.php
+++ b/tests/Composer/Test/Repository/PathRepositoryTest.php
@@ -17,7 +17,7 @@ use Composer\Test\TestCase;
 
 class PathRepositoryTest extends TestCase
 {
-    public function testLoadPackageFromFileSystemWithIncorrectPath()
+    public function testLoadPackageFromFileSystemWithIncorrectPath(): void
     {
         self::expectException('RuntimeException');
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
@@ -30,7 +30,7 @@ class PathRepositoryTest extends TestCase
         $repository->getPackages();
     }
 
-    public function testLoadPackageFromFileSystemWithVersion()
+    public function testLoadPackageFromFileSystemWithVersion(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
@@ -46,7 +46,7 @@ class PathRepositoryTest extends TestCase
         $this->assertTrue($repository->hasPackage($this->getPackage('test/path-versioned', '0.0.2')));
     }
 
-    public function testLoadPackageFromFileSystemWithoutVersion()
+    public function testLoadPackageFromFileSystemWithoutVersion(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
@@ -67,7 +67,7 @@ class PathRepositoryTest extends TestCase
         $this->assertNotEmpty($packageVersion);
     }
 
-    public function testLoadPackageFromFileSystemWithWildcard()
+    public function testLoadPackageFromFileSystemWithWildcard(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
@@ -92,7 +92,7 @@ class PathRepositoryTest extends TestCase
         $this->assertEquals(array('test/path-unversioned', 'test/path-versioned'), $names);
     }
 
-    public function testLoadPackageWithExplicitVersions()
+    public function testLoadPackageWithExplicitVersions(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
@@ -127,7 +127,7 @@ class PathRepositoryTest extends TestCase
     /**
      * Verify relative repository URLs remain relative, see #4439
      */
-    public function testUrlRemainsRelative()
+    public function testUrlRemainsRelative(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
@@ -155,7 +155,7 @@ class PathRepositoryTest extends TestCase
         $this->assertSame($relativeUrl, $package->getDistUrl());
     }
 
-    public function testReferenceNone()
+    public function testReferenceNone(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
@@ -176,7 +176,7 @@ class PathRepositoryTest extends TestCase
         }
     }
 
-    public function testReferenceConfig()
+    public function testReferenceConfig(): void
     {
         $ioInterface = $this->getMockBuilder('Composer\IO\IOInterface')
             ->getMock();
diff --git a/tests/Composer/Test/Repository/PlatformRepositoryTest.php b/tests/Composer/Test/Repository/PlatformRepositoryTest.php
index 05f030eac..68418d72b 100644
--- a/tests/Composer/Test/Repository/PlatformRepositoryTest.php
+++ b/tests/Composer/Test/Repository/PlatformRepositoryTest.php
@@ -21,7 +21,7 @@ use PHPUnit\Framework\Assert;
 
 class PlatformRepositoryTest extends TestCase
 {
-    public function testHhvmPackage()
+    public function testHhvmPackage(): void
     {
         $hhvmDetector = $this->getMockBuilder('Composer\Platform\HhvmDetector')->getMock();
         $platformRepository = new PlatformRepository(array(), array(), null, $hhvmDetector);
@@ -36,7 +36,7 @@ class PlatformRepositoryTest extends TestCase
         self::assertSame('2.1.0', $hhvm->getPrettyVersion());
     }
 
-    public function providePhpFlavorTestCases()
+    public function providePhpFlavorTestCases(): array
     {
         return array(
             array(
@@ -120,7 +120,7 @@ class PlatformRepositoryTest extends TestCase
      * @param array<string, string> $packages
      * @param list<array{string, list<string>, string|bool}>  $functions
      */
-    public function testPhpVersion(array $constants, array $packages, array $functions = array())
+    public function testPhpVersion(array $constants, array $packages, array $functions = array()): void
     {
         $runtime = $this->getMockBuilder('Composer\Platform\Runtime')->getMock();
         $runtime
@@ -128,18 +128,14 @@ class PlatformRepositoryTest extends TestCase
             ->willReturn(array());
         $runtime
             ->method('hasConstant')
-            ->willReturnMap(
-                array_map(function ($constant) {
-                    return array($constant, null, true);
-                }, array_keys($constants))
-            );
+            ->willReturnCallback(function ($constant, $class = null) use ($constants): bool {
+                return isset($constants[ltrim($class.'::'.$constant, ':')]);
+            });
         $runtime
             ->method('getConstant')
-            ->willReturnMap(
-                array_map(function ($constant, $value) {
-                    return array($constant, null, $value);
-                }, array_keys($constants), array_values($constants))
-            );
+            ->willReturnCallback(function ($constant, $class = null) use ($constants) {
+                return $constants[ltrim($class.'::'.$constant, ':')] ?? null;
+            });
         $runtime
             ->method('invoke')
             ->willReturnMap($functions);
@@ -152,7 +148,7 @@ class PlatformRepositoryTest extends TestCase
         }
     }
 
-    public function testInetPtonRegression()
+    public function testInetPtonRegression(): void
     {
         $runtime = $this->getMockBuilder('Composer\Platform\Runtime')->getMock();
 
@@ -163,29 +159,26 @@ class PlatformRepositoryTest extends TestCase
             ->willReturn(false);
         $runtime
             ->method('hasConstant')
-            ->willReturnMap(
-                array(
-                    array('PHP_ZTS', false),
-                    array('AF_INET6', false),
-                )
-            );
+            ->willReturn(false); // suppressing PHP_ZTS & AF_INET6
+
+        $constants = [
+            'PHP_VERSION' => '7.0.0',
+            'PHP_DEBUG' => false,
+        ];
+        $runtime
+            ->method('getConstant')
+            ->willReturnCallback(function ($constant, $class = null) use ($constants) {
+                return $constants[ltrim($class.'::'.$constant, ':')] ?? null;
+            });
         $runtime
             ->method('getExtensions')
             ->willReturn(array());
-        $runtime
-            ->method('getConstant')
-            ->willReturnMap(
-                array(
-                    array('PHP_VERSION', null, '7.0.0'),
-                    array('PHP_DEBUG', null, false),
-                )
-            );
         $repository = new PlatformRepository(array(), array(), $runtime);
         $package = $repository->findPackage('php-ipv6', '*');
         self::assertNull($package);
     }
 
-    public static function provideLibraryTestCases()
+    public static function provideLibraryTestCases(): array
     {
         return array(
             'amqp' => array(
@@ -1097,7 +1090,7 @@ Linked Version => 1.2.11',
         array $functions = array(),
         array $constants = array(),
         array $classDefinitions = array()
-    ) {
+    ): void {
         $extensions = (array) $extensions;
 
         $extensionVersion = '100.200.300';
@@ -1110,7 +1103,7 @@ Linked Version => 1.2.11',
         $runtime
             ->method('getExtensionVersion')
             ->willReturnMap(
-                array_map(function ($extension) use ($extensionVersion) {
+                array_map(function ($extension) use ($extensionVersion): array {
                     return array($extension, $extensionVersion);
                 }, $extensions)
             );
@@ -1118,7 +1111,7 @@ Linked Version => 1.2.11',
         $runtime
             ->method('getExtensionInfo')
             ->willReturnMap(
-                array_map(function ($extension) use ($info) {
+                array_map(function ($extension) use ($info): array {
                     return array($extension, $info);
                 }, $extensions)
             );
@@ -1130,55 +1123,57 @@ Linked Version => 1.2.11',
         $constants[] = array('PHP_VERSION', null, '7.1.0');
         $runtime
             ->method('hasConstant')
-            ->willReturnMap(
-                array_map(
-                    function ($constantDefintion) {
-                        return array($constantDefintion[0], $constantDefintion[1], true);
-                    },
-                    $constants
-                )
-            );
+            ->willReturnCallback(function ($constant, $class = null) use ($constants): bool {
+                foreach ($constants as $definition) {
+                    if ($definition[0] === $constant && $definition[1] === $class) {
+                        return true;
+                    }
+                }
+
+                return false;
+            });
         $runtime
             ->method('getConstant')
             ->willReturnMap($constants);
 
         $runtime
             ->method('hasClass')
-            ->willReturnMap(
-                array_map(
-                    function ($classDefinition) {
-                        return array($classDefinition[0], true);
-                    },
-                    $classDefinitions
-                )
-            );
+            ->willReturnCallback(function ($class) use ($classDefinitions): bool {
+                foreach ($classDefinitions as $definition) {
+                    if ($definition[0] === $class) {
+                        return true;
+                    }
+                }
+
+                return false;
+            });
         $runtime
             ->method('construct')
             ->willReturnMap($classDefinitions);
 
         $platformRepository = new PlatformRepository(array(), array(), $runtime);
 
-        $expectations = array_map(function ($expectation) {
+        $expectations = array_map(function ($expectation): array {
             return array_replace(array(null, array(), array()), (array) $expectation);
         }, $expectations);
 
         $libraries = array_map(
-            function ($package) {
+            function ($package): string {
                 return $package['name'];
             },
             array_filter(
                 $platformRepository->search('lib', PlatformRepository::SEARCH_NAME),
-                function ($package) {
+                function ($package): bool {
                     return strpos($package['name'], 'lib-') === 0;
                 }
             )
         );
-        $expectedLibraries = array_merge(array_keys(array_filter($expectations, function ($expectation) {
+        $expectedLibraries = array_merge(array_keys(array_filter($expectations, function ($expectation): bool {
             return $expectation[0] !== false;
         })));
         self::assertCount(count(array_filter($expectedLibraries)), $libraries, sprintf('Expected: %s, got %s', var_export($expectedLibraries, true), var_export($libraries, true)));
 
-        $expectations = array_merge($expectations, array_combine(array_map(function ($extension) {
+        $expectations = array_merge($expectations, array_combine(array_map(function ($extension): string {
             return 'ext-'.$extension;
         }, $extensions), array_fill(0, count($extensions), array($extensionVersion, array(), array()))));
 
@@ -1204,7 +1199,7 @@ Linked Version => 1.2.11',
      *
      * @return void
      */
-    private function assertPackageLinks($context, array $expectedLinks, PackageInterface $sourcePackage, array $links)
+    private function assertPackageLinks($context, array $expectedLinks, PackageInterface $sourcePackage, array $links): void
     {
         self::assertCount(count($expectedLinks), $links, sprintf('%s: expected package count to match', $context));
 
@@ -1215,7 +1210,7 @@ Linked Version => 1.2.11',
         }
     }
 
-    public function testComposerPlatformVersion()
+    public function testComposerPlatformVersion(): void
     {
         $runtime = $this->getMockBuilder('Composer\Platform\Runtime')->getMock();
         $runtime
@@ -1236,7 +1231,7 @@ Linked Version => 1.2.11',
         self::assertNotNull($package, 'Composer package exists');
     }
 
-    public static function providePlatformPackages()
+    public static function providePlatformPackages(): array
     {
         return array(
             array('php', true),
@@ -1268,7 +1263,7 @@ Linked Version => 1.2.11',
      * @param bool $expectation
      * @dataProvider providePlatformPackages
      */
-    public function testValidPlatformPackages($packageName, $expectation)
+    public function testValidPlatformPackages($packageName, $expectation): void
     {
         self::assertSame($expectation, PlatformRepository::isPlatformPackage($packageName));
     }
@@ -1285,7 +1280,7 @@ class ResourceBundleStub
      *
      * @return ResourceBundleStub
      */
-    public static function create($locale, $bundleName, $fallback)
+    public static function create($locale, $bundleName, $fallback): ResourceBundleStub
     {
         Assert::assertSame(3, func_num_args());
         Assert::assertSame('root', $locale);
@@ -1300,7 +1295,7 @@ class ResourceBundleStub
      *
      * @return string
      */
-    public function get($field)
+    public function get($field): string
     {
         Assert::assertSame(1, func_num_args());
         Assert::assertSame('Version', $field);
@@ -1328,7 +1323,7 @@ class ImagickStub
      * @return array<string, string>
      * @phpstan-return array{versionString: string}
      */
-    public function getVersion()
+    public function getVersion(): array
     {
         Assert::assertSame(0, func_num_args());
 
diff --git a/tests/Composer/Test/Repository/RepositoryFactoryTest.php b/tests/Composer/Test/Repository/RepositoryFactoryTest.php
index ab725b18f..3928d0288 100644
--- a/tests/Composer/Test/Repository/RepositoryFactoryTest.php
+++ b/tests/Composer/Test/Repository/RepositoryFactoryTest.php
@@ -17,7 +17,7 @@ use Composer\Test\TestCase;
 
 class RepositoryFactoryTest extends TestCase
 {
-    public function testManagerWithAllRepositoryTypes()
+    public function testManagerWithAllRepositoryTypes(): void
     {
         $manager = RepositoryFactory::manager(
             $this->getMockBuilder('Composer\IO\IOInterface')->getMock(),
@@ -55,19 +55,18 @@ class RepositoryFactoryTest extends TestCase
      * @param int|string            $index
      * @param array<string, string> $config
      * @param array<string, mixed>  $existingRepos
-     * @param int|string            $expected
      *
      * @phpstan-param array{url?: string} $config
      */
-    public function testGenerateRepositoryName($index, array $config, array $existingRepos, $expected)
+    public function testGenerateRepositoryName($index, array $config, array $existingRepos, string $expected): void
     {
         $this->assertSame($expected, RepositoryFactory::generateRepositoryName($index, $config, $existingRepos));
     }
 
-    public function generateRepositoryNameProvider()
+    public function generateRepositoryNameProvider(): array
     {
         return array(
-            array(0, array(), array(), 0),
+            array(0, array(), array(), '0'),
             array(0, array(), array(array()), '02'),
             array(0, array('url' => 'https://example.org'), array(), 'example.org'),
             array(0, array('url' => 'https://example.org'), array('example.org' => array()), 'example.org2'),
diff --git a/tests/Composer/Test/Repository/RepositoryManagerTest.php b/tests/Composer/Test/Repository/RepositoryManagerTest.php
index 1596c0a40..14db37204 100644
--- a/tests/Composer/Test/Repository/RepositoryManagerTest.php
+++ b/tests/Composer/Test/Repository/RepositoryManagerTest.php
@@ -35,7 +35,7 @@ class RepositoryManagerTest extends TestCase
         }
     }
 
-    public function testPrepend()
+    public function testPrepend(): void
     {
         $rm = new RepositoryManager(
             $this->getMockBuilder('Composer\IO\IOInterface')->getMock(),
@@ -59,7 +59,7 @@ class RepositoryManagerTest extends TestCase
      * @param array<string, mixed> $options
      * @param class-string<\Throwable>|null $exception
      */
-    public function testRepoCreation($type, $options, ?string $exception = null)
+    public function testRepoCreation($type, $options, ?string $exception = null): void
     {
         if ($exception !== null) {
             self::expectException($exception);
@@ -76,7 +76,7 @@ class RepositoryManagerTest extends TestCase
         $config
             ->expects($this->any())
             ->method('get')
-            ->will($this->returnCallback(function ($arg) use ($tmpdir) {
+            ->will($this->returnCallback(function ($arg) use ($tmpdir): ?string {
                 return 'cache-repo-dir' === $arg ? $tmpdir : null;
             }))
         ;
@@ -95,7 +95,7 @@ class RepositoryManagerTest extends TestCase
         $this->assertInstanceOf('Composer\Repository\RepositoryInterface', $rm->createRepository($type, $options));
     }
 
-    public function provideRepoCreationTestCases()
+    public function provideRepoCreationTestCases(): array
     {
         $cases = array(
             array('composer', array('url' => 'http://example.org')),
@@ -115,7 +115,7 @@ class RepositoryManagerTest extends TestCase
         return $cases;
     }
 
-    public function testFilterRepoWrapping()
+    public function testFilterRepoWrapping(): void
     {
         $rm = new RepositoryManager(
             $this->getMockBuilder('Composer\IO\IOInterface')->getMock(),
diff --git a/tests/Composer/Test/Repository/Vcs/FossilDriverTest.php b/tests/Composer/Test/Repository/Vcs/FossilDriverTest.php
index 06c6bd5b6..cd51b6c3c 100644
--- a/tests/Composer/Test/Repository/Vcs/FossilDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/FossilDriverTest.php
@@ -46,7 +46,7 @@ class FossilDriverTest extends TestCase
         $fs->removeDirectory($this->home);
     }
 
-    public static function supportProvider()
+    public static function supportProvider(): array
     {
         return array(
             array('http://fossil.kd2.org/kd2fw/', true),
@@ -61,7 +61,7 @@ class FossilDriverTest extends TestCase
      * @param string $url
      * @param bool   $assertion
      */
-    public function testSupport($url, $assertion)
+    public function testSupport($url, $assertion): void
     {
         $config = new Config();
         $result = FossilDriver::supports($this->getMockBuilder('Composer\IO\IOInterface')->getMock(), $config, $url);
diff --git a/tests/Composer/Test/Repository/Vcs/GitBitbucketDriverTest.php b/tests/Composer/Test/Repository/Vcs/GitBitbucketDriverTest.php
index 66de8b7eb..144852f5a 100644
--- a/tests/Composer/Test/Repository/Vcs/GitBitbucketDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/GitBitbucketDriverTest.php
@@ -64,7 +64,7 @@ class GitBitbucketDriverTest extends TestCase
      *
      * @phpstan-param array{url: string}&array<string, mixed> $repoConfig
      */
-    private function getDriver(array $repoConfig)
+    private function getDriver(array $repoConfig): GitBitbucketDriver
     {
         $driver = new GitBitbucketDriver(
             $repoConfig,
@@ -79,7 +79,7 @@ class GitBitbucketDriverTest extends TestCase
         return $driver;
     }
 
-    public function testGetRootIdentifierWrongScmType()
+    public function testGetRootIdentifierWrongScmType(): void
     {
         self::expectException('RuntimeException');
         self::expectExceptionMessage('https://bitbucket.org/user/repo.git does not appear to be a git repository, use https://bitbucket.org/user/repo but remember that Bitbucket no longer supports the mercurial repositories. https://bitbucket.org/blog/sunsetting-mercurial-support-in-bitbucket');
@@ -99,7 +99,7 @@ class GitBitbucketDriverTest extends TestCase
         $driver->getRootIdentifier();
     }
 
-    public function testDriver()
+    public function testDriver(): \Composer\Repository\Vcs\GitBitbucketDriver
     {
         $driver = $this->getDriver(array('url' => 'https://bitbucket.org/user/repo.git'));
 
@@ -195,7 +195,7 @@ class GitBitbucketDriverTest extends TestCase
      * @depends testDriver
      * @param \Composer\Repository\Vcs\VcsDriverInterface $driver
      */
-    public function testGetParams($driver)
+    public function testGetParams($driver): void
     {
         $url = 'https://bitbucket.org/user/repo.git';
 
@@ -217,7 +217,7 @@ class GitBitbucketDriverTest extends TestCase
         );
     }
 
-    public function testInitializeInvalidRepositoryUrl()
+    public function testInitializeInvalidRepositoryUrl(): void
     {
         $this->expectException('\InvalidArgumentException');
 
@@ -225,7 +225,7 @@ class GitBitbucketDriverTest extends TestCase
         $driver->initialize();
     }
 
-    public function testSupports()
+    public function testSupports(): void
     {
         $this->assertTrue(
             GitBitbucketDriver::supports($this->io, $this->config, 'https://bitbucket.org/user/repo.git')
diff --git a/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php b/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php
index 7a721a1be..8482b28bd 100644
--- a/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/GitHubDriverTest.php
@@ -44,7 +44,7 @@ class GitHubDriverTest extends TestCase
         $fs->removeDirectory($this->home);
     }
 
-    public function testPrivateRepository()
+    public function testPrivateRepository(): void
     {
         $repoUrl = 'http://github.com/composer/packagist';
         $repoApiUrl = 'https://api.github.com/repos/composer/packagist';
@@ -105,7 +105,7 @@ class GitHubDriverTest extends TestCase
         $this->assertEquals('SOMESHA', $source['reference']);
     }
 
-    public function testPublicRepository()
+    public function testPublicRepository(): void
     {
         $repoUrl = 'http://github.com/composer/packagist';
         $repoApiUrl = 'https://api.github.com/repos/composer/packagist';
@@ -147,7 +147,7 @@ class GitHubDriverTest extends TestCase
         $this->assertEquals($sha, $source['reference']);
     }
 
-    public function testPublicRepository2()
+    public function testPublicRepository2(): void
     {
         $repoUrl = 'http://github.com/composer/packagist';
         $repoApiUrl = 'https://api.github.com/repos/composer/packagist';
@@ -196,7 +196,7 @@ class GitHubDriverTest extends TestCase
         $this->assertArrayNotHasKey('abandoned', $data);
     }
 
-    public function testPublicRepositoryArchived()
+    public function testPublicRepositoryArchived(): void
     {
         $repoUrl = 'http://github.com/composer/packagist';
         $repoApiUrl = 'https://api.github.com/repos/composer/packagist';
@@ -233,7 +233,7 @@ class GitHubDriverTest extends TestCase
         $this->assertTrue($data['abandoned']);
     }
 
-    public function testPrivateRepositoryNoInteraction()
+    public function testPrivateRepositoryNoInteraction(): void
     {
         $repoUrl = 'http://github.com/composer/packagist';
         $repoApiUrl = 'https://api.github.com/repos/composer/packagist';
@@ -305,7 +305,7 @@ class GitHubDriverTest extends TestCase
     /**
      * @return void
      */
-    public function initializeInvalidReoUrl()
+    public function initializeInvalidReoUrl(): void
     {
         $this->expectException('\InvalidArgumentException');
 
@@ -327,7 +327,7 @@ class GitHubDriverTest extends TestCase
      * @param bool $expected
      * @param string $repoUrl
      */
-    public function testSupports($expected, $repoUrl)
+    public function testSupports($expected, $repoUrl): void
     {
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
 
@@ -337,7 +337,7 @@ class GitHubDriverTest extends TestCase
     /**
      * @return list<array{bool, string}>
      */
-    public function supportsProvider()
+    public function supportsProvider(): array
     {
         return array(
             array(false, 'https://github.com/acme'),
@@ -353,7 +353,7 @@ class GitHubDriverTest extends TestCase
      *
      * @return void
      */
-    protected function setAttribute($object, $attribute, $value)
+    protected function setAttribute($object, $attribute, $value): void
     {
         $attr = new \ReflectionProperty($object, $attribute);
         $attr->setAccessible(true);
diff --git a/tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php b/tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php
index e2b79774d..b2706d613 100644
--- a/tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php
@@ -77,7 +77,7 @@ class GitLabDriverTest extends TestCase
         $fs->removeDirectory($this->home);
     }
 
-    public function provideInitializeUrls()
+    public function provideInitializeUrls(): array
     {
         return array(
             array('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject'),
@@ -92,7 +92,7 @@ class GitLabDriverTest extends TestCase
      * @param string $url
      * @param string $apiUrl
      */
-    public function testInitialize($url, $apiUrl)
+    public function testInitialize($url, $apiUrl): \Composer\Repository\Vcs\GitLabDriver
     {
         // @link http://doc.gitlab.com/ce/api/projects.html#get-single-project
         $projectData = <<<JSON
@@ -135,7 +135,7 @@ JSON;
      * @param string $url
      * @param string $apiUrl
      */
-    public function testInitializePublicProject($url, $apiUrl)
+    public function testInitializePublicProject($url, $apiUrl): \Composer\Repository\Vcs\GitLabDriver
     {
         // @link http://doc.gitlab.com/ce/api/projects.html#get-single-project
         $projectData = <<<JSON
@@ -176,7 +176,7 @@ JSON;
      * @param string $url
      * @param string $apiUrl
      */
-    public function testInitializePublicProjectAsAnonymous($url, $apiUrl)
+    public function testInitializePublicProjectAsAnonymous($url, $apiUrl): \Composer\Repository\Vcs\GitLabDriver
     {
         // @link http://doc.gitlab.com/ce/api/projects.html#get-single-project
         $projectData = <<<JSON
@@ -215,7 +215,7 @@ JSON;
      *
      * @group gitlabHttpPort
      */
-    public function testInitializeWithPortNumber()
+    public function testInitializeWithPortNumber(): void
     {
         $domain = 'gitlab.mycompany.com';
         $port = '5443';
@@ -249,7 +249,7 @@ JSON;
         $this->assertEquals($url, $driver->getUrl());
     }
 
-    public function testGetDist()
+    public function testGetDist(): void
     {
         $driver = $this->testInitialize('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject');
 
@@ -264,7 +264,7 @@ JSON;
         $this->assertEquals($expected, $driver->getDist($reference));
     }
 
-    public function testGetSource()
+    public function testGetSource(): void
     {
         $driver = $this->testInitialize('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject');
 
@@ -278,7 +278,7 @@ JSON;
         $this->assertEquals($expected, $driver->getSource($reference));
     }
 
-    public function testGetSource_GivenPublicProject()
+    public function testGetSource_GivenPublicProject(): void
     {
         $driver = $this->testInitializePublicProject('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject');
 
@@ -292,7 +292,7 @@ JSON;
         $this->assertEquals($expected, $driver->getSource($reference));
     }
 
-    public function testGetTags()
+    public function testGetTags(): void
     {
         $driver = $this->testInitialize('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject');
 
@@ -333,7 +333,7 @@ JSON;
         $this->assertEquals($expected, $driver->getTags(), 'Tags are cached');
     }
 
-    public function testGetPaginatedRefs()
+    public function testGetPaginatedRefs(): void
     {
         $driver = $this->testInitialize('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject');
 
@@ -395,7 +395,7 @@ JSON;
         $this->assertEquals($expected, $driver->getBranches(), 'Branches are cached');
     }
 
-    public function testGetBranches()
+    public function testGetBranches(): void
     {
         $driver = $this->testInitialize('https://gitlab.com/mygroup/myproject', 'https://gitlab.com/api/v4/projects/mygroup%2Fmyproject');
 
@@ -444,12 +444,12 @@ JSON;
      * @param string $url
      * @param bool   $expected
      */
-    public function testSupports($url, $expected)
+    public function testSupports($url, $expected): void
     {
         $this->assertSame($expected, GitLabDriver::supports($this->io, $this->config, $url));
     }
 
-    public function dataForTestSupports()
+    public function dataForTestSupports(): array
     {
         return array(
             array('http://gitlab.com/foo/bar', true),
@@ -473,7 +473,7 @@ JSON;
         );
     }
 
-    public function testGitlabSubDirectory()
+    public function testGitlabSubDirectory(): void
     {
         $url = 'https://mycompany.com/gitlab/mygroup/my-pro.ject';
         $apiUrl = 'https://mycompany.com/gitlab/api/v4/projects/mygroup%2Fmy-pro%2Eject';
@@ -505,7 +505,7 @@ JSON;
         $this->assertEquals($apiUrl, $driver->getApiUrl(), 'API URL is derived from the repository URL');
     }
 
-    public function testGitlabSubGroup()
+    public function testGitlabSubGroup(): void
     {
         $url = 'https://gitlab.com/mygroup/mysubgroup/myproject';
         $apiUrl = 'https://gitlab.com/api/v4/projects/mygroup%2Fmysubgroup%2Fmyproject';
@@ -537,7 +537,7 @@ JSON;
         $this->assertEquals($apiUrl, $driver->getApiUrl(), 'API URL is derived from the repository URL');
     }
 
-    public function testGitlabSubDirectorySubGroup()
+    public function testGitlabSubDirectorySubGroup(): void
     {
         $url = 'https://mycompany.com/gitlab/mygroup/mysubgroup/myproject';
         $apiUrl = 'https://mycompany.com/gitlab/api/v4/projects/mygroup%2Fmysubgroup%2Fmyproject';
@@ -569,7 +569,7 @@ JSON;
         $this->assertEquals($apiUrl, $driver->getApiUrl(), 'API URL is derived from the repository URL');
     }
 
-    public function testForwardsOptions()
+    public function testForwardsOptions(): void
     {
         $options = array(
             'ssl' => array(
@@ -607,7 +607,7 @@ JSON;
         $driver->initialize();
     }
 
-    public function testProtocolOverrideRepositoryUrlGeneration()
+    public function testProtocolOverrideRepositoryUrlGeneration(): void
     {
         // @link http://doc.gitlab.com/ce/api/projects.html#get-single-project
         $projectData = <<<JSON
diff --git a/tests/Composer/Test/Repository/Vcs/HgDriverTest.php b/tests/Composer/Test/Repository/Vcs/HgDriverTest.php
index 68b9c9e65..fd94cf945 100644
--- a/tests/Composer/Test/Repository/Vcs/HgDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/HgDriverTest.php
@@ -50,14 +50,14 @@ class HgDriverTest extends TestCase
      *
      * @param string $repositoryUrl
      */
-    public function testSupports($repositoryUrl)
+    public function testSupports($repositoryUrl): void
     {
         $this->assertTrue(
             HgDriver::supports($this->io, $this->config, $repositoryUrl)
         );
     }
 
-    public function supportsDataProvider()
+    public function supportsDataProvider(): array
     {
         return array(
             array('ssh://bitbucket.org/user/repo'),
diff --git a/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php b/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
index a918b7ff9..00acd67df 100644
--- a/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
@@ -89,7 +89,7 @@ class PerforceDriverTest extends TestCase
     /**
      * @return void
      */
-    protected function overrideDriverInternalPerforce(Perforce $perforce)
+    protected function overrideDriverInternalPerforce(Perforce $perforce): void
     {
         $reflectionClass = new \ReflectionClass($this->driver);
         $property = $reflectionClass->getProperty('perforce');
@@ -102,7 +102,7 @@ class PerforceDriverTest extends TestCase
      *
      * @return Config
      */
-    protected function getTestConfig($testPath)
+    protected function getTestConfig($testPath): Config
     {
         $config = new Config();
         $config->merge(array('config' => array('home' => $testPath)));
@@ -136,7 +136,7 @@ class PerforceDriverTest extends TestCase
         return $this->getMockBuilder('Composer\Util\Perforce')->disableOriginalConstructor()->getMock();
     }
 
-    public function testInitializeCapturesVariablesFromRepoConfig()
+    public function testInitializeCapturesVariablesFromRepoConfig(): void
     {
         $driver = new PerforceDriver($this->repoConfig, $this->io, $this->config, $this->httpDownloader, $this->process);
         $driver->initialize();
@@ -145,7 +145,7 @@ class PerforceDriverTest extends TestCase
         $this->assertEquals(self::TEST_BRANCH, $driver->getBranch());
     }
 
-    public function testInitializeLogsInAndConnectsClient()
+    public function testInitializeLogsInAndConnectsClient(): void
     {
         $this->perforce->expects($this->once())->method('p4Login');
         $this->perforce->expects($this->once())->method('checkStream');
@@ -158,7 +158,7 @@ class PerforceDriverTest extends TestCase
      * @depends testInitializeCapturesVariablesFromRepoConfig
      * @depends testInitializeLogsInAndConnectsClient
      */
-    public function testHasComposerFileReturnsFalseOnNoComposerFile()
+    public function testHasComposerFileReturnsFalseOnNoComposerFile(): void
     {
         $identifier = 'TEST_IDENTIFIER';
         $formatted_depot_path = '//' . self::TEST_DEPOT . '/' . $identifier;
@@ -172,7 +172,7 @@ class PerforceDriverTest extends TestCase
      * @depends testInitializeCapturesVariablesFromRepoConfig
      * @depends testInitializeLogsInAndConnectsClient
      */
-    public function testHasComposerFileReturnsTrueWithOneOrMoreComposerFiles()
+    public function testHasComposerFileReturnsTrueWithOneOrMoreComposerFiles(): void
     {
         $identifier = 'TEST_IDENTIFIER';
         $formatted_depot_path = '//' . self::TEST_DEPOT . '/' . $identifier;
@@ -189,13 +189,13 @@ class PerforceDriverTest extends TestCase
      *
      * @return void
      */
-    public function testSupportsReturnsFalseNoDeepCheck()
+    public function testSupportsReturnsFalseNoDeepCheck(): void
     {
         $this->expectOutputString('');
         $this->assertFalse(PerforceDriver::supports($this->io, $this->config, 'existing.url'));
     }
 
-    public function testCleanup()
+    public function testCleanup(): void
     {
         $this->perforce->expects($this->once())->method('cleanupClientSpec');
         $this->driver->cleanup();
diff --git a/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php b/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php
index e3b32b9bf..f08891a41 100644
--- a/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php
@@ -47,7 +47,7 @@ class SvnDriverTest extends TestCase
         $fs->removeDirectory($this->home);
     }
 
-    public function testWrongCredentialsInUrl()
+    public function testWrongCredentialsInUrl(): void
     {
         self::expectException('RuntimeException');
         self::expectExceptionMessage("Repository https://till:secret@corp.svn.local/repo could not be processed, wrong credentials provided (svn: OPTIONS of 'https://corp.svn.local/repo': authorization failed: Could not authenticate to server: rejected Basic challenge (https://corp.svn.local/))");
@@ -84,7 +84,7 @@ class SvnDriverTest extends TestCase
         $svn->initialize();
     }
 
-    public static function supportProvider()
+    public static function supportProvider(): array
     {
         return array(
             array('http://svn.apache.org', true),
@@ -100,7 +100,7 @@ class SvnDriverTest extends TestCase
      * @param string $url
      * @param bool   $assertion
      */
-    public function testSupport($url, $assertion)
+    public function testSupport($url, $assertion): void
     {
         $config = new Config();
         $result = SvnDriver::supports($this->getMockBuilder('Composer\IO\IOInterface')->getMock(), $config, $url);
diff --git a/tests/Composer/Test/Repository/VcsRepositoryTest.php b/tests/Composer/Test/Repository/VcsRepositoryTest.php
index 470c9bced..819dcd08d 100644
--- a/tests/Composer/Test/Repository/VcsRepositoryTest.php
+++ b/tests/Composer/Test/Repository/VcsRepositoryTest.php
@@ -42,7 +42,7 @@ class VcsRepositoryTest extends TestCase
     /**
      * @return void
      */
-    protected function initialize()
+    protected function initialize(): void
     {
         $locator = new ExecutableFinder();
         if (!$locator->find('git')) {
@@ -63,7 +63,7 @@ class VcsRepositoryTest extends TestCase
 
         // init
         $process = new ProcessExecutor;
-        $exec = function ($command) use ($process) {
+        $exec = function ($command) use ($process): void {
             $cwd = getcwd();
             if ($process->execute($command, $output, $cwd) !== 0) {
                 throw new \RuntimeException('Failed to execute '.$command.': '.$process->getErrorOutput());
@@ -147,7 +147,7 @@ class VcsRepositoryTest extends TestCase
         $fs->removeDirectory(self::$gitRepo);
     }
 
-    public function testLoadVersions()
+    public function testLoadVersions(): void
     {
         $expected = array(
             '0.6.0' => true,
diff --git a/tests/Composer/Test/Script/EventTest.php b/tests/Composer/Test/Script/EventTest.php
index 8ef3c916b..724d0e886 100644
--- a/tests/Composer/Test/Script/EventTest.php
+++ b/tests/Composer/Test/Script/EventTest.php
@@ -19,7 +19,7 @@ use Composer\Test\TestCase;
 
 class EventTest extends TestCase
 {
-    public function testEventSetsOriginatingEvent()
+    public function testEventSetsOriginatingEvent(): void
     {
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $composer = $this->createComposerInstance();
@@ -43,7 +43,7 @@ class EventTest extends TestCase
         );
     }
 
-    public function testEventCalculatesNestedOriginatingEvent()
+    public function testEventCalculatesNestedOriginatingEvent(): void
     {
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $composer = $this->createComposerInstance();
@@ -71,7 +71,7 @@ class EventTest extends TestCase
     /**
      * @return Composer
      */
-    private function createComposerInstance()
+    private function createComposerInstance(): Composer
     {
         $composer = new Composer;
         $config = new Config;
diff --git a/tests/Composer/Test/TestCase.php b/tests/Composer/Test/TestCase.php
index e6a171621..a8136edb5 100644
--- a/tests/Composer/Test/TestCase.php
+++ b/tests/Composer/Test/TestCase.php
@@ -67,7 +67,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
     /**
      * @return string
      */
-    public static function getUniqueTmpDirectory()
+    public static function getUniqueTmpDirectory(): string
     {
         $attempts = 5;
         $root = sys_get_temp_dir();
@@ -86,7 +86,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
     /**
      * @return VersionParser
      */
-    protected static function getVersionParser()
+    protected static function getVersionParser(): VersionParser
     {
         if (!self::$parser) {
             self::$parser = new VersionParser();
@@ -100,7 +100,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      * @param string $version
      * @return Constraint
      */
-    protected function getVersionConstraint($operator, $version)
+    protected function getVersionConstraint($operator, $version): Constraint
     {
         $constraint = new Constraint(
             $operator,
@@ -122,6 +122,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      * @return CompletePackage|CompleteAliasPackage|RootPackage|RootAliasPackage
      *
      * @phpstan-param class-string<PackageClass> $class
+     * @phpstan-return PackageClass
      */
     protected function getPackage($name, $version, $class = 'Composer\Package\CompletePackage')
     {
@@ -134,7 +135,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      * @param string $version
      * @return AliasPackage|RootAliasPackage|CompleteAliasPackage
      */
-    protected function getAliasPackage(Package $package, $version)
+    protected function getAliasPackage(Package $package, $version): \Composer\Package\AliasPackage
     {
         $normVersion = self::getVersionParser()->normalize($version);
 
@@ -152,7 +153,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      * @param array<string, array<string, string>> $config
      * @return void
      */
-    protected function configureLinks(PackageInterface $package, array $config)
+    protected function configureLinks(PackageInterface $package, array $config): void
     {
         $arrayLoader = new ArrayLoader();
 
@@ -175,7 +176,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      * @param  string $directory
      * @return void
      */
-    protected static function ensureDirectoryExistsAndClear($directory)
+    protected static function ensureDirectoryExistsAndClear($directory): void
     {
         $fs = new Filesystem();
 
@@ -195,7 +196,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      *
      * @throws \PHPUnit\Framework\SkippedTestError
      */
-    protected function skipIfNotExecutable($executableName)
+    protected function skipIfNotExecutable($executableName): void
     {
         if (!isset(self::$executableCache[$executableName])) {
             $finder = new ExecutableFinder();
@@ -214,7 +215,7 @@ abstract class TestCase extends \PHPUnit\Framework\TestCase
      *
      * @return string The transformed command
      */
-    protected function getCmd($cmd)
+    protected function getCmd($cmd): string
     {
         if (Platform::isWindows()) {
             $cmd = Preg::replaceCallback("/('[^']*')/", function ($m) {
diff --git a/tests/Composer/Test/Util/AuthHelperTest.php b/tests/Composer/Test/Util/AuthHelperTest.php
index a65d022c1..d12f87066 100644
--- a/tests/Composer/Test/Util/AuthHelperTest.php
+++ b/tests/Composer/Test/Util/AuthHelperTest.php
@@ -43,7 +43,7 @@ class AuthHelperTest extends TestCase
         $this->authHelper = new AuthHelper($this->io, $this->config);
     }
 
-    public function testAddAuthenticationHeaderWithoutAuthCredentials()
+    public function testAddAuthenticationHeaderWithoutAuthCredentials(): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -63,7 +63,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function testAddAuthenticationHeaderWithBearerPassword()
+    public function testAddAuthenticationHeaderWithBearerPassword(): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -86,7 +86,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function testAddAuthenticationHeaderWithGithubToken()
+    public function testAddAuthenticationHeaderWithGithubToken(): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -113,7 +113,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function testAddAuthenticationHeaderWithGitlabOathToken()
+    public function testAddAuthenticationHeaderWithGitlabOathToken(): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -145,7 +145,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function gitlabPrivateTokenProvider()
+    public function gitlabPrivateTokenProvider(): array
     {
         return array(
           array('private-token'),
@@ -158,7 +158,7 @@ class AuthHelperTest extends TestCase
      *
      * @param string $password
      */
-    public function testAddAuthenticationHeaderWithGitlabPrivateToken($password)
+    public function testAddAuthenticationHeaderWithGitlabPrivateToken($password): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -190,7 +190,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function testAddAuthenticationHeaderWithBitbucketOathToken()
+    public function testAddAuthenticationHeaderWithBitbucketOathToken(): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -222,7 +222,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function bitbucketPublicUrlProvider()
+    public function bitbucketPublicUrlProvider(): array
     {
         return array(
             array('https://bitbucket.org/user/repo/downloads/whatever'),
@@ -235,7 +235,7 @@ class AuthHelperTest extends TestCase
      *
      * @param string $url
      */
-    public function testAddAuthenticationHeaderWithBitbucketPublicUrl($url)
+    public function testAddAuthenticationHeaderWithBitbucketPublicUrl($url): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -260,7 +260,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function basicHttpAuthenticationProvider()
+    public function basicHttpAuthenticationProvider(): array
     {
         return array(
             array(
@@ -299,7 +299,7 @@ class AuthHelperTest extends TestCase
      *
      * @phpstan-param array{username: string|null, password: string|null} $auth
      */
-    public function testAddAuthenticationHeaderWithBasicHttpAuthentication($url, $origin, $auth)
+    public function testAddAuthenticationHeaderWithBasicHttpAuthentication($url, $origin, $auth): void
     {
         $headers = array(
             'Accept-Encoding: gzip',
@@ -337,12 +337,12 @@ class AuthHelperTest extends TestCase
      *
      * @param string $url
      */
-    public function testIsPublicBitBucketDownloadWithBitbucketPublicUrl($url)
+    public function testIsPublicBitBucketDownloadWithBitbucketPublicUrl($url): void
     {
         $this->assertTrue($this->authHelper->isPublicBitBucketDownload($url));
     }
 
-    public function testIsPublicBitBucketDownloadWithNonBitbucketPublicUrl()
+    public function testIsPublicBitBucketDownloadWithNonBitbucketPublicUrl(): void
     {
         $this->assertFalse(
             $this->authHelper->isPublicBitBucketDownload(
@@ -351,7 +351,7 @@ class AuthHelperTest extends TestCase
         );
     }
 
-    public function testStoreAuthAutomatically()
+    public function testStoreAuthAutomatically(): void
     {
         $origin = 'github.com';
         $storeAuth = true;
@@ -383,7 +383,7 @@ class AuthHelperTest extends TestCase
         $this->authHelper->storeAuth($origin, $storeAuth);
     }
 
-    public function testStoreAuthWithPromptYesAnswer()
+    public function testStoreAuthWithPromptYesAnswer(): void
     {
         $origin = 'github.com';
         $storeAuth = 'prompt';
@@ -416,7 +416,7 @@ class AuthHelperTest extends TestCase
                 null,
                 'y'
             )
-            ->willReturnCallback(function ($question, $validator, $attempts, $default) use ($answer) {
+            ->willReturnCallback(function ($question, $validator, $attempts, $default) use ($answer): string {
                 $validator($answer);
 
                 return $answer;
@@ -435,7 +435,7 @@ class AuthHelperTest extends TestCase
         $this->authHelper->storeAuth($origin, $storeAuth);
     }
 
-    public function testStoreAuthWithPromptNoAnswer()
+    public function testStoreAuthWithPromptNoAnswer(): void
     {
         $origin = 'github.com';
         $storeAuth = 'prompt';
@@ -464,7 +464,7 @@ class AuthHelperTest extends TestCase
                 null,
                 'y'
             )
-            ->willReturnCallback(function ($question, $validator, $attempts, $default) use ($answer) {
+            ->willReturnCallback(function ($question, $validator, $attempts, $default) use ($answer): string {
                 $validator($answer);
 
                 return $answer;
@@ -473,7 +473,7 @@ class AuthHelperTest extends TestCase
         $this->authHelper->storeAuth($origin, $storeAuth);
     }
 
-    public function testStoreAuthWithPromptInvalidAnswer()
+    public function testStoreAuthWithPromptInvalidAnswer(): void
     {
         self::expectException('RuntimeException');
 
@@ -504,7 +504,7 @@ class AuthHelperTest extends TestCase
                 null,
                 'y'
             )
-            ->willReturnCallback(function ($question, $validator, $attempts, $default) use ($answer) {
+            ->willReturnCallback(function ($question, $validator, $attempts, $default) use ($answer): string {
                 $validator($answer);
 
                 return $answer;
@@ -521,7 +521,7 @@ class AuthHelperTest extends TestCase
      *
      * @phpstan-param array{username: string|null, password: string|null} $auth
      */
-    private function expectsAuthentication($origin, $auth)
+    private function expectsAuthentication($origin, $auth): void
     {
         $this->io->expects($this->once())
             ->method('hasAuthentication')
diff --git a/tests/Composer/Test/Util/BitbucketTest.php b/tests/Composer/Test/Util/BitbucketTest.php
index afc7f7aa2..643b1f64e 100644
--- a/tests/Composer/Test/Util/BitbucketTest.php
+++ b/tests/Composer/Test/Util/BitbucketTest.php
@@ -68,7 +68,7 @@ class BitbucketTest extends TestCase
         $this->bitbucket = new Bitbucket($this->io, $this->config, null, $this->httpDownloader, $this->time);
     }
 
-    public function testRequestAccessTokenWithValidOAuthConsumer()
+    public function testRequestAccessTokenWithValidOAuthConsumer(): void
     {
         $this->io->expects($this->once())
             ->method('setAuthentication')
@@ -111,7 +111,7 @@ class BitbucketTest extends TestCase
         );
     }
 
-    public function testRequestAccessTokenWithValidOAuthConsumerAndValidStoredAccessToken()
+    public function testRequestAccessTokenWithValidOAuthConsumerAndValidStoredAccessToken(): \Composer\Util\Bitbucket
     {
         $this->config->expects($this->once())
             ->method('get')
@@ -135,7 +135,7 @@ class BitbucketTest extends TestCase
         return $this->bitbucket;
     }
 
-    public function testRequestAccessTokenWithValidOAuthConsumerAndExpiredAccessToken()
+    public function testRequestAccessTokenWithValidOAuthConsumerAndExpiredAccessToken(): void
     {
         $this->config->expects($this->once())
             ->method('get')
@@ -187,7 +187,7 @@ class BitbucketTest extends TestCase
         );
     }
 
-    public function testRequestAccessTokenWithUsernameAndPassword()
+    public function testRequestAccessTokenWithUsernameAndPassword(): void
     {
         $this->io->expects($this->once())
             ->method('setAuthentication')
@@ -232,7 +232,7 @@ class BitbucketTest extends TestCase
         $this->assertEquals('', $this->bitbucket->requestToken($this->origin, $this->username, $this->password));
     }
 
-    public function testRequestAccessTokenWithUsernameAndPasswordWithUnauthorizedResponse()
+    public function testRequestAccessTokenWithUsernameAndPasswordWithUnauthorizedResponse(): void
     {
         $this->config->expects($this->once())
             ->method('get')
@@ -267,7 +267,7 @@ class BitbucketTest extends TestCase
         $this->assertEquals('', $this->bitbucket->requestToken($this->origin, $this->username, $this->password));
     }
 
-    public function testRequestAccessTokenWithUsernameAndPasswordWithNotFoundResponse()
+    public function testRequestAccessTokenWithUsernameAndPasswordWithNotFoundResponse(): void
     {
         self::expectException('Composer\Downloader\TransportException');
         $this->config->expects($this->once())
@@ -297,7 +297,7 @@ class BitbucketTest extends TestCase
         $this->bitbucket->requestToken($this->origin, $this->username, $this->password);
     }
 
-    public function testUsernamePasswordAuthenticationFlow()
+    public function testUsernamePasswordAuthenticationFlow(): void
     {
         $this->io
             ->expects($this->atLeastOnce())
@@ -338,7 +338,7 @@ class BitbucketTest extends TestCase
         $this->assertTrue($this->bitbucket->authorizeOAuthInteractively($this->origin, $this->message));
     }
 
-    public function testAuthorizeOAuthInteractivelyWithEmptyUsername()
+    public function testAuthorizeOAuthInteractivelyWithEmptyUsername(): void
     {
         $authConfigSourceMock = $this->getMockBuilder('Composer\Config\ConfigSourceInterface')->getMock();
         $this->config->expects($this->atLeastOnce())
@@ -353,7 +353,7 @@ class BitbucketTest extends TestCase
         $this->assertFalse($this->bitbucket->authorizeOAuthInteractively($this->origin, $this->message));
     }
 
-    public function testAuthorizeOAuthInteractivelyWithEmptyPassword()
+    public function testAuthorizeOAuthInteractivelyWithEmptyPassword(): void
     {
         $authConfigSourceMock = $this->getMockBuilder('Composer\Config\ConfigSourceInterface')->getMock();
         $this->config->expects($this->atLeastOnce())
@@ -371,7 +371,7 @@ class BitbucketTest extends TestCase
         $this->assertFalse($this->bitbucket->authorizeOAuthInteractively($this->origin, $this->message));
     }
 
-    public function testAuthorizeOAuthInteractivelyWithRequestAccessTokenFailure()
+    public function testAuthorizeOAuthInteractivelyWithRequestAccessTokenFailure(): void
     {
         $authConfigSourceMock = $this->getMockBuilder('Composer\Config\ConfigSourceInterface')->getMock();
         $this->config->expects($this->atLeastOnce())
@@ -411,7 +411,7 @@ class BitbucketTest extends TestCase
      *
      * @return void
      */
-    private function setExpectationsForStoringAccessToken($removeBasicAuth = false)
+    private function setExpectationsForStoringAccessToken($removeBasicAuth = false): void
     {
         $configSourceMock = $this->getMockBuilder('Composer\Config\ConfigSourceInterface')->getMock();
         $this->config->expects($this->once())
@@ -446,7 +446,7 @@ class BitbucketTest extends TestCase
         }
     }
 
-    public function testGetTokenWithoutAccessToken()
+    public function testGetTokenWithoutAccessToken(): void
     {
         $this->assertSame('', $this->bitbucket->getToken());
     }
@@ -456,17 +456,17 @@ class BitbucketTest extends TestCase
      *
      * @param Bitbucket $bitbucket
      */
-    public function testGetTokenWithAccessToken(Bitbucket $bitbucket)
+    public function testGetTokenWithAccessToken(Bitbucket $bitbucket): void
     {
         $this->assertSame($this->token, $bitbucket->getToken());
     }
 
-    public function testAuthorizeOAuthWithWrongOriginUrl()
+    public function testAuthorizeOAuthWithWrongOriginUrl(): void
     {
         $this->assertFalse($this->bitbucket->authorizeOAuth('non-' . $this->origin));
     }
 
-    public function testAuthorizeOAuthWithoutAvailableGitConfigToken()
+    public function testAuthorizeOAuthWithoutAvailableGitConfigToken(): void
     {
         $process = $this->getProcessExecutorMock();
         $process->expects(array(), false, array('return' => -1));
@@ -476,7 +476,7 @@ class BitbucketTest extends TestCase
         $this->assertFalse($bitbucket->authorizeOAuth($this->origin));
     }
 
-    public function testAuthorizeOAuthWithAvailableGitConfigToken()
+    public function testAuthorizeOAuthWithAvailableGitConfigToken(): void
     {
         $process = $this->getProcessExecutorMock();
 
diff --git a/tests/Composer/Test/Util/ConfigValidatorTest.php b/tests/Composer/Test/Util/ConfigValidatorTest.php
index 0e53e1c54..c755ea6e5 100644
--- a/tests/Composer/Test/Util/ConfigValidatorTest.php
+++ b/tests/Composer/Test/Util/ConfigValidatorTest.php
@@ -24,7 +24,7 @@ class ConfigValidatorTest extends TestCase
     /**
      * Test ConfigValidator warns on commit reference
      */
-    public function testConfigValidatorCommitRefWarning()
+    public function testConfigValidatorCommitRefWarning(): void
     {
         $configValidator = new ConfigValidator(new NullIO());
         list(, , $warnings) = $configValidator->validate(__DIR__ . '/Fixtures/composer_commit-ref.json');
@@ -35,7 +35,7 @@ class ConfigValidatorTest extends TestCase
         );
     }
 
-    public function testConfigValidatorWarnsOnScriptDescriptionForNonexistentScript()
+    public function testConfigValidatorWarnsOnScriptDescriptionForNonexistentScript(): void
     {
         $configValidator = new ConfigValidator(new NullIO());
         list(, , $warnings) = $configValidator->validate(__DIR__ . '/Fixtures/composer_scripts-descriptions.json');
@@ -46,7 +46,7 @@ class ConfigValidatorTest extends TestCase
         );
     }
 
-    public function testConfigValidatorWarnsOnUnnecessaryProvideReplace()
+    public function testConfigValidatorWarnsOnUnnecessaryProvideReplace(): void
     {
         $configValidator = new ConfigValidator(new NullIO());
         list(, , $warnings) = $configValidator->validate(__DIR__ . '/Fixtures/composer_provide-replace-requirements.json');
diff --git a/tests/Composer/Test/Util/ErrorHandlerTest.php b/tests/Composer/Test/Util/ErrorHandlerTest.php
index 6dc0c8ec6..4def526a4 100644
--- a/tests/Composer/Test/Util/ErrorHandlerTest.php
+++ b/tests/Composer/Test/Util/ErrorHandlerTest.php
@@ -34,7 +34,7 @@ class ErrorHandlerTest extends TestCase
     /**
      * Test ErrorHandler handles notices
      */
-    public function testErrorHandlerCaptureNotice()
+    public function testErrorHandlerCaptureNotice(): void
     {
         if (PHP_VERSION_ID >= 80000) {
             self::expectException('\ErrorException');
@@ -52,7 +52,7 @@ class ErrorHandlerTest extends TestCase
     /**
      * Test ErrorHandler handles warnings
      */
-    public function testErrorHandlerCaptureWarning()
+    public function testErrorHandlerCaptureWarning(): void
     {
         if (PHP_VERSION_ID >= 80000) {
             self::expectException('TypeError');
@@ -70,7 +70,7 @@ class ErrorHandlerTest extends TestCase
      * Test ErrorHandler handles warnings
      * @doesNotPerformAssertions
      */
-    public function testErrorHandlerRespectsAtOperator()
+    public function testErrorHandlerRespectsAtOperator(): void
     {
         @trigger_error('test', E_USER_NOTICE);
     }
diff --git a/tests/Composer/Test/Util/FilesystemTest.php b/tests/Composer/Test/Util/FilesystemTest.php
index 2c7fa5657..dc6706b27 100644
--- a/tests/Composer/Test/Util/FilesystemTest.php
+++ b/tests/Composer/Test/Util/FilesystemTest.php
@@ -59,13 +59,13 @@ class FilesystemTest extends TestCase
      * @param string $expected
      * @param bool   $static
      */
-    public function testFindShortestPathCode($a, $b, $directory, $expected, $static = false)
+    public function testFindShortestPathCode($a, $b, $directory, $expected, $static = false): void
     {
         $fs = new Filesystem;
         $this->assertEquals($expected, $fs->findShortestPathCode($a, $b, $directory, $static));
     }
 
-    public function providePathCouplesAsCode()
+    public function providePathCouplesAsCode(): array
     {
         return array(
             array('/foo/bar', '/foo/bar', false, "__FILE__"),
@@ -122,13 +122,13 @@ class FilesystemTest extends TestCase
      * @param string $expected
      * @param bool   $directory
      */
-    public function testFindShortestPath($a, $b, $expected, $directory = false)
+    public function testFindShortestPath($a, $b, $expected, $directory = false): void
     {
         $fs = new Filesystem;
         $this->assertEquals($expected, $fs->findShortestPath($a, $b, $directory));
     }
 
-    public function providePathCouples()
+    public function providePathCouples(): array
     {
         return array(
             array('/foo/bar', '/foo/bar', "./bar"),
@@ -173,7 +173,7 @@ class FilesystemTest extends TestCase
     /**
      * @group GH-1339
      */
-    public function testRemoveDirectoryPhp()
+    public function testRemoveDirectoryPhp(): void
     {
         @mkdir($this->workingDir . "/level1/level2", 0777, true);
         file_put_contents($this->workingDir . "/level1/level2/hello.txt", "hello world");
@@ -183,7 +183,7 @@ class FilesystemTest extends TestCase
         $this->assertFileDoesNotExist($this->workingDir . "/level1/level2/hello.txt");
     }
 
-    public function testFileSize()
+    public function testFileSize(): void
     {
         file_put_contents($this->testFile, 'Hello');
 
@@ -191,7 +191,7 @@ class FilesystemTest extends TestCase
         $this->assertGreaterThanOrEqual(5, $fs->size($this->testFile));
     }
 
-    public function testDirectorySize()
+    public function testDirectorySize(): void
     {
         @mkdir($this->workingDir, 0777, true);
         file_put_contents($this->workingDir."/file1.txt", 'Hello');
@@ -207,13 +207,13 @@ class FilesystemTest extends TestCase
      * @param string $expected
      * @param string $actual
      */
-    public function testNormalizePath($expected, $actual)
+    public function testNormalizePath($expected, $actual): void
     {
         $fs = new Filesystem;
         $this->assertEquals($expected, $fs->normalizePath($actual));
     }
 
-    public function provideNormalizedPaths()
+    public function provideNormalizedPaths(): array
     {
         return array(
             array('../foo', '../foo'),
@@ -243,7 +243,7 @@ class FilesystemTest extends TestCase
      * @link https://github.com/composer/composer/issues/3157
      * @requires function symlink
      */
-    public function testUnlinkSymlinkedDirectory()
+    public function testUnlinkSymlinkedDirectory(): void
     {
         $basepath = $this->workingDir;
         $symlinked = $basepath . "/linked";
@@ -270,7 +270,7 @@ class FilesystemTest extends TestCase
      * @link https://github.com/composer/composer/issues/3144
      * @requires function symlink
      */
-    public function testRemoveSymlinkedDirectoryWithTrailingSlash()
+    public function testRemoveSymlinkedDirectoryWithTrailingSlash(): void
     {
         @mkdir($this->workingDir . "/real", 0777, true);
         touch($this->workingDir . "/real/FILE");
@@ -299,7 +299,7 @@ class FilesystemTest extends TestCase
         $this->assertFileDoesNotExist($symlinked);
     }
 
-    public function testJunctions()
+    public function testJunctions(): void
     {
         @mkdir($this->workingDir . '/real/nesting/testing', 0777, true);
         $fs = new Filesystem();
@@ -329,7 +329,7 @@ class FilesystemTest extends TestCase
         $this->assertFalse(is_dir($junction), $junction . ' is not a directory');
     }
 
-    public function testCopy()
+    public function testCopy(): void
     {
         @mkdir($this->workingDir . '/foo/bar', 0777, true);
         @mkdir($this->workingDir . '/foo/baz', 0777, true);
@@ -354,7 +354,7 @@ class FilesystemTest extends TestCase
         $this->assertTrue(is_file($this->workingDir . '/testfile.file'));
     }
 
-    public function testCopyThenRemove()
+    public function testCopyThenRemove(): void
     {
         @mkdir($this->workingDir . '/foo/bar', 0777, true);
         @mkdir($this->workingDir . '/foo/baz', 0777, true);
diff --git a/tests/Composer/Test/Util/GitHubTest.php b/tests/Composer/Test/Util/GitHubTest.php
index 0da22733a..a4151f489 100644
--- a/tests/Composer/Test/Util/GitHubTest.php
+++ b/tests/Composer/Test/Util/GitHubTest.php
@@ -27,7 +27,7 @@ class GitHubTest extends TestCase
     /** @var string */
     private $origin = 'github.com';
 
-    public function testUsernamePasswordAuthenticationFlow()
+    public function testUsernamePasswordAuthenticationFlow(): void
     {
         $io = $this->getIOMock();
         $io
@@ -65,7 +65,7 @@ class GitHubTest extends TestCase
         $this->assertTrue($github->authorizeOAuthInteractively($this->origin, $this->message));
     }
 
-    public function testUsernamePasswordFailure()
+    public function testUsernamePasswordFailure(): void
     {
         $io = $this->getIOMock();
         $io
diff --git a/tests/Composer/Test/Util/GitLabTest.php b/tests/Composer/Test/Util/GitLabTest.php
index 397b164ad..61bc35c72 100644
--- a/tests/Composer/Test/Util/GitLabTest.php
+++ b/tests/Composer/Test/Util/GitLabTest.php
@@ -31,7 +31,7 @@ class GitLabTest extends TestCase
     /** @var string */
     private $token = 'gitlabtoken';
 
-    public function testUsernamePasswordAuthenticationFlow()
+    public function testUsernamePasswordAuthenticationFlow(): void
     {
         $io = $this->getIOMock();
         $io
@@ -70,7 +70,7 @@ class GitLabTest extends TestCase
         $this->assertTrue($gitLab->authorizeOAuthInteractively('http', $this->origin, $this->message));
     }
 
-    public function testUsernamePasswordFailure()
+    public function testUsernamePasswordFailure(): void
     {
         self::expectException('RuntimeException');
         self::expectExceptionMessage('Invalid GitLab credentials 5 times in a row, aborting.');
diff --git a/tests/Composer/Test/Util/GitTest.php b/tests/Composer/Test/Util/GitTest.php
index 9379d5b2e..e6df4e1ed 100644
--- a/tests/Composer/Test/Util/GitTest.php
+++ b/tests/Composer/Test/Util/GitTest.php
@@ -47,9 +47,9 @@ class GitTest extends TestCase
      * @param string $protocol
      * @param string $expectedUrl
      */
-    public function testRunCommandPublicGitHubRepositoryNotInitialClone($protocol, $expectedUrl)
+    public function testRunCommandPublicGitHubRepositoryNotInitialClone($protocol, $expectedUrl): void
     {
-        $commandCallable = function ($url) use ($expectedUrl) {
+        $commandCallable = function ($url) use ($expectedUrl): string {
             $this->assertSame($expectedUrl, $url);
 
             return 'git command';
@@ -62,7 +62,7 @@ class GitTest extends TestCase
         $this->git->runCommand($commandCallable, 'https://github.com/acme/repo', null, true);
     }
 
-    public function publicGithubNoCredentialsProvider()
+    public function publicGithubNoCredentialsProvider(): array
     {
         return array(
             array('ssh', 'git@github.com:acme/repo'),
@@ -70,11 +70,11 @@ class GitTest extends TestCase
         );
     }
 
-    public function testRunCommandPrivateGitHubRepositoryNotInitialCloneNotInteractiveWithoutAuthentication()
+    public function testRunCommandPrivateGitHubRepositoryNotInitialCloneNotInteractiveWithoutAuthentication(): void
     {
         self::expectException('RuntimeException');
 
-        $commandCallable = function ($url) {
+        $commandCallable = function ($url): string {
             $this->assertSame('https://github.com/acme/repo', $url);
 
             return 'git command';
@@ -99,9 +99,9 @@ class GitTest extends TestCase
      * @param string $expectedUrl
      * @param int    $expectedFailuresBeforeSuccess
      */
-    public function testRunCommandPrivateGitHubRepositoryNotInitialCloneNotInteractiveWithAuthentication($gitUrl, $protocol, $gitHubToken, $expectedUrl, $expectedFailuresBeforeSuccess)
+    public function testRunCommandPrivateGitHubRepositoryNotInitialCloneNotInteractiveWithAuthentication($gitUrl, $protocol, $gitHubToken, $expectedUrl, $expectedFailuresBeforeSuccess): void
     {
-        $commandCallable = function ($url) use ($expectedUrl) {
+        $commandCallable = function ($url) use ($expectedUrl): string {
             if ($url !== $expectedUrl) {
                 return 'git command failing';
             }
@@ -135,7 +135,7 @@ class GitTest extends TestCase
         $this->git->runCommand($commandCallable, $gitUrl, null, true);
     }
 
-    public function privateGithubWithCredentialsProvider()
+    public function privateGithubWithCredentialsProvider(): array
     {
         return array(
             array('git@github.com:acme/repo.git', 'ssh', 'MY_GITHUB_TOKEN', 'https://token:MY_GITHUB_TOKEN@github.com/acme/repo.git', 1),
@@ -148,7 +148,7 @@ class GitTest extends TestCase
      *
      * @return void
      */
-    private function mockConfig($protocol)
+    private function mockConfig($protocol): void
     {
         $this->config
             ->method('get')
diff --git a/tests/Composer/Test/Util/Http/ProxyHelperTest.php b/tests/Composer/Test/Util/Http/ProxyHelperTest.php
index 72e1d2f6d..15fd85069 100644
--- a/tests/Composer/Test/Util/Http/ProxyHelperTest.php
+++ b/tests/Composer/Test/Util/Http/ProxyHelperTest.php
@@ -49,7 +49,7 @@ class ProxyHelperTest extends TestCase
      *
      * @param string $url
      */
-    public function testThrowsOnMalformedUrl($url)
+    public function testThrowsOnMalformedUrl($url): void
     {
         $_SERVER['http_proxy'] = $url;
 
@@ -57,7 +57,7 @@ class ProxyHelperTest extends TestCase
         ProxyHelper::getProxyData();
     }
 
-    public function dataMalformed()
+    public function dataMalformed(): array
     {
         return array(
             'no-host' => array('localhost'),
@@ -71,7 +71,7 @@ class ProxyHelperTest extends TestCase
      * @param string $url
      * @param string $expected
      */
-    public function testUrlFormatting($url, $expected)
+    public function testUrlFormatting($url, $expected): void
     {
         $_SERVER['http_proxy'] = $url;
 
@@ -79,7 +79,7 @@ class ProxyHelperTest extends TestCase
         $this->assertSame($expected, $httpProxy);
     }
 
-    public function dataFormatting()
+    public function dataFormatting(): array
     {
         // url, expected
         return array(
@@ -96,7 +96,7 @@ class ProxyHelperTest extends TestCase
      * @param string               $expected
      * @param int                  $index
      */
-    public function testLowercaseOverridesUppercase(array $server, $expected, $index)
+    public function testLowercaseOverridesUppercase(array $server, $expected, $index): void
     {
         $_SERVER = array_merge($_SERVER, $server);
 
@@ -104,7 +104,7 @@ class ProxyHelperTest extends TestCase
         $this->assertSame($expected, $list[$index]);
     }
 
-    public function dataCaseOverrides()
+    public function dataCaseOverrides(): array
     {
         // server, expected, list index
         return array(
@@ -121,7 +121,7 @@ class ProxyHelperTest extends TestCase
      * @param string               $expected
      * @param int                  $index
      */
-    public function testCGIUpperCaseOverridesHttp(array $server, $expected, $index)
+    public function testCGIUpperCaseOverridesHttp(array $server, $expected, $index): void
     {
         $_SERVER = array_merge($_SERVER, $server);
 
@@ -129,7 +129,7 @@ class ProxyHelperTest extends TestCase
         $this->assertSame($expected, $list[$index]);
     }
 
-    public function dataCGIOverrides()
+    public function dataCGIOverrides(): array
     {
         // server, expected, list index
         return array(
@@ -138,7 +138,7 @@ class ProxyHelperTest extends TestCase
         );
     }
 
-    public function testNoHttpsProxyUsesHttpProxy()
+    public function testNoHttpsProxyUsesHttpProxy(): void
     {
         $_SERVER['http_proxy'] = 'http://http.com';
 
@@ -146,7 +146,7 @@ class ProxyHelperTest extends TestCase
         $this->assertSame('http://http.com:80', $httpsProxy);
     }
 
-    public function testNoHttpProxyDoesNotUseHttpsProxy()
+    public function testNoHttpProxyDoesNotUseHttpsProxy(): void
     {
         $_SERVER['https_proxy'] = 'http://https.com';
 
@@ -162,12 +162,12 @@ class ProxyHelperTest extends TestCase
      *
      * @phpstan-param array{http: array{proxy: string, header?: string}} $expected
      */
-    public function testGetContextOptions($url, $expected)
+    public function testGetContextOptions($url, $expected): void
     {
         $this->assertEquals($expected, ProxyHelper::getContextOptions($url));
     }
 
-    public function dataContextOptions()
+    public function dataContextOptions(): array
     {
         // url, expected
         return array(
@@ -190,7 +190,7 @@ class ProxyHelperTest extends TestCase
      * @param string  $requestUrl
      * @param mixed[] $expected
      */
-    public function testSetRequestFullUri($requestUrl, $expected)
+    public function testSetRequestFullUri($requestUrl, $expected): void
     {
         $options = array();
         ProxyHelper::setRequestFullUri($requestUrl, $options);
@@ -198,7 +198,7 @@ class ProxyHelperTest extends TestCase
         $this->assertEquals($expected, $options);
     }
 
-    public function dataRequestFullUri()
+    public function dataRequestFullUri(): array
     {
         $options = array('http' => array('request_fulluri' => true));
 
diff --git a/tests/Composer/Test/Util/Http/ProxyManagerTest.php b/tests/Composer/Test/Util/Http/ProxyManagerTest.php
index 16d926039..729e12da3 100644
--- a/tests/Composer/Test/Util/Http/ProxyManagerTest.php
+++ b/tests/Composer/Test/Util/Http/ProxyManagerTest.php
@@ -46,7 +46,7 @@ class ProxyManagerTest extends TestCase
         ProxyManager::reset();
     }
 
-    public function testInstantiation()
+    public function testInstantiation(): void
     {
         $originalInstance = ProxyManager::getInstance();
         $this->assertInstanceOf('Composer\Util\Http\ProxyManager', $originalInstance);
@@ -59,7 +59,7 @@ class ProxyManagerTest extends TestCase
         $this->assertFalse($sameInstance === $newInstance);
     }
 
-    public function testGetProxyForRequestThrowsOnBadProxyUrl()
+    public function testGetProxyForRequestThrowsOnBadProxyUrl(): void
     {
         $_SERVER['http_proxy'] = 'localhost';
         $proxyManager = ProxyManager::getInstance();
@@ -77,7 +77,7 @@ class ProxyManagerTest extends TestCase
      * @param bool                 $expectedSecure
      * @param string               $expectedMessage
      */
-    public function testGetProxyForRequest($server, $url, $expectedUrl, $expectedOptions, $expectedSecure, $expectedMessage)
+    public function testGetProxyForRequest($server, $url, $expectedUrl, $expectedOptions, $expectedSecure, $expectedMessage): void
     {
         $_SERVER = array_merge($_SERVER, $server);
         $proxyManager = ProxyManager::getInstance();
@@ -100,7 +100,7 @@ class ProxyManagerTest extends TestCase
         $this->assertTrue($condition, 'lastProxy check');
     }
 
-    public function dataRequest()
+    public function dataRequest(): array
     {
         $server = array(
             'http_proxy' => 'http://user:p%40ss@proxy.com',
@@ -139,7 +139,7 @@ class ProxyManagerTest extends TestCase
      * @param bool                 $expectedStatus
      * @param string               $expectedMessage
      */
-    public function testGetStatus($server, $expectedStatus, $expectedMessage)
+    public function testGetStatus($server, $expectedStatus, $expectedMessage): void
     {
         $_SERVER = array_merge($_SERVER, $server);
         $proxyManager = ProxyManager::getInstance();
@@ -156,7 +156,7 @@ class ProxyManagerTest extends TestCase
         $this->assertTrue($condition, 'message check');
     }
 
-    public function dataStatus()
+    public function dataStatus(): array
     {
         // server, expectedStatus, expectedMessage
         return array(
diff --git a/tests/Composer/Test/Util/Http/RequestProxyTest.php b/tests/Composer/Test/Util/Http/RequestProxyTest.php
index 33f5664a8..120ff8263 100644
--- a/tests/Composer/Test/Util/Http/RequestProxyTest.php
+++ b/tests/Composer/Test/Util/Http/RequestProxyTest.php
@@ -23,14 +23,14 @@ class RequestProxyTest extends TestCase
      * @param string $url
      * @param bool   $expectedSecure
      */
-    public function testIsSecure($url, $expectedSecure)
+    public function testIsSecure($url, $expectedSecure): void
     {
         $proxy = new RequestProxy($url, array(), '');
 
         $this->assertSame($expectedSecure, $proxy->isSecure());
     }
 
-    public function dataSecure()
+    public function dataSecure(): array
     {
         // url, secure
         return array(
@@ -47,7 +47,7 @@ class RequestProxyTest extends TestCase
      * @param string $format
      * @param string $expected
      */
-    public function testGetFormattedUrlFormat($url, $format, $expected)
+    public function testGetFormattedUrlFormat($url, $format, $expected): void
     {
         $proxy = new RequestProxy($url, array(), $url);
 
@@ -55,7 +55,7 @@ class RequestProxyTest extends TestCase
         $this->assertSame($expected, $message);
     }
 
-    public function dataProxyUrl()
+    public function dataProxyUrl(): array
     {
         $format = 'proxy (%s)';
 
diff --git a/tests/Composer/Test/Util/HttpDownloaderTest.php b/tests/Composer/Test/Util/HttpDownloaderTest.php
index e35ab514e..fe49c1ca5 100644
--- a/tests/Composer/Test/Util/HttpDownloaderTest.php
+++ b/tests/Composer/Test/Util/HttpDownloaderTest.php
@@ -38,7 +38,7 @@ class HttpDownloaderTest extends TestCase
     /**
      * @group slow
      */
-    public function testCaptureAuthenticationParamsFromUrl()
+    public function testCaptureAuthenticationParamsFromUrl(): void
     {
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $io->expects($this->once())
@@ -53,7 +53,7 @@ class HttpDownloaderTest extends TestCase
         }
     }
 
-    public function testOutputWarnings()
+    public function testOutputWarnings(): void
     {
         $io = new BufferIO();
         HttpDownloader::outputWarnings($io, '$URL', array());
diff --git a/tests/Composer/Test/Util/IniHelperTest.php b/tests/Composer/Test/Util/IniHelperTest.php
index a6d83bb09..a3e8bc8e6 100644
--- a/tests/Composer/Test/Util/IniHelperTest.php
+++ b/tests/Composer/Test/Util/IniHelperTest.php
@@ -26,7 +26,7 @@ class IniHelperTest extends TestCase
      */
     public static $envOriginal;
 
-    public function testWithNoIni()
+    public function testWithNoIni(): void
     {
         $paths = array(
             '',
@@ -37,7 +37,7 @@ class IniHelperTest extends TestCase
         $this->assertEquals($paths, IniHelper::getAll());
     }
 
-    public function testWithLoadedIniOnly()
+    public function testWithLoadedIniOnly(): void
     {
         $paths = array(
             'loaded.ini',
@@ -47,7 +47,7 @@ class IniHelperTest extends TestCase
         $this->assertStringContainsString('loaded.ini', IniHelper::getMessage());
     }
 
-    public function testWithLoadedIniAndAdditional()
+    public function testWithLoadedIniAndAdditional(): void
     {
         $paths = array(
             'loaded.ini',
@@ -60,7 +60,7 @@ class IniHelperTest extends TestCase
         $this->assertEquals($paths, IniHelper::getAll());
     }
 
-    public function testWithoutLoadedIniAndAdditional()
+    public function testWithoutLoadedIniAndAdditional(): void
     {
         $paths = array(
             '',
@@ -96,7 +96,7 @@ class IniHelperTest extends TestCase
      *
      * @return void
      */
-    protected function setEnv(array $paths)
+    protected function setEnv(array $paths): void
     {
         putenv('COMPOSER_ORIGINAL_INIS='.implode(PATH_SEPARATOR, $paths));
     }
diff --git a/tests/Composer/Test/Util/MetadataMinifierTest.php b/tests/Composer/Test/Util/MetadataMinifierTest.php
index fa69e928d..44e28c9b2 100644
--- a/tests/Composer/Test/Util/MetadataMinifierTest.php
+++ b/tests/Composer/Test/Util/MetadataMinifierTest.php
@@ -19,7 +19,7 @@ use PHPUnit\Framework\TestCase;
 
 class MetadataMinifierTest extends TestCase
 {
-    public function testMinifyExpand()
+    public function testMinifyExpand(): void
     {
         $package1 = new CompletePackage('foo/bar', '2.0.0.0', '2.0.0');
         $package1->setScripts(array('foo' => array('bar')));
diff --git a/tests/Composer/Test/Util/NoProxyPatternTest.php b/tests/Composer/Test/Util/NoProxyPatternTest.php
index 940eff5cc..c70c9774d 100644
--- a/tests/Composer/Test/Util/NoProxyPatternTest.php
+++ b/tests/Composer/Test/Util/NoProxyPatternTest.php
@@ -24,14 +24,14 @@ class NoProxyPatternTest extends TestCase
      * @param string $url
      * @param bool   $expected
      */
-    public function testHostName($noproxy, $url, $expected)
+    public function testHostName($noproxy, $url, $expected): void
     {
         $matcher = new NoProxyPattern($noproxy);
         $url = $this->getUrl($url);
         $this->assertEquals($expected, $matcher->test($url));
     }
 
-    public function dataHostName()
+    public function dataHostName(): array
     {
         $noproxy = 'foobar.com, .barbaz.net';
 
@@ -55,14 +55,14 @@ class NoProxyPatternTest extends TestCase
      * @param string $url
      * @param bool   $expected
      */
-    public function testIpAddress($noproxy, $url, $expected)
+    public function testIpAddress($noproxy, $url, $expected): void
     {
         $matcher = new NoProxyPattern($noproxy);
         $url = $this->getUrl($url);
         $this->assertEquals($expected, $matcher->test($url));
     }
 
-    public function dataIpAddress()
+    public function dataIpAddress(): array
     {
         $noproxy = '192.168.1.1, 2001:db8::52:0:1';
 
@@ -84,14 +84,14 @@ class NoProxyPatternTest extends TestCase
      * @param string $url
      * @param bool   $expected
      */
-    public function testIpRange($noproxy, $url, $expected)
+    public function testIpRange($noproxy, $url, $expected): void
     {
         $matcher = new NoProxyPattern($noproxy);
         $url = $this->getUrl($url);
         $this->assertEquals($expected, $matcher->test($url));
     }
 
-    public function dataIpRange()
+    public function dataIpRange(): array
     {
         $noproxy = '10.0.0.0/30, 2002:db8:a::45/121';
 
@@ -113,14 +113,14 @@ class NoProxyPatternTest extends TestCase
      * @param string $url
      * @param bool   $expected
      */
-    public function testPort($noproxy, $url, $expected)
+    public function testPort($noproxy, $url, $expected): void
     {
         $matcher = new NoProxyPattern($noproxy);
         $url = $this->getUrl($url);
         $this->assertEquals($expected, $matcher->test($url));
     }
 
-    public function dataPort()
+    public function dataPort(): array
     {
         $noproxy = '192.168.1.2:81, 192.168.1.3:80, [2001:db8::52:0:2]:443, [2001:db8::52:0:3]:80';
 
@@ -140,7 +140,7 @@ class NoProxyPatternTest extends TestCase
      *
      * @return string
      */
-    private function getUrl($url)
+    private function getUrl($url): string
     {
         if (parse_url($url, PHP_URL_SCHEME)) {
             return $url;
diff --git a/tests/Composer/Test/Util/PackageSorterTest.php b/tests/Composer/Test/Util/PackageSorterTest.php
index 7ee94982c..4f25f5633 100644
--- a/tests/Composer/Test/Util/PackageSorterTest.php
+++ b/tests/Composer/Test/Util/PackageSorterTest.php
@@ -20,7 +20,7 @@ use Composer\Semver\Constraint\MatchAllConstraint;
 
 class PackageSorterTest extends TestCase
 {
-    public function testSortingDoesNothingWithNoDependencies()
+    public function testSortingDoesNothingWithNoDependencies(): void
     {
         $packages[] = $this->createPackage('foo/bar1', array());
         $packages[] = $this->createPackage('foo/bar2', array());
@@ -32,7 +32,7 @@ class PackageSorterTest extends TestCase
         self::assertSame($packages, $sortedPackages);
     }
 
-    public function sortingOrdersDependenciesHigherThanPackageDataProvider()
+    public function sortingOrdersDependenciesHigherThanPackageDataProvider(): array
     {
         return array(
             'one package is dep' => array(
@@ -108,10 +108,10 @@ class PackageSorterTest extends TestCase
      * @param Package[] $packages
      * @param string[]  $expectedOrderedList
      */
-    public function testSortingOrdersDependenciesHigherThanPackage($packages, $expectedOrderedList)
+    public function testSortingOrdersDependenciesHigherThanPackage($packages, $expectedOrderedList): void
     {
         $sortedPackages = PackageSorter::sortPackages($packages);
-        $sortedPackageNames = array_map(function ($package) {
+        $sortedPackageNames = array_map(function ($package): string {
             return $package->getName();
         }, $sortedPackages);
 
@@ -124,7 +124,7 @@ class PackageSorterTest extends TestCase
      *
      * @return Package
      */
-    private function createPackage($name, $requires)
+    private function createPackage($name, $requires): Package
     {
         $package = new Package($name, '1.0.0.0', '1.0.0');
 
diff --git a/tests/Composer/Test/Util/PerforceTest.php b/tests/Composer/Test/Util/PerforceTest.php
index 817e4fe48..4e3f7d5b9 100644
--- a/tests/Composer/Test/Util/PerforceTest.php
+++ b/tests/Composer/Test/Util/PerforceTest.php
@@ -50,7 +50,7 @@ class PerforceTest extends TestCase
     /**
      * @return array<string, string>
      */
-    public function getTestRepoConfig()
+    public function getTestRepoConfig(): array
     {
         return array(
             'depot' => self::TEST_DEPOT,
@@ -73,12 +73,12 @@ class PerforceTest extends TestCase
      *
      * @return void
      */
-    protected function createNewPerforceWithWindowsFlag($flag)
+    protected function createNewPerforceWithWindowsFlag($flag): void
     {
         $this->perforce = new Perforce($this->repoConfig, self::TEST_PORT, self::TEST_PATH, $this->processExecutor, $flag, $this->io);
     }
 
-    public function testGetClientWithoutStream()
+    public function testGetClientWithoutStream(): void
     {
         $client = $this->perforce->getClient();
 
@@ -86,7 +86,7 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $client);
     }
 
-    public function testGetClientFromStream()
+    public function testGetClientFromStream(): void
     {
         $this->setPerforceToStream();
 
@@ -96,13 +96,13 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $client);
     }
 
-    public function testGetStreamWithoutStream()
+    public function testGetStreamWithoutStream(): void
     {
         $stream = $this->perforce->getStream();
         $this->assertEquals("//depot", $stream);
     }
 
-    public function testGetStreamWithStream()
+    public function testGetStreamWithStream(): void
     {
         $this->setPerforceToStream();
 
@@ -110,26 +110,26 @@ class PerforceTest extends TestCase
         $this->assertEquals('//depot/branch', $stream);
     }
 
-    public function testGetStreamWithoutLabelWithStreamWithoutLabel()
+    public function testGetStreamWithoutLabelWithStreamWithoutLabel(): void
     {
         $stream = $this->perforce->getStreamWithoutLabel('//depot/branch');
         $this->assertEquals('//depot/branch', $stream);
     }
 
-    public function testGetStreamWithoutLabelWithStreamWithLabel()
+    public function testGetStreamWithoutLabelWithStreamWithLabel(): void
     {
         $stream = $this->perforce->getStreamWithoutLabel('//depot/branching@label');
         $this->assertEquals('//depot/branching', $stream);
     }
 
-    public function testGetClientSpec()
+    public function testGetClientSpec(): void
     {
         $clientSpec = $this->perforce->getP4ClientSpec();
         $expected = 'path/composer_perforce_TEST_depot.p4.spec';
         $this->assertEquals($expected, $clientSpec);
     }
 
-    public function testGenerateP4Command()
+    public function testGenerateP4Command(): void
     {
         $command = 'do something';
         $p4Command = $this->perforce->generateP4Command($command);
@@ -137,13 +137,13 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $p4Command);
     }
 
-    public function testQueryP4UserWithUserAlreadySet()
+    public function testQueryP4UserWithUserAlreadySet(): void
     {
         $this->perforce->queryP4user();
         $this->assertEquals(self::TEST_P4USER, $this->perforce->getUser());
     }
 
-    public function testQueryP4UserWithUserSetInP4VariablesWithWindowsOS()
+    public function testQueryP4UserWithUserSetInP4VariablesWithWindowsOS(): void
     {
         $this->createNewPerforceWithWindowsFlag(true);
         $this->perforce->setUser(null);
@@ -156,7 +156,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_P4VARIABLE_USER', $this->perforce->getUser());
     }
 
-    public function testQueryP4UserWithUserSetInP4VariablesNotWindowsOS()
+    public function testQueryP4UserWithUserSetInP4VariablesNotWindowsOS(): void
     {
         $this->createNewPerforceWithWindowsFlag(false);
         $this->perforce->setUser(null);
@@ -170,7 +170,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_P4VARIABLE_USER', $this->perforce->getUser());
     }
 
-    public function testQueryP4UserQueriesForUser()
+    public function testQueryP4UserQueriesForUser(): void
     {
         $this->perforce->setUser(null);
         $expectedQuestion = 'Enter P4 User:';
@@ -181,7 +181,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_QUERY_USER', $this->perforce->getUser());
     }
 
-    public function testQueryP4UserStoresResponseToQueryForUserWithWindows()
+    public function testQueryP4UserStoresResponseToQueryForUserWithWindows(): void
     {
         $this->createNewPerforceWithWindowsFlag(true);
         $this->perforce->setUser(null);
@@ -203,7 +203,7 @@ class PerforceTest extends TestCase
         $this->perforce->queryP4user();
     }
 
-    public function testQueryP4UserStoresResponseToQueryForUserWithoutWindows()
+    public function testQueryP4UserStoresResponseToQueryForUserWithoutWindows(): void
     {
         $this->createNewPerforceWithWindowsFlag(false);
         $this->perforce->setUser(null);
@@ -223,7 +223,7 @@ class PerforceTest extends TestCase
         $this->perforce->queryP4user();
     }
 
-    public function testQueryP4PasswordWithPasswordAlreadySet()
+    public function testQueryP4PasswordWithPasswordAlreadySet(): void
     {
         $repoConfig = array(
             'depot' => 'depot',
@@ -236,7 +236,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_PASSWORD', $password);
     }
 
-    public function testQueryP4PasswordWithPasswordSetInP4VariablesWithWindowsOS()
+    public function testQueryP4PasswordWithPasswordSetInP4VariablesWithWindowsOS(): void
     {
         $this->createNewPerforceWithWindowsFlag(true);
 
@@ -249,7 +249,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_P4VARIABLE_PASSWORD', $password);
     }
 
-    public function testQueryP4PasswordWithPasswordSetInP4VariablesNotWindowsOS()
+    public function testQueryP4PasswordWithPasswordSetInP4VariablesNotWindowsOS(): void
     {
         $this->createNewPerforceWithWindowsFlag(false);
 
@@ -262,7 +262,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_P4VARIABLE_PASSWORD', $password);
     }
 
-    public function testQueryP4PasswordQueriesForPassword()
+    public function testQueryP4PasswordQueriesForPassword(): void
     {
         $expectedQuestion = 'Enter password for Perforce user user: ';
         $this->io->expects($this->once())
@@ -274,7 +274,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('TEST_QUERY_PASSWORD', $password);
     }
 
-    public function testWriteP4ClientSpecWithoutStream()
+    public function testWriteP4ClientSpecWithoutStream(): void
     {
         $stream = fopen('php://memory', 'w+');
         if (false === $stream) {
@@ -297,7 +297,7 @@ class PerforceTest extends TestCase
         fclose($stream);
     }
 
-    public function testWriteP4ClientSpecWithStream()
+    public function testWriteP4ClientSpecWithStream(): void
     {
         $this->setPerforceToStream();
         $stream = fopen('php://memory', 'w+');
@@ -321,7 +321,7 @@ class PerforceTest extends TestCase
         fclose($stream);
     }
 
-    public function testIsLoggedIn()
+    public function testIsLoggedIn(): void
     {
         $this->processExecutor->expects(
             [['cmd' => 'p4 -u user -p port login -s']],
@@ -330,7 +330,7 @@ class PerforceTest extends TestCase
         $this->perforce->isLoggedIn();
     }
 
-    public function testConnectClient()
+    public function testConnectClient(): void
     {
         $this->processExecutor->expects(
             ['p4 -u user -c composer_perforce_TEST_depot -p port client -i < path/composer_perforce_TEST_depot.p4.spec'],
@@ -340,7 +340,7 @@ class PerforceTest extends TestCase
         $this->perforce->connectClient();
     }
 
-    public function testGetBranchesWithStream()
+    public function testGetBranchesWithStream(): void
     {
         $this->setPerforceToStream();
 
@@ -362,7 +362,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('//depot/branch@1234', $branches['master']);
     }
 
-    public function testGetBranchesWithoutStream()
+    public function testGetBranchesWithoutStream(): void
     {
         $this->processExecutor->expects(
             [
@@ -378,7 +378,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('//depot@5678', $branches['master']);
     }
 
-    public function testGetTagsWithoutStream()
+    public function testGetTagsWithoutStream(): void
     {
         $this->processExecutor->expects(
             [
@@ -395,7 +395,7 @@ class PerforceTest extends TestCase
         $this->assertEquals('//depot@0.0.2', $tags['0.0.2']);
     }
 
-    public function testGetTagsWithStream()
+    public function testGetTagsWithStream(): void
     {
         $this->setPerforceToStream();
 
@@ -414,14 +414,14 @@ class PerforceTest extends TestCase
         $this->assertEquals('//depot/branch@0.0.2', $tags['0.0.2']);
     }
 
-    public function testCheckStreamWithoutStream()
+    public function testCheckStreamWithoutStream(): void
     {
         $result = $this->perforce->checkStream();
         $this->assertFalse($result);
         $this->assertFalse($this->perforce->isStream());
     }
 
-    public function testCheckStreamWithStream()
+    public function testCheckStreamWithStream(): void
     {
         $this->processExecutor->expects(
             [
@@ -438,7 +438,7 @@ class PerforceTest extends TestCase
         $this->assertTrue($this->perforce->isStream());
     }
 
-    public function testGetComposerInformationWithoutLabelWithoutStream()
+    public function testGetComposerInformationWithoutLabelWithoutStream(): void
     {
         $this->processExecutor->expects(
             [
@@ -460,7 +460,7 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $result);
     }
 
-    public function testGetComposerInformationWithLabelWithoutStream()
+    public function testGetComposerInformationWithLabelWithoutStream(): void
     {
         $this->processExecutor->expects(
             [
@@ -487,7 +487,7 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $result);
     }
 
-    public function testGetComposerInformationWithoutLabelWithStream()
+    public function testGetComposerInformationWithoutLabelWithStream(): void
     {
         $this->setPerforceToStream();
 
@@ -512,7 +512,7 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $result);
     }
 
-    public function testGetComposerInformationWithLabelWithStream()
+    public function testGetComposerInformationWithLabelWithStream(): void
     {
         $this->processExecutor->expects(
             [
@@ -541,7 +541,7 @@ class PerforceTest extends TestCase
         $this->assertEquals($expected, $result);
     }
 
-    public function testSyncCodeBaseWithoutStream()
+    public function testSyncCodeBaseWithoutStream(): void
     {
         $this->processExecutor->expects(
             ['p4 -u user -c composer_perforce_TEST_depot -p port sync -f @label'],
@@ -551,7 +551,7 @@ class PerforceTest extends TestCase
         $this->perforce->syncCodeBase('label');
     }
 
-    public function testSyncCodeBaseWithStream()
+    public function testSyncCodeBaseWithStream(): void
     {
         $this->setPerforceToStream();
 
@@ -563,7 +563,7 @@ class PerforceTest extends TestCase
         $this->perforce->syncCodeBase('label');
     }
 
-    public function testCheckServerExists()
+    public function testCheckServerExists(): void
     {
         $this->processExecutor->expects(
             ['p4 -p '.ProcessExecutor::escape('perforce.does.exist:port').' info -s'],
@@ -581,7 +581,7 @@ class PerforceTest extends TestCase
      *
      * @return void
      */
-    public function testCheckServerClientError()
+    public function testCheckServerClientError(): void
     {
         $processExecutor = $this->getMockBuilder('Composer\Util\ProcessExecutor')->getMock();
 
@@ -598,7 +598,7 @@ class PerforceTest extends TestCase
     /**
      * @return string
      */
-    public static function getComposerJson()
+    public static function getComposerJson(): string
     {
         return JsonFile::encode([
             'name' => 'test/perforce',
@@ -615,7 +615,7 @@ class PerforceTest extends TestCase
      *
      * @return string[]
      */
-    private function getExpectedClientSpec($withStream)
+    private function getExpectedClientSpec($withStream): array
     {
         $expectedArray = array(
             'Client: composer_perforce_TEST_depot',
@@ -650,12 +650,12 @@ class PerforceTest extends TestCase
     /**
      * @return void
      */
-    private function setPerforceToStream()
+    private function setPerforceToStream(): void
     {
         $this->perforce->setStream('//depot/branch');
     }
 
-    public function testCleanupClientSpecShouldDeleteClient()
+    public function testCleanupClientSpecShouldDeleteClient(): void
     {
         $fs = $this->getMockBuilder('Composer\Util\Filesystem')->getMock();
         $this->perforce->setFilesystem($fs);
diff --git a/tests/Composer/Test/Util/PlatformTest.php b/tests/Composer/Test/Util/PlatformTest.php
index 70777d69a..f05d3a736 100644
--- a/tests/Composer/Test/Util/PlatformTest.php
+++ b/tests/Composer/Test/Util/PlatformTest.php
@@ -22,7 +22,7 @@ use Composer\Test\TestCase;
  */
 class PlatformTest extends TestCase
 {
-    public function testExpandPath()
+    public function testExpandPath(): void
     {
         putenv('TESTENV=/home/test');
         $this->assertEquals('/home/test/myPath', Platform::expandPath('%TESTENV%/myPath'));
@@ -30,7 +30,7 @@ class PlatformTest extends TestCase
         $this->assertEquals((getenv('HOME') ?: getenv('USERPROFILE')) . '/test', Platform::expandPath('~/test'));
     }
 
-    public function testIsWindows()
+    public function testIsWindows(): void
     {
         // Compare 2 common tests for Windows to the built-in Windows test
         $this->assertEquals(('\\' === DIRECTORY_SEPARATOR), Platform::isWindows());
diff --git a/tests/Composer/Test/Util/ProcessExecutorTest.php b/tests/Composer/Test/Util/ProcessExecutorTest.php
index ff2acd82e..c0ef2f43e 100644
--- a/tests/Composer/Test/Util/ProcessExecutorTest.php
+++ b/tests/Composer/Test/Util/ProcessExecutorTest.php
@@ -25,14 +25,14 @@ use Symfony\Component\Console\Output\StreamOutput;
 
 class ProcessExecutorTest extends TestCase
 {
-    public function testExecuteCapturesOutput()
+    public function testExecuteCapturesOutput(): void
     {
         $process = new ProcessExecutor;
         $process->execute('echo foo', $output);
         $this->assertEquals("foo".PHP_EOL, $output);
     }
 
-    public function testExecuteOutputsIfNotCaptured()
+    public function testExecuteOutputsIfNotCaptured(): void
     {
         $process = new ProcessExecutor;
         ob_start();
@@ -41,7 +41,7 @@ class ProcessExecutorTest extends TestCase
         $this->assertEquals("foo".PHP_EOL, $output);
     }
 
-    public function testUseIOIsNotNullAndIfNotCaptured()
+    public function testUseIOIsNotNullAndIfNotCaptured(): void
     {
         $io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
         $io->expects($this->once())
@@ -52,14 +52,14 @@ class ProcessExecutorTest extends TestCase
         $process->execute('echo foo');
     }
 
-    public function testExecuteCapturesStderr()
+    public function testExecuteCapturesStderr(): void
     {
         $process = new ProcessExecutor;
         $process->execute('cat foo', $output);
         $this->assertNotNull($process->getErrorOutput());
     }
 
-    public function testTimeout()
+    public function testTimeout(): void
     {
         ProcessExecutor::setTimeout(1);
         $process = new ProcessExecutor;
@@ -73,14 +73,14 @@ class ProcessExecutorTest extends TestCase
      * @param string $command
      * @param string $expectedCommandOutput
      */
-    public function testHidePasswords($command, $expectedCommandOutput)
+    public function testHidePasswords($command, $expectedCommandOutput): void
     {
         $process = new ProcessExecutor($buffer = new BufferIO('', StreamOutput::VERBOSITY_DEBUG));
         $process->execute($command, $output);
         $this->assertEquals('Executing command (CWD): ' . $expectedCommandOutput, trim($buffer->getOutput()));
     }
 
-    public function hidePasswordProvider()
+    public function hidePasswordProvider(): array
     {
         return array(
             array('echo https://foo:bar@example.org/', 'echo https://foo:***@example.org/'),
@@ -91,14 +91,14 @@ class ProcessExecutorTest extends TestCase
         );
     }
 
-    public function testDoesntHidePorts()
+    public function testDoesntHidePorts(): void
     {
         $process = new ProcessExecutor($buffer = new BufferIO('', StreamOutput::VERBOSITY_DEBUG));
         $process->execute('echo https://localhost:1234/', $output);
         $this->assertEquals('Executing command (CWD): echo https://localhost:1234/', trim($buffer->getOutput()));
     }
 
-    public function testSplitLines()
+    public function testSplitLines(): void
     {
         $process = new ProcessExecutor;
         $this->assertEquals(array(), $process->splitLines(''));
@@ -109,7 +109,7 @@ class ProcessExecutorTest extends TestCase
         $this->assertEquals(array('foo', 'bar'), $process->splitLines("foo\r\nbar\n"));
     }
 
-    public function testConsoleIODoesNotFormatSymfonyConsoleStyle()
+    public function testConsoleIODoesNotFormatSymfonyConsoleStyle(): void
     {
         $output = new BufferedOutput(OutputInterface::VERBOSITY_NORMAL, true);
         $process = new ProcessExecutor(new ConsoleIO(new ArrayInput(array()), $output, new HelperSet(array())));
@@ -118,7 +118,7 @@ class ProcessExecutorTest extends TestCase
         $this->assertSame('<error>foo</error>'.PHP_EOL, $output->fetch());
     }
 
-    public function testExecuteAsyncCancel()
+    public function testExecuteAsyncCancel(): void
     {
         $process = new ProcessExecutor($buffer = new BufferIO('', StreamOutput::VERBOSITY_DEBUG));
         $process->enableAsync();
@@ -141,7 +141,7 @@ class ProcessExecutorTest extends TestCase
      * @param string            $win
      * @param string            $unix
      */
-    public function testEscapeArgument($argument, $win, $unix)
+    public function testEscapeArgument($argument, $win, $unix): void
     {
         $expected = defined('PHP_WINDOWS_VERSION_BUILD') ? $win : $unix;
         $this->assertSame($expected, ProcessExecutor::escape($argument));
@@ -151,7 +151,7 @@ class ProcessExecutorTest extends TestCase
      * Each named test is an array of:
      *   argument, win-expected, unix-expected
      */
-    public function dataEscapeArguments()
+    public function dataEscapeArguments(): array
     {
         return array(
             // empty argument - must be quoted
diff --git a/tests/Composer/Test/Util/RemoteFilesystemTest.php b/tests/Composer/Test/Util/RemoteFilesystemTest.php
index a63e24bb3..06df39b88 100644
--- a/tests/Composer/Test/Util/RemoteFilesystemTest.php
+++ b/tests/Composer/Test/Util/RemoteFilesystemTest.php
@@ -24,7 +24,7 @@ use ReflectionProperty;
 
 class RemoteFilesystemTest extends TestCase
 {
-    public function testGetOptionsForUrl()
+    public function testGetOptionsForUrl(): void
     {
         $io = $this->getIOInterfaceMock();
         $io
@@ -37,7 +37,7 @@ class RemoteFilesystemTest extends TestCase
         $this->assertTrue(isset($res['http']['header']) && is_array($res['http']['header']), 'getOptions must return an array with headers');
     }
 
-    public function testGetOptionsForUrlWithAuthorization()
+    public function testGetOptionsForUrlWithAuthorization(): void
     {
         $io = $this->getIOInterfaceMock();
         $io
@@ -62,7 +62,7 @@ class RemoteFilesystemTest extends TestCase
         $this->assertTrue($found, 'getOptions must have an Authorization header');
     }
 
-    public function testGetOptionsForUrlWithStreamOptions()
+    public function testGetOptionsForUrlWithStreamOptions(): void
     {
         $io = $this->getIOInterfaceMock();
         $io
@@ -88,7 +88,7 @@ class RemoteFilesystemTest extends TestCase
         );
     }
 
-    public function testGetOptionsForUrlWithCallOptionsKeepsHeader()
+    public function testGetOptionsForUrlWithCallOptionsKeepsHeader(): void
     {
         $io = $this->getIOInterfaceMock();
         $io
@@ -121,14 +121,14 @@ class RemoteFilesystemTest extends TestCase
         $this->assertGreaterThan(1, count($res['http']['header']));
     }
 
-    public function testCallbackGetFileSize()
+    public function testCallbackGetFileSize(): void
     {
         $fs = new RemoteFilesystem($this->getIOInterfaceMock(), $this->getConfigMock());
         $this->callCallbackGet($fs, STREAM_NOTIFY_FILE_SIZE_IS, 0, '', 0, 0, 20);
         $this->assertAttributeEqualsCustom(20, 'bytesMax', $fs);
     }
 
-    public function testCallbackGetNotifyProgress()
+    public function testCallbackGetNotifyProgress(): void
     {
         $io = $this->getIOInterfaceMock();
         $io
@@ -144,7 +144,7 @@ class RemoteFilesystemTest extends TestCase
         $this->assertAttributeEqualsCustom(50, 'lastProgress', $fs);
     }
 
-    public function testCallbackGetPassesThrough404()
+    public function testCallbackGetPassesThrough404(): void
     {
         $fs = new RemoteFilesystem($this->getIOInterfaceMock(), $this->getConfigMock());
 
@@ -152,14 +152,14 @@ class RemoteFilesystemTest extends TestCase
         $this->assertTrue(true, 'callbackGet must pass through 404');
     }
 
-    public function testGetContents()
+    public function testGetContents(): void
     {
         $fs = new RemoteFilesystem($this->getIOInterfaceMock(), $this->getConfigMock());
 
         $this->assertStringContainsString('testGetContents', $fs->getContents('http://example.org', 'file://'.__FILE__));
     }
 
-    public function testCopy()
+    public function testCopy(): void
     {
         $fs = new RemoteFilesystem($this->getIOInterfaceMock(), $this->getConfigMock());
 
@@ -170,13 +170,13 @@ class RemoteFilesystemTest extends TestCase
         unlink($file);
     }
 
-    public function testCopyWithNoRetryOnFailure()
+    public function testCopyWithNoRetryOnFailure(): void
     {
         self::expectException('Composer\Downloader\TransportException');
         $fs = $this->getRemoteFilesystemWithMockedMethods(array('getRemoteContents'));
 
         $fs->expects($this->once())->method('getRemoteContents')
-            ->willReturnCallback(function ($originUrl, $fileUrl, $ctx, &$http_response_header) {
+            ->willReturnCallback(function ($originUrl, $fileUrl, $ctx, &$http_response_header): string {
                 $http_response_header = array('http/1.1 401 unauthorized');
 
                 return '';
@@ -194,7 +194,7 @@ class RemoteFilesystemTest extends TestCase
         );
     }
 
-    public function testCopyWithSuccessOnRetry()
+    public function testCopyWithSuccessOnRetry(): void
     {
         $authHelper = $this->getAuthHelperWithMockedMethods(array('promptAuthIfNeeded'));
         $fs = $this->getRemoteFilesystemWithMockedMethods(array('getRemoteContents'), $authHelper);
@@ -241,7 +241,7 @@ class RemoteFilesystemTest extends TestCase
     /**
      * @group TLS
      */
-    public function testGetOptionsForUrlCreatesSecureTlsDefaults()
+    public function testGetOptionsForUrlCreatesSecureTlsDefaults(): void
     {
         $io = $this->getIOInterfaceMock();
 
@@ -265,7 +265,7 @@ class RemoteFilesystemTest extends TestCase
      *
      * @return string[][]
      */
-    public function provideBitbucketPublicDownloadUrls()
+    public function provideBitbucketPublicDownloadUrls(): array
     {
         return array(
             array('https://bitbucket.org/seldaek/composer-live-test-repo/downloads/composer-unit-test-download-me.txt', '1234'),
@@ -279,7 +279,7 @@ class RemoteFilesystemTest extends TestCase
      * @param string $contents
      * @dataProvider provideBitbucketPublicDownloadUrls
      */
-    public function testBitBucketPublicDownload($url, $contents)
+    public function testBitBucketPublicDownload($url, $contents): void
     {
         /** @var ConsoleIO $io */
         $io = $this
@@ -302,7 +302,7 @@ class RemoteFilesystemTest extends TestCase
      * @param string $contents
      * @dataProvider provideBitbucketPublicDownloadUrls
      */
-    public function testBitBucketPublicDownloadWithAuthConfigured($url, $contents)
+    public function testBitBucketPublicDownloadWithAuthConfigured($url, $contents): void
     {
         /** @var MockObject|ConsoleIO $io */
         $io = $this
@@ -313,7 +313,7 @@ class RemoteFilesystemTest extends TestCase
         $domains = array();
         $io
             ->method('hasAuthentication')
-            ->willReturnCallback(function ($arg) use (&$domains) {
+            ->willReturnCallback(function ($arg) use (&$domains): bool {
                 $domains[] = $arg;
                 // first time is called with bitbucket.org, then it redirects to bbuseruploads.s3.amazonaws.com so next time we have no auth configured
                 return $arg === 'bitbucket.org';
@@ -343,7 +343,7 @@ class RemoteFilesystemTest extends TestCase
      *
      * @return mixed[]
      */
-    private function callGetOptionsForUrl(IOInterface $io, array $args = array(), array $options = array(), $fileUrl = '')
+    private function callGetOptionsForUrl(IOInterface $io, array $args = array(), array $options = array(), $fileUrl = ''): array
     {
         $fs = new RemoteFilesystem($io, $this->getConfigMock(), $options);
         $ref = new ReflectionMethod($fs, 'getOptionsForUrl');
@@ -385,7 +385,7 @@ class RemoteFilesystemTest extends TestCase
      *
      * @return void
      */
-    private function callCallbackGet(RemoteFilesystem $fs, $notificationCode, $severity, $message, $messageCode, $bytesTransferred, $bytesMax)
+    private function callCallbackGet(RemoteFilesystem $fs, $notificationCode, $severity, $message, $messageCode, $bytesTransferred, $bytesMax): void
     {
         $ref = new ReflectionMethod($fs, 'callbackGet');
         $ref->setAccessible(true);
@@ -399,7 +399,7 @@ class RemoteFilesystemTest extends TestCase
      *
      * @return void
      */
-    private function setAttribute($object, $attribute, $value)
+    private function setAttribute($object, $attribute, $value): void
     {
         $attr = new ReflectionProperty($object, $attribute);
         $attr->setAccessible(true);
@@ -413,7 +413,7 @@ class RemoteFilesystemTest extends TestCase
      *
      * @return void
      */
-    private function assertAttributeEqualsCustom($value, $attribute, $object)
+    private function assertAttributeEqualsCustom($value, $attribute, $object): void
     {
         $attr = new ReflectionProperty($object, $attribute);
         $attr->setAccessible(true);
diff --git a/tests/Composer/Test/Util/SilencerTest.php b/tests/Composer/Test/Util/SilencerTest.php
index 9072a7cda..58a81eb92 100644
--- a/tests/Composer/Test/Util/SilencerTest.php
+++ b/tests/Composer/Test/Util/SilencerTest.php
@@ -25,7 +25,7 @@ class SilencerTest extends TestCase
     /**
      * Test succeeds when no warnings are emitted externally, and original level is restored.
      */
-    public function testSilencer()
+    public function testSilencer(): void
     {
         $before = error_reporting();
 
@@ -49,12 +49,12 @@ class SilencerTest extends TestCase
     /**
      * Test whether exception from silent callbacks are correctly forwarded.
      */
-    public function testSilencedException()
+    public function testSilencedException(): void
     {
         $verification = microtime();
         self::expectException('RuntimeException');
         self::expectExceptionMessage($verification);
-        Silencer::call(function () use ($verification) {
+        Silencer::call(function () use ($verification): void {
             throw new \RuntimeException($verification);
         });
     }
diff --git a/tests/Composer/Test/Util/StreamContextFactoryTest.php b/tests/Composer/Test/Util/StreamContextFactoryTest.php
index ca86d16ce..0d90672ba 100644
--- a/tests/Composer/Test/Util/StreamContextFactoryTest.php
+++ b/tests/Composer/Test/Util/StreamContextFactoryTest.php
@@ -39,7 +39,7 @@ class StreamContextFactoryTest extends TestCase
      * @param mixed[] $expectedParams
      * @param mixed[] $defaultParams
      */
-    public function testGetContext($expectedOptions, $defaultOptions, $expectedParams, $defaultParams)
+    public function testGetContext($expectedOptions, $defaultOptions, $expectedParams, $defaultParams): void
     {
         $context = StreamContextFactory::getContext('http://example.org', $defaultOptions, $defaultParams);
         $options = stream_context_get_options($context);
@@ -49,7 +49,7 @@ class StreamContextFactoryTest extends TestCase
         $this->assertEquals($expectedParams, $params);
     }
 
-    public function dataGetContext()
+    public function dataGetContext(): array
     {
         return array(
             array(
@@ -58,13 +58,13 @@ class StreamContextFactoryTest extends TestCase
             ),
             array(
                 $a = array('http' => array('method' => 'GET', 'max_redirects' => 20, 'follow_location' => 1, 'header' => array('User-Agent: foo'))), array('http' => array('method' => 'GET', 'header' => 'User-Agent: foo')),
-                array('options' => $a, 'notification' => $f = function () {
+                array('options' => $a, 'notification' => $f = function (): void {
                 }), array('notification' => $f),
             ),
         );
     }
 
-    public function testHttpProxy()
+    public function testHttpProxy(): void
     {
         $_SERVER['http_proxy'] = 'http://username:p%40ssword@proxyserver.net:3128/';
         $_SERVER['HTTP_PROXY'] = 'http://proxyserver/';
@@ -82,7 +82,7 @@ class StreamContextFactoryTest extends TestCase
         )), $options);
     }
 
-    public function testHttpProxyWithNoProxy()
+    public function testHttpProxyWithNoProxy(): void
     {
         $_SERVER['http_proxy'] = 'http://username:password@proxyserver.net:3128/';
         $_SERVER['no_proxy'] = 'foo,example.org';
@@ -98,7 +98,7 @@ class StreamContextFactoryTest extends TestCase
         )), $options);
     }
 
-    public function testHttpProxyWithNoProxyWildcard()
+    public function testHttpProxyWithNoProxyWildcard(): void
     {
         $_SERVER['http_proxy'] = 'http://username:password@proxyserver.net:3128/';
         $_SERVER['no_proxy'] = '*';
@@ -114,7 +114,7 @@ class StreamContextFactoryTest extends TestCase
         )), $options);
     }
 
-    public function testOptionsArePreserved()
+    public function testOptionsArePreserved(): void
     {
         $_SERVER['http_proxy'] = 'http://username:password@proxyserver.net:3128/';
 
@@ -131,7 +131,7 @@ class StreamContextFactoryTest extends TestCase
         )), $options);
     }
 
-    public function testHttpProxyWithoutPort()
+    public function testHttpProxyWithoutPort(): void
     {
         $_SERVER['http_proxy'] = 'http://username:password@proxyserver.net';
 
@@ -147,7 +147,7 @@ class StreamContextFactoryTest extends TestCase
         )), $options);
     }
 
-    public function testHttpsProxyOverride()
+    public function testHttpsProxyOverride(): void
     {
         if (!extension_loaded('openssl')) {
             $this->markTestSkipped('Requires openssl');
@@ -167,7 +167,7 @@ class StreamContextFactoryTest extends TestCase
      * @param string $expected
      * @param string $proxy
      */
-    public function testSSLProxy($expected, $proxy)
+    public function testSSLProxy($expected, $proxy): void
     {
         $_SERVER['http_proxy'] = $proxy;
 
@@ -192,7 +192,7 @@ class StreamContextFactoryTest extends TestCase
         }
     }
 
-    public function dataSSLProxy()
+    public function dataSSLProxy(): array
     {
         return array(
             array('ssl://proxyserver:443', 'https://proxyserver/'),
@@ -200,7 +200,7 @@ class StreamContextFactoryTest extends TestCase
         );
     }
 
-    public function testEnsureThatfixHttpHeaderFieldMovesContentTypeToEndOfOptions()
+    public function testEnsureThatfixHttpHeaderFieldMovesContentTypeToEndOfOptions(): void
     {
         $options = array(
             'http' => array(
@@ -222,7 +222,7 @@ class StreamContextFactoryTest extends TestCase
         $this->assertEquals(end($expectedOptions['http']['header']), end($ctxoptions['http']['header']));
     }
 
-    public function testInitOptionsDoesIncludeProxyAuthHeaders()
+    public function testInitOptionsDoesIncludeProxyAuthHeaders(): void
     {
         $_SERVER['http_proxy'] = 'http://username:password@proxyserver.net:3128/';
 
@@ -233,7 +233,7 @@ class StreamContextFactoryTest extends TestCase
         $this->assertTrue(false !== stripos($headers, 'Proxy-Authorization'));
     }
 
-    public function testInitOptionsForCurlDoesNotIncludeProxyAuthHeaders()
+    public function testInitOptionsForCurlDoesNotIncludeProxyAuthHeaders(): void
     {
         if (!extension_loaded('curl')) {
             $this->markTestSkipped('The curl is not available.');
diff --git a/tests/Composer/Test/Util/SvnTest.php b/tests/Composer/Test/Util/SvnTest.php
index 0ab43dd45..7aa7c73b2 100644
--- a/tests/Composer/Test/Util/SvnTest.php
+++ b/tests/Composer/Test/Util/SvnTest.php
@@ -27,7 +27,7 @@ class SvnTest extends TestCase
      *
      * @dataProvider urlProvider
      */
-    public function testCredentials($url, $expect)
+    public function testCredentials($url, $expect): void
     {
         $svn = new Svn($url, new NullIO, new Config());
         $reflMethod = new \ReflectionMethod('Composer\\Util\\Svn', 'getCredentialString');
@@ -36,7 +36,7 @@ class SvnTest extends TestCase
         $this->assertEquals($expect, $reflMethod->invoke($svn));
     }
 
-    public function urlProvider()
+    public function urlProvider(): array
     {
         return array(
             array('http://till:test@svn.example.org/', $this->getCmd(" --username 'till' --password 'test' ")),
@@ -45,7 +45,7 @@ class SvnTest extends TestCase
         );
     }
 
-    public function testInteractiveString()
+    public function testInteractiveString(): void
     {
         $url = 'http://svn.example.org';
 
@@ -59,7 +59,7 @@ class SvnTest extends TestCase
         );
     }
 
-    public function testCredentialsFromConfig()
+    public function testCredentialsFromConfig(): void
     {
         $url = 'http://svn.apache.org';
 
@@ -79,7 +79,7 @@ class SvnTest extends TestCase
         $this->assertEquals($this->getCmd(" --username 'foo' --password 'bar' "), $reflMethod->invoke($svn));
     }
 
-    public function testCredentialsFromConfigWithCacheCredentialsTrue()
+    public function testCredentialsFromConfigWithCacheCredentialsTrue(): void
     {
         $url = 'http://svn.apache.org';
 
@@ -102,7 +102,7 @@ class SvnTest extends TestCase
         $this->assertEquals($this->getCmd(" --username 'foo' --password 'bar' "), $reflMethod->invoke($svn));
     }
 
-    public function testCredentialsFromConfigWithCacheCredentialsFalse()
+    public function testCredentialsFromConfigWithCacheCredentialsFalse(): void
     {
         $url = 'http://svn.apache.org';
 
diff --git a/tests/Composer/Test/Util/TarTest.php b/tests/Composer/Test/Util/TarTest.php
index c15470071..b0e697819 100644
--- a/tests/Composer/Test/Util/TarTest.php
+++ b/tests/Composer/Test/Util/TarTest.php
@@ -20,44 +20,44 @@ use Composer\Test\TestCase;
  */
 class TarTest extends TestCase
 {
-    public function testReturnsNullifTheTarIsNotFound()
+    public function testReturnsNullifTheTarIsNotFound(): void
     {
         $result = Tar::getComposerJson(__DIR__.'/Fixtures/Tar/invalid.zip');
 
         $this->assertNull($result);
     }
 
-    public function testReturnsNullIfTheTarIsEmpty()
+    public function testReturnsNullIfTheTarIsEmpty(): void
     {
         $result = Tar::getComposerJson(__DIR__.'/Fixtures/Tar/empty.tar.gz');
         $this->assertNull($result);
     }
 
-    public function testThrowsExceptionIfTheTarHasNoComposerJson()
+    public function testThrowsExceptionIfTheTarHasNoComposerJson(): void
     {
         self::expectException('RuntimeException');
         Tar::getComposerJson(__DIR__.'/Fixtures/Tar/nojson.tar.gz');
     }
 
-    public function testThrowsExceptionIfTheComposerJsonIsInASubSubfolder()
+    public function testThrowsExceptionIfTheComposerJsonIsInASubSubfolder(): void
     {
         self::expectException('RuntimeException');
         Tar::getComposerJson(__DIR__.'/Fixtures/Tar/subfolders.tar.gz');
     }
 
-    public function testReturnsComposerJsonInTarRoot()
+    public function testReturnsComposerJsonInTarRoot(): void
     {
         $result = Tar::getComposerJson(__DIR__.'/Fixtures/Tar/root.tar.gz');
         $this->assertEquals("{\n    \"name\": \"foo/bar\"\n}\n", $result);
     }
 
-    public function testReturnsComposerJsonInFirstFolder()
+    public function testReturnsComposerJsonInFirstFolder(): void
     {
         $result = Tar::getComposerJson(__DIR__.'/Fixtures/Tar/folder.tar.gz');
         $this->assertEquals("{\n    \"name\": \"foo/bar\"\n}\n", $result);
     }
 
-    public function testMultipleTopLevelDirsIsInvalid()
+    public function testMultipleTopLevelDirsIsInvalid(): void
     {
         self::expectException('RuntimeException');
         Tar::getComposerJson(__DIR__.'/Fixtures/Tar/multiple.tar.gz');
diff --git a/tests/Composer/Test/Util/TlsHelperTest.php b/tests/Composer/Test/Util/TlsHelperTest.php
index 3b9549951..1bc7d9151 100644
--- a/tests/Composer/Test/Util/TlsHelperTest.php
+++ b/tests/Composer/Test/Util/TlsHelperTest.php
@@ -24,7 +24,7 @@ class TlsHelperTest extends TestCase
      * @param string   $hostname
      * @param string[] $certNames
      */
-    public function testCheckCertificateHost($expectedResult, $hostname, $certNames)
+    public function testCheckCertificateHost($expectedResult, $hostname, $certNames): void
     {
         $certificate['subject']['commonName'] = $expectedCn = array_shift($certNames);
         $certificate['extensions']['subjectAltName'] = $certNames ? 'DNS:'.implode(',DNS:', $certNames) : '';
@@ -41,7 +41,7 @@ class TlsHelperTest extends TestCase
         }
     }
 
-    public function dataCheckCertificateHost()
+    public function dataCheckCertificateHost(): array
     {
         return array(
             array(true, 'getcomposer.org', array('getcomposer.org')),
@@ -67,7 +67,7 @@ class TlsHelperTest extends TestCase
         );
     }
 
-    public function testGetCertificateNames()
+    public function testGetCertificateNames(): void
     {
         $certificate['subject']['commonName'] = 'example.net';
         $certificate['extensions']['subjectAltName'] = 'DNS: example.com, IP: 127.0.0.1, DNS: getcomposer.org, Junk: blah, DNS: composer.example.org';
diff --git a/tests/Composer/Test/Util/UrlTest.php b/tests/Composer/Test/Util/UrlTest.php
index 66165b4b8..650d53875 100644
--- a/tests/Composer/Test/Util/UrlTest.php
+++ b/tests/Composer/Test/Util/UrlTest.php
@@ -26,7 +26,7 @@ class UrlTest extends TestCase
      * @param array<string, mixed> $conf
      * @param string               $ref
      */
-    public function testUpdateDistReference($url, $expectedUrl, $conf = array(), $ref = 'newref')
+    public function testUpdateDistReference($url, $expectedUrl, $conf = array(), $ref = 'newref'): void
     {
         $config = new Config();
         $config->merge(array('config' => $conf));
@@ -34,7 +34,7 @@ class UrlTest extends TestCase
         $this->assertSame($expectedUrl, Url::updateDistReference($config, $url, $ref));
     }
 
-    public static function distRefsProvider()
+    public static function distRefsProvider(): array
     {
         return array(
             // github
@@ -70,12 +70,12 @@ class UrlTest extends TestCase
      * @param string $expected
      * @param string $url
      */
-    public function testSanitize($expected, $url)
+    public function testSanitize($expected, $url): void
     {
         $this->assertSame($expected, Url::sanitize($url));
     }
 
-    public static function sanitizeProvider()
+    public static function sanitizeProvider(): array
     {
         return array(
             // with scheme
diff --git a/tests/Composer/Test/Util/ZipTest.php b/tests/Composer/Test/Util/ZipTest.php
index b55208c3e..fa92fd897 100644
--- a/tests/Composer/Test/Util/ZipTest.php
+++ b/tests/Composer/Test/Util/ZipTest.php
@@ -20,7 +20,7 @@ use Composer\Test\TestCase;
  */
 class ZipTest extends TestCase
 {
-    public function testThrowsExceptionIfZipExtensionIsNotLoaded()
+    public function testThrowsExceptionIfZipExtensionIsNotLoaded(): void
     {
         if (extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is loaded.');
@@ -32,7 +32,7 @@ class ZipTest extends TestCase
         Zip::getComposerJson('');
     }
 
-    public function testReturnsNullifTheZipIsNotFound()
+    public function testReturnsNullifTheZipIsNotFound(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -43,7 +43,7 @@ class ZipTest extends TestCase
         $this->assertNull($result);
     }
 
-    public function testReturnsNullIfTheZipIsEmpty()
+    public function testReturnsNullIfTheZipIsEmpty(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -54,7 +54,7 @@ class ZipTest extends TestCase
         $this->assertNull($result);
     }
 
-    public function testThrowsExceptionIfTheZipHasNoComposerJson()
+    public function testThrowsExceptionIfTheZipHasNoComposerJson(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -66,7 +66,7 @@ class ZipTest extends TestCase
         Zip::getComposerJson(__DIR__.'/Fixtures/Zip/nojson.zip');
     }
 
-    public function testThrowsExceptionIfTheComposerJsonIsInASubSubfolder()
+    public function testThrowsExceptionIfTheComposerJsonIsInASubSubfolder(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -78,7 +78,7 @@ class ZipTest extends TestCase
         Zip::getComposerJson(__DIR__.'/Fixtures/Zip/subfolders.zip');
     }
 
-    public function testReturnsComposerJsonInZipRoot()
+    public function testReturnsComposerJsonInZipRoot(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -89,7 +89,7 @@ class ZipTest extends TestCase
         $this->assertEquals("{\n    \"name\": \"foo/bar\"\n}\n", $result);
     }
 
-    public function testReturnsComposerJsonInFirstFolder()
+    public function testReturnsComposerJsonInFirstFolder(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -99,7 +99,7 @@ class ZipTest extends TestCase
         $this->assertEquals("{\n    \"name\": \"foo/bar\"\n}\n", $result);
     }
 
-    public function testMultipleTopLevelDirsIsInvalid()
+    public function testMultipleTopLevelDirsIsInvalid(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');
@@ -111,7 +111,7 @@ class ZipTest extends TestCase
         Zip::getComposerJson(__DIR__.'/Fixtures/Zip/multiple.zip');
     }
 
-    public function testReturnsComposerJsonFromFirstSubfolder()
+    public function testReturnsComposerJsonFromFirstSubfolder(): void
     {
         if (!extension_loaded('zip')) {
             $this->markTestSkipped('The PHP zip extension is not loaded.');