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/src/Composer/Autoload/ClassMapGenerator.php b/src/Composer/Autoload/ClassMapGenerator.php
index cb948945f..9d0c73f1f 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();
 
diff --git a/src/Composer/Autoload/PhpFileCleaner.php b/src/Composer/Autoload/PhpFileCleaner.php
index 7941f463c..17341a935 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[$type[0]] = array(
@@ -152,7 +152,7 @@ class PhpFileCleaner
     /**
      * @return void
      */
-    private function skipToPhp()
+    private function skipToPhp(): void
     {
         while ($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);
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..314c06554 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')
diff --git a/src/Composer/Command/BaseDependencyCommand.php b/src/Composer/Command/BaseDependencyCommand.php
index 4b24202ed..3cd693370 100644
--- a/src/Composer/Command/BaseDependencyCommand.php
+++ b/src/Composer/Command/BaseDependencyCommand.php
@@ -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..9071f3b92 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);
 
@@ -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..5d6ce71bd 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
     {
         $this
             ->setName('create-project')
@@ -443,7 +443,7 @@ EOT
             @mkdir($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/FundCommand.php b/src/Composer/Command/FundCommand.php
index 6e551a66a..5d397f058 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.')
diff --git a/src/Composer/Command/GlobalCommand.php b/src/Composer/Command/GlobalCommand.php
index 91aae03f0..074bc283c 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')
diff --git a/src/Composer/Command/HomeCommand.php b/src/Composer/Command/HomeCommand.php
index a44546c55..eaab9fdab 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')
@@ -128,7 +128,7 @@ EOT
      * @param string $url
      * @return void
      */
-    private function openBrowser($url)
+    private function openBrowser($url): void
     {
         $url = ProcessExecutor::escape($url);
 
diff --git a/src/Composer/Command/LicensesCommand.php b/src/Composer/Command/LicensesCommand.php
index 5a9bfa16b..de51bba87 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')
diff --git a/src/Composer/Command/OutdatedCommand.php b/src/Composer/Command/OutdatedCommand.php
index bdf60f664..af62009bd 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')
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..18c8e4560 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')
diff --git a/src/Composer/Command/RunScriptCommand.php b/src/Composer/Command/RunScriptCommand.php
index f35688c15..64a00bc64 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')
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..b473658b1 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');
@@ -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();
diff --git a/src/Composer/Command/StatusCommand.php b/src/Composer/Command/StatusCommand.php
index eed32b623..99cd983d1 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')
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/ValidateCommand.php b/src/Composer/Command/ValidateCommand.php
index 700461bcf..ee9ffa811 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;
 
diff --git a/src/Composer/Compiler.php b/src/Composer/Compiler.php
index 362deeb34..619ac4ea8 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);
@@ -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);
diff --git a/src/Composer/Composer.php b/src/Composer/Composer.php
index 749c9e35d..8f8e4e74f 100644
--- a/src/Composer/Composer.php
+++ b/src/Composer/Composer.php
@@ -144,7 +144,7 @@ class Composer
     /**
      * @return void
      */
-    public function setPackage(RootPackageInterface $package)
+    public function setPackage(RootPackageInterface $package): void
     {
         $this->package = $package;
     }
@@ -160,7 +160,7 @@ class Composer
     /**
      * @return void
      */
-    public function setConfig(Config $config)
+    public function setConfig(Config $config): void
     {
         $this->config = $config;
     }
@@ -176,7 +176,7 @@ class Composer
     /**
      * @return void
      */
-    public function setLocker(Locker $locker)
+    public function setLocker(Locker $locker): void
     {
         $this->locker = $locker;
     }
@@ -192,7 +192,7 @@ class Composer
     /**
      * @return void
      */
-    public function setLoop(Loop $loop)
+    public function setLoop(Loop $loop): void
     {
         $this->loop = $loop;
     }
@@ -208,7 +208,7 @@ class Composer
     /**
      * @return void
      */
-    public function setRepositoryManager(RepositoryManager $manager)
+    public function setRepositoryManager(RepositoryManager $manager): void
     {
         $this->repositoryManager = $manager;
     }
@@ -224,7 +224,7 @@ class Composer
     /**
      * @return void
      */
-    public function setDownloadManager(DownloadManager $manager)
+    public function setDownloadManager(DownloadManager $manager): void
     {
         $this->downloadManager = $manager;
     }
@@ -240,7 +240,7 @@ class Composer
     /**
      * @return void
      */
-    public function setArchiveManager(ArchiveManager $manager)
+    public function setArchiveManager(ArchiveManager $manager): void
     {
         $this->archiveManager = $manager;
     }
@@ -256,7 +256,7 @@ class Composer
     /**
      * @return void
      */
-    public function setInstallationManager(InstallationManager $manager)
+    public function setInstallationManager(InstallationManager $manager): void
     {
         $this->installationManager = $manager;
     }
@@ -272,7 +272,7 @@ class Composer
     /**
      * @return void
      */
-    public function setPluginManager(PluginManager $manager)
+    public function setPluginManager(PluginManager $manager): void
     {
         $this->pluginManager = $manager;
     }
@@ -288,7 +288,7 @@ class Composer
     /**
      * @return void
      */
-    public function setEventDispatcher(EventDispatcher $eventDispatcher)
+    public function setEventDispatcher(EventDispatcher $eventDispatcher): void
     {
         $this->eventDispatcher = $eventDispatcher;
     }
@@ -304,7 +304,7 @@ class Composer
     /**
      * @return void
      */
-    public function setAutoloadGenerator(AutoloadGenerator $autoloadGenerator)
+    public function setAutoloadGenerator(AutoloadGenerator $autoloadGenerator): void
     {
         $this->autoloadGenerator = $autoloadGenerator;
     }
diff --git a/src/Composer/Config.php b/src/Composer/Config.php
index a83f70175..1dd2f0773 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;
     }
@@ -156,7 +156,7 @@ class Config
     /**
      * @return void
      */
-    public function setAuthConfigSource(ConfigSourceInterface $source)
+    public function setAuthConfigSource(ConfigSourceInterface $source): void
     {
         $this->authConfigSource = $source;
     }
@@ -177,7 +177,7 @@ class Config
      *
      * @return void
      */
-    public function merge($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($config['config'])) {
@@ -471,7 +471,7 @@ class Config
      *
      * @return void
      */
-    private function setSourceOfConfigValue($configValue, $path, $source)
+    private function setSourceOfConfigValue($configValue, $path, $source): void
     {
         $this->sourceOfConfigValue[$path] = $source;
 
@@ -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/Console/GithubActionError.php b/src/Composer/Console/GithubActionError.php
index 6cd49c20d..6f8710261 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);
diff --git a/src/Composer/DependencyResolver/Decisions.php b/src/Composer/DependencyResolver/Decisions.php
index 2df464bc1..686ce8fcb 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);
         $this->decisionQueue[] = array(
@@ -177,7 +177,7 @@ class Decisions implements \Iterator, \Countable
     /**
      * @return void
      */
-    public function reset()
+    public function reset(): void
     {
         while ($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);
@@ -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);
 
diff --git a/src/Composer/DependencyResolver/LockTransaction.php b/src/Composer/DependencyResolver/LockTransaction.php
index a3abe1e2a..e46b66c1a 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);
 
diff --git a/src/Composer/DependencyResolver/MultiConflictRule.php b/src/Composer/DependencyResolver/MultiConflictRule.php
index deda73010..35f92be9b 100644
--- a/src/Composer/DependencyResolver/MultiConflictRule.php
+++ b/src/Composer/DependencyResolver/MultiConflictRule.php
@@ -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/Pool.php b/src/Composer/DependencyResolver/Pool.php
index 0fa5783e4..3888ef8a5 100644
--- a/src/Composer/DependencyResolver/Pool.php
+++ b/src/Composer/DependencyResolver/Pool.php
@@ -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;
 
diff --git a/src/Composer/DependencyResolver/PoolBuilder.php b/src/Composer/DependencyResolver/PoolBuilder.php
index 8ca7050fd..c0b2ed4d2 100644
--- a/src/Composer/DependencyResolver/PoolBuilder.php
+++ b/src/Composer/DependencyResolver/PoolBuilder.php
@@ -298,7 +298,7 @@ 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)) {
@@ -357,7 +357,7 @@ 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;
@@ -394,7 +394,7 @@ 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;
@@ -568,7 +568,7 @@ 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);
@@ -600,7 +600,7 @@ 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
@@ -666,7 +666,7 @@ 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);
 
diff --git a/src/Composer/DependencyResolver/PoolOptimizer.php b/src/Composer/DependencyResolver/PoolOptimizer.php
index 7e1e32fdc..420c1ec66 100644
--- a/src/Composer/DependencyResolver/PoolOptimizer.php
+++ b/src/Composer/DependencyResolver/PoolOptimizer.php
@@ -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) {
@@ -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();
@@ -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/Request.php b/src/Composer/DependencyResolver/Request.php
index 3b680ad0b..7499b1f99 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;
diff --git a/src/Composer/DependencyResolver/Rule.php b/src/Composer/DependencyResolver/Rule.php
index 133e7f7a0..c5f324038 100644
--- a/src/Composer/DependencyResolver/Rule.php
+++ b/src/Composer/DependencyResolver/Rule.php
@@ -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);
     }
@@ -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);
     }
diff --git a/src/Composer/DependencyResolver/RuleSet.php b/src/Composer/DependencyResolver/RuleSet.php
index d83320c52..65369c1b1 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: ' . $type);
diff --git a/src/Composer/DependencyResolver/RuleSetGenerator.php b/src/Composer/DependencyResolver/RuleSetGenerator.php
index fef50b0eb..3dee4d5dc 100644
--- a/src/Composer/DependencyResolver/RuleSetGenerator.php
+++ b/src/Composer/DependencyResolver/RuleSetGenerator.php
@@ -152,7 +152,7 @@ class RuleSetGenerator
      *
      * @return void
      */
-    private function addRule($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
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..16b8c5235 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;
@@ -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..aef4dd135 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();
 
@@ -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..45814c36e 100644
--- a/src/Composer/DependencyResolver/Solver.php
+++ b/src/Composer/DependencyResolver/Solver.php
@@ -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($packageName)) {
@@ -261,7 +261,7 @@ class Solver
      *
      * @return void
      */
-    private function revert($level)
+    private function revert($level): void
     {
         while (!$this->decisions->isEmpty()) {
             $literal = $this->decisions->lastLiteral();
@@ -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;
@@ -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/Transaction.php b/src/Composer/DependencyResolver/Transaction.php
index ef9c74174..265474123 100644
--- a/src/Composer/DependencyResolver/Transaction.php
+++ b/src/Composer/DependencyResolver/Transaction.php
@@ -69,7 +69,7 @@ class Transaction
      * @param PackageInterface[] $resultPackages
      * @return void
      */
-    private function setResultPackageMaps($resultPackages)
+    private function setResultPackageMaps($resultPackages): void
     {
         $packageSort = function (PackageInterface $a, PackageInterface $b) {
             // sort alias packages by the same name behind their non alias version
diff --git a/src/Composer/Downloader/GzipDownloader.php b/src/Composer/Downloader/GzipDownloader.php
index 8b5caf474..11b607e79 100644
--- a/src/Composer/Downloader/GzipDownloader.php
+++ b/src/Composer/Downloader/GzipDownloader.php
@@ -59,7 +59,7 @@ class GzipDownloader extends ArchiveDownloader
      *
      * @return void
      */
-    private function extractUsingExt($file, $targetFilepath)
+    private function extractUsingExt($file, $targetFilepath): void
     {
         $archiveFile = gzopen($file, 'rb');
         $targetFile = fopen($targetFilepath, 'wb');
diff --git a/src/Composer/Downloader/PerforceDownloader.php b/src/Composer/Downloader/PerforceDownloader.php
index c7d0e7985..8f2427727 100644
--- a/src/Composer/Downloader/PerforceDownloader.php
+++ b/src/Composer/Downloader/PerforceDownloader.php
@@ -73,7 +73,7 @@ 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);
@@ -126,7 +126,7 @@ class PerforceDownloader extends VcsDownloader
     /**
      * @return void
      */
-    public function setPerforce(Perforce $perforce)
+    public function setPerforce(Perforce $perforce): void
     {
         $this->perforce = $perforce;
     }
diff --git a/src/Composer/Downloader/TransportException.php b/src/Composer/Downloader/TransportException.php
index 92be261a6..2f5a28361 100644
--- a/src/Composer/Downloader/TransportException.php
+++ b/src/Composer/Downloader/TransportException.php
@@ -31,7 +31,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setHeaders($headers)
+    public function setHeaders($headers): void
     {
         $this->headers = $headers;
     }
@@ -49,7 +49,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setResponse($response)
+    public function setResponse($response): void
     {
         $this->response = $response;
     }
@@ -67,7 +67,7 @@ class TransportException extends \RuntimeException
      *
      * @return void
      */
-    public function setStatusCode($statusCode)
+    public function setStatusCode($statusCode): void
     {
         $this->statusCode = $statusCode;
     }
@@ -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/IO/BufferIO.php b/src/Composer/IO/BufferIO.php
index 627abec9d..bc01787b1 100644
--- a/src/Composer/IO/BufferIO.php
+++ b/src/Composer/IO/BufferIO.php
@@ -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/NullIO.php b/src/Composer/IO/NullIO.php
index 89c574e43..cd8e5fc12 100644
--- a/src/Composer/IO/NullIO.php
+++ b/src/Composer/IO/NullIO.php
@@ -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
     {
     }
 
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index 8bd88e5b9..43af3fde5 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -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) {
@@ -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) {
diff --git a/src/Composer/Installer/BinaryInstaller.php b/src/Composer/Installer/BinaryInstaller.php
index 417e9be2b..d1902d8a2 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();
 
@@ -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);
diff --git a/src/Composer/Installer/SuggestedPackagesReporter.php b/src/Composer/Installer/SuggestedPackagesReporter.php
index f8fb9d887..840e63f51 100644
--- a/src/Composer/Installer/SuggestedPackagesReporter.php
+++ b/src/Composer/Installer/SuggestedPackagesReporter.php
@@ -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) {
diff --git a/src/Composer/Package/AliasPackage.php b/src/Composer/Package/AliasPackage.php
index 4dd2bd9aa..8b47879c4 100644
--- a/src/Composer/Package/AliasPackage.php
+++ b/src/Composer/Package/AliasPackage.php
@@ -249,7 +249,7 @@ class AliasPackage extends BasePackage
         return $this->aliasOf->getExtra();
     }
 
-    public function setInstallationSource($type)
+    public function setInstallationSource($type): void
     {
         $this->aliasOf->setInstallationSource($type);
     }
@@ -279,12 +279,12 @@ class AliasPackage extends BasePackage
         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);
     }
@@ -314,7 +314,7 @@ class AliasPackage extends BasePackage
         return $this->aliasOf->getDistReference();
     }
 
-    public function setDistReference($reference)
+    public function setDistReference($reference): void
     {
         $this->aliasOf->setDistReference($reference);
     }
@@ -324,7 +324,7 @@ class AliasPackage extends BasePackage
         return $this->aliasOf->getDistSha1Checksum();
     }
 
-    public function setTransportOptions(array $options)
+    public function setTransportOptions(array $options): void
     {
         $this->aliasOf->setTransportOptions($options);
     }
@@ -334,7 +334,7 @@ class AliasPackage extends BasePackage
         return $this->aliasOf->getTransportOptions();
     }
 
-    public function setDistMirrors($mirrors)
+    public function setDistMirrors($mirrors): void
     {
         $this->aliasOf->setDistMirrors($mirrors);
     }
@@ -384,17 +384,17 @@ class AliasPackage extends BasePackage
         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/ArchiveManager.php b/src/Composer/Package/Archiver/ArchiveManager.php
index a6682fb3b..197c7fe8e 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[] = $archiver;
     }
diff --git a/src/Composer/Package/Comparer/Comparer.php b/src/Composer/Package/Comparer/Comparer.php
index b24661ad3..d7c2a5fc7 100644
--- a/src/Composer/Package/Comparer/Comparer.php
+++ b/src/Composer/Package/Comparer/Comparer.php
@@ -31,7 +31,7 @@ class Comparer
      *
      * @return void
      */
-    public function setSource($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..ee8f92d91 100644
--- a/src/Composer/Package/CompleteAliasPackage.php
+++ b/src/Composer/Package/CompleteAliasPackage.php
@@ -45,7 +45,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getScripts();
     }
 
-    public function setScripts(array $scripts)
+    public function setScripts(array $scripts): void
     {
         $this->aliasOf->setScripts($scripts);
     }
@@ -55,7 +55,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getRepositories();
     }
 
-    public function setRepositories(array $repositories)
+    public function setRepositories(array $repositories): void
     {
         $this->aliasOf->setRepositories($repositories);
     }
@@ -65,7 +65,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getLicense();
     }
 
-    public function setLicense(array $license)
+    public function setLicense(array $license): void
     {
         $this->aliasOf->setLicense($license);
     }
@@ -75,7 +75,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getKeywords();
     }
 
-    public function setKeywords(array $keywords)
+    public function setKeywords(array $keywords): void
     {
         $this->aliasOf->setKeywords($keywords);
     }
@@ -85,7 +85,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getDescription();
     }
 
-    public function setDescription($description)
+    public function setDescription($description): void
     {
         $this->aliasOf->setDescription($description);
     }
@@ -95,7 +95,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getHomepage();
     }
 
-    public function setHomepage($homepage)
+    public function setHomepage($homepage): void
     {
         $this->aliasOf->setHomepage($homepage);
     }
@@ -105,7 +105,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getAuthors();
     }
 
-    public function setAuthors(array $authors)
+    public function setAuthors(array $authors): void
     {
         $this->aliasOf->setAuthors($authors);
     }
@@ -115,7 +115,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getSupport();
     }
 
-    public function setSupport(array $support)
+    public function setSupport(array $support): void
     {
         $this->aliasOf->setSupport($support);
     }
@@ -125,7 +125,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getFunding();
     }
 
-    public function setFunding(array $funding)
+    public function setFunding(array $funding): void
     {
         $this->aliasOf->setFunding($funding);
     }
@@ -140,7 +140,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getReplacementPackage();
     }
 
-    public function setAbandoned($abandoned)
+    public function setAbandoned($abandoned): void
     {
         $this->aliasOf->setAbandoned($abandoned);
     }
@@ -150,7 +150,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         return $this->aliasOf->getArchiveName();
     }
 
-    public function setArchiveName($name)
+    public function setArchiveName($name): void
     {
         $this->aliasOf->setArchiveName($name);
     }
@@ -160,7 +160,7 @@ class CompleteAliasPackage extends AliasPackage implements CompletePackageInterf
         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/RootAliasPackage.php b/src/Composer/Package/RootAliasPackage.php
index 4817653dc..2ae19890d 100644
--- a/src/Composer/Package/RootAliasPackage.php
+++ b/src/Composer/Package/RootAliasPackage.php
@@ -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..3f23bb8ca 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;
     }
@@ -53,7 +53,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * @inheritDoc
      */
-    public function setStabilityFlags(array $stabilityFlags)
+    public function setStabilityFlags(array $stabilityFlags): void
     {
         $this->stabilityFlags = $stabilityFlags;
     }
@@ -69,7 +69,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setPreferStable($preferStable)
+    public function setPreferStable($preferStable): void
     {
         $this->preferStable = $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;
     }
@@ -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;
     }
@@ -117,7 +117,7 @@ class RootPackage extends CompletePackage implements RootPackageInterface
     /**
      * {@inerhitDoc}
      */
-    public function setAliases(array $aliases)
+    public function setAliases(array $aliases): void
     {
         $this->aliases = $aliases;
     }
diff --git a/src/Composer/Platform/HhvmDetector.php b/src/Composer/Platform/HhvmDetector.php
index 7fee999a6..181f15de2 100644
--- a/src/Composer/Platform/HhvmDetector.php
+++ b/src/Composer/Platform/HhvmDetector.php
@@ -34,7 +34,7 @@ class HhvmDetector
     /**
      * @return void
      */
-    public function reset()
+    public function reset(): void
     {
         self::$hhvmVersion = null;
     }
diff --git a/src/Composer/Plugin/PluginManager.php b/src/Composer/Plugin/PluginManager.php
index 4f974d9f8..5f3390ef7 100644
--- a/src/Composer/Plugin/PluginManager.php
+++ b/src/Composer/Plugin/PluginManager.php
@@ -92,7 +92,7 @@ class PluginManager
      *
      * @return void
      */
-    public function loadInstalledPlugins()
+    public function loadInstalledPlugins(): void
     {
         if ($this->disablePlugins) {
             return;
@@ -111,7 +111,7 @@ class PluginManager
      *
      * @return void
      */
-    public function deactivateInstalledPlugins()
+    public function deactivateInstalledPlugins(): void
     {
         if ($this->disablePlugins) {
             return;
@@ -159,7 +159,7 @@ 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 +318,7 @@ class PluginManager
      *
      * @throws \UnexpectedValueException
      */
-    public function deactivatePackage(PackageInterface $package)
+    public function deactivatePackage(PackageInterface $package): void
     {
         if ($this->disablePlugins) {
             return;
@@ -349,7 +349,7 @@ class PluginManager
      *
      * @throws \UnexpectedValueException
      */
-    public function uninstallPackage(PackageInterface $package)
+    public function uninstallPackage(PackageInterface $package): void
     {
         if ($this->disablePlugins) {
             return;
@@ -392,7 +392,7 @@ 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 +429,7 @@ 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 +454,7 @@ 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 +476,7 @@ 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 +503,7 @@ 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));
diff --git a/src/Composer/Plugin/PreFileDownloadEvent.php b/src/Composer/Plugin/PreFileDownloadEvent.php
index 9e675d438..7dd56d416 100644
--- a/src/Composer/Plugin/PreFileDownloadEvent.php
+++ b/src/Composer/Plugin/PreFileDownloadEvent.php
@@ -95,7 +95,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return void
      */
-    public function setProcessedUrl($processedUrl)
+    public function setProcessedUrl($processedUrl): void
     {
         $this->processedUrl = $processedUrl;
     }
@@ -117,7 +117,7 @@ class PreFileDownloadEvent extends Event
      *
      * @return void
      */
-    public function setCustomCacheKey($customCacheKey)
+    public function setCustomCacheKey($customCacheKey): void
     {
         $this->customCacheKey = $customCacheKey;
     }
@@ -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..497e1d462 100644
--- a/src/Composer/Plugin/PrePoolCreateEvent.php
+++ b/src/Composer/Plugin/PrePoolCreateEvent.php
@@ -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/Repository/CompositeRepository.php b/src/Composer/Repository/CompositeRepository.php
index 83603081c..b25d0a0e1 100644
--- a/src/Composer/Repository/CompositeRepository.php
+++ b/src/Composer/Repository/CompositeRepository.php
@@ -166,7 +166,7 @@ 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 +195,7 @@ 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/InstalledRepository.php b/src/Composer/Repository/InstalledRepository.php
index db0967ba7..b5ceb983a 100644
--- a/src/Composer/Repository/InstalledRepository.php
+++ b/src/Composer/Repository/InstalledRepository.php
@@ -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/PackageRepository.php b/src/Composer/Repository/PackageRepository.php
index b787931ca..da3a01e2c 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();
 
diff --git a/src/Composer/Repository/PathRepository.php b/src/Composer/Repository/PathRepository.php
index 725a50360..bc86a9201 100644
--- a/src/Composer/Repository/PathRepository.php
+++ b/src/Composer/Repository/PathRepository.php
@@ -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();
 
diff --git a/src/Composer/Repository/PlatformRepository.php b/src/Composer/Repository/PlatformRepository.php
index b1c98b61c..c1495f64d 100644
--- a/src/Composer/Repository/PlatformRepository.php
+++ b/src/Composer/Repository/PlatformRepository.php
@@ -106,7 +106,7 @@ class PlatformRepository extends ArrayRepository
         return $this->disabledPackages;
     }
 
-    protected function initialize()
+    protected function initialize(): void
     {
         parent::initialize();
 
@@ -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));
@@ -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;
 
@@ -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);
diff --git a/src/Composer/Repository/RepositorySet.php b/src/Composer/Repository/RepositorySet.php
index eb640df2b..18b988b46 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;
     }
@@ -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.");
diff --git a/src/Composer/Repository/Vcs/FossilDriver.php b/src/Composer/Repository/Vcs/FossilDriver.php
index 66a50bdff..d15dd1e99 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);
diff --git a/src/Composer/Repository/Vcs/GitLabDriver.php b/src/Composer/Repository/Vcs/GitLabDriver.php
index 2e7d36cc2..795f29e14 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;
     }
@@ -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 = $this->getApiUrl();
@@ -452,7 +452,7 @@ class GitLabDriver extends VcsDriver
      *
      * @return void
      */
-    protected function setupGitDriver($url)
+    protected function setupGitDriver($url): void
     {
         $this->gitDriver = new GitDriver(
             array('url' => $url),
diff --git a/src/Composer/SelfUpdate/Versions.php b/src/Composer/SelfUpdate/Versions.php
index 9e617fb13..b6ba9c48f 100644
--- a/src/Composer/SelfUpdate/Versions.php
+++ b/src/Composer/SelfUpdate/Versions.php
@@ -63,7 +63,7 @@ class Versions
      *
      * @return void
      */
-    public function setChannel($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::$channels));
diff --git a/src/Composer/Util/AuthHelper.php b/src/Composer/Util/AuthHelper.php
index 28c4aba68..b9e178875 100644
--- a/src/Composer/Util/AuthHelper.php
+++ b/src/Composer/Util/AuthHelper.php
@@ -41,7 +41,7 @@ class AuthHelper
      *
      * @return void
      */
-    public function storeAuth($origin, $storeAuth)
+    public function storeAuth($origin, $storeAuth): void
     {
         $store = false;
         $configSource = $this->config->getAuthConfigSource();
diff --git a/src/Composer/Util/Bitbucket.php b/src/Composer/Util/Bitbucket.php
index 62a46c1c5..e0104a64b 100644
--- a/src/Composer/Util/Bitbucket.php
+++ b/src/Composer/Util/Bitbucket.php
@@ -223,7 +223,7 @@ class Bitbucket
      *
      * @return void
      */
-    private function storeInAuthConfig($originUrl, $consumerKey, $consumerSecret)
+    private function storeInAuthConfig($originUrl, $consumerKey, $consumerSecret): void
     {
         $this->config->getConfigSource()->removeConfigSetting('bitbucket-oauth.'.$originUrl);
 
diff --git a/src/Composer/Util/ErrorHandler.php b/src/Composer/Util/ErrorHandler.php
index 5e2bc1790..fc60bf3f1 100644
--- a/src/Composer/Util/ErrorHandler.php
+++ b/src/Composer/Util/ErrorHandler.php
@@ -76,7 +76,7 @@ class ErrorHandler
      *
      * @return void
      */
-    public static function register(IOInterface $io = null)
+    public static function register(IOInterface $io = null): void
     {
         set_error_handler(array(__CLASS__, 'handle'));
         error_reporting(E_ALL | E_STRICT);
diff --git a/src/Composer/Util/Hg.php b/src/Composer/Util/Hg.php
index 73e3a8e52..34dc00623 100644
--- a/src/Composer/Util/Hg.php
+++ b/src/Composer/Util/Hg.php
@@ -53,7 +53,7 @@ class Hg
      *
      * @return void
      */
-    public function runCommand($commandCallable, $url, $cwd)
+    public function runCommand($commandCallable, $url, $cwd): void
     {
         $this->config->prohibitUrlByConfig($url, $this->io);
 
diff --git a/src/Composer/Util/Http/CurlDownloader.php b/src/Composer/Util/Http/CurlDownloader.php
index a53fd7fb7..8ae18ee4c 100644
--- a/src/Composer/Util/Http/CurlDownloader.php
+++ b/src/Composer/Util/Http/CurlDownloader.php
@@ -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;
 
@@ -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'].'~');
@@ -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 && $code != CURLM_CALL_MULTI_PERFORM) {
             throw new \RuntimeException(
diff --git a/src/Composer/Util/Http/ProxyHelper.php b/src/Composer/Util/Http/ProxyHelper.php
index febcfc73d..bfdfe2e4a 100644
--- a/src/Composer/Util/Http/ProxyHelper.php
+++ b/src/Composer/Util/Http/ProxyHelper.php
@@ -97,7 +97,7 @@ class ProxyHelper
      *
      * @return void
      */
-    public static function setRequestFullUri($requestUrl, array &$options)
+    public static function setRequestFullUri($requestUrl, array &$options): void
     {
         if ('http' === parse_url($requestUrl, PHP_URL_SCHEME)) {
             $options['http']['request_fulluri'] = true;
diff --git a/src/Composer/Util/Http/ProxyManager.php b/src/Composer/Util/Http/ProxyManager.php
index 352ab8d35..43bcea749 100644
--- a/src/Composer/Util/Http/ProxyManager.php
+++ b/src/Composer/Util/Http/ProxyManager.php
@@ -72,7 +72,7 @@ class ProxyManager
      *
      * @return void
      */
-    public static function reset()
+    public static function reset(): void
     {
         self::$instance = null;
     }
@@ -133,7 +133,7 @@ class ProxyManager
      *
      * @return void
      */
-    private function initProxyData()
+    private function initProxyData(): void
     {
         try {
             list($httpProxy, $httpsProxy, $noProxy) = ProxyHelper::getProxyData();
diff --git a/src/Composer/Util/Http/Response.php b/src/Composer/Util/Http/Response.php
index 39c68d6f2..1ce384473 100644
--- a/src/Composer/Util/Http/Response.php
+++ b/src/Composer/Util/Http/Response.php
@@ -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;
diff --git a/src/Composer/Util/Loop.php b/src/Composer/Util/Loop.php
index b103011a4..2be438a50 100644
--- a/src/Composer/Util/Loop.php
+++ b/src/Composer/Util/Loop.php
@@ -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/PackageSorter.php b/src/Composer/Util/PackageSorter.php
index df2d89a4b..a28e95b11 100644
--- a/src/Composer/Util/PackageSorter.php
+++ b/src/Composer/Util/PackageSorter.php
@@ -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..b57b7dda8 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]);
@@ -202,7 +202,7 @@ class Platform
     /**
      * @return void
      */
-    public static function workaroundFilesystemIssues()
+    public static function workaroundFilesystemIssues(): void
     {
         if (self::isVirtualBoxGuest()) {
             usleep(200000);
diff --git a/src/Composer/Util/Silencer.php b/src/Composer/Util/Silencer.php
index 361ef41f6..b4164b1cc 100644
--- a/src/Composer/Util/Silencer.php
+++ b/src/Composer/Util/Silencer.php
@@ -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/Svn.php b/src/Composer/Util/Svn.php
index 865479e9a..c04f075df 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');
@@ -194,7 +194,7 @@ class Svn
      * @param  bool $cacheCredentials
      * @return void
      */
-    public function setCacheCredentials($cacheCredentials)
+    public function setCacheCredentials($cacheCredentials): void
     {
         $this->cacheCredentials = $cacheCredentials;
     }
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/tests/Composer/Test/AllFunctionalTest.php b/tests/Composer/Test/AllFunctionalTest.php
index d1f6394e4..896079f89 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;
         });
 
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..ca5af8273 100644
--- a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
+++ b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
@@ -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';
@@ -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);
@@ -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..7b1f7ab7b 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');
@@ -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..8f4fd1395 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));
     }
@@ -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..fcae19051 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';
 
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..bcc193662 100644
--- a/tests/Composer/Test/Config/JsonConfigSourceTest.php
+++ b/tests/Composer/Test/Config/JsonConfigSourceTest.php
@@ -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);
diff --git a/tests/Composer/Test/ConfigTest.php b/tests/Composer/Test/ConfigTest.php
index fc829c159..0aad6141c 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) {
@@ -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);
@@ -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..3939fcdd2 100644
--- a/tests/Composer/Test/DependencyResolver/PoolBuilderTest.php
+++ b/tests/Composer/Test/DependencyResolver/PoolBuilderTest.php
@@ -44,7 +44,7 @@ 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';
diff --git a/tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php b/tests/Composer/Test/DependencyResolver/PoolOptimizerTest.php
index 442b156ef..6b3e863bb 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();
 
diff --git a/tests/Composer/Test/DependencyResolver/PoolTest.php b/tests/Composer/Test/DependencyResolver/PoolTest.php
index 350a5cab8..1e7fd5724 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();
 
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..c12e1e6f4 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');
@@ -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');
diff --git a/tests/Composer/Test/Downloader/DownloadManagerTest.php b/tests/Composer/Test/Downloader/DownloadManagerTest.php
index 7c84a1c9b..dd7e38ba9 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) {
@@ -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();
         $package
diff --git a/tests/Composer/Test/Downloader/FileDownloaderTest.php b/tests/Composer/Test/Downloader/FileDownloaderTest.php
index 2ade50ce3..691a38a72 100644
--- a/tests/Composer/Test/Downloader/FileDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/FileDownloaderTest.php
@@ -57,7 +57,7 @@ class FileDownloaderTest extends TestCase
         return new FileDownloader($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);
         });
 
@@ -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);
         });
 
@@ -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..a59ee038c 100644
--- a/tests/Composer/Test/Downloader/FossilDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/FossilDownloaderTest.php
@@ -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..57d55da1b 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
         $refl = new \ReflectionProperty('Composer\Util\Git', 'version');
@@ -92,7 +92,7 @@ class GitDownloaderTest extends TestCase
         return new GitDownloader($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())
@@ -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();
 
diff --git a/tests/Composer/Test/Downloader/HgDownloaderTest.php b/tests/Composer/Test/Downloader/HgDownloaderTest.php
index b2e555bab..f3a839303 100644
--- a/tests/Composer/Test/Downloader/HgDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/HgDownloaderTest.php
@@ -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..c6b99b5a6 100644
--- a/tests/Composer/Test/Downloader/PerforceDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/PerforceDownloaderTest.php
@@ -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..92b9ab5e2 100644
--- a/tests/Composer/Test/Downloader/ZipDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/ZipDownloaderTest.php
@@ -56,7 +56,7 @@ 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 +68,7 @@ class ZipDownloaderTest extends TestCase
         }
     }
 
-    public function testErrorMessages()
+    public function testErrorMessages(): void
     {
         if (!class_exists('ZipArchive')) {
             $this->markTestSkipped('zip extension missing');
@@ -106,7 +106,7 @@ 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 +129,7 @@ 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 +152,7 @@ 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 +173,7 @@ 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 +202,7 @@ class ZipDownloaderTest extends TestCase
         $this->wait($promise);
     }
 
-    public function testSystemUnzipOnlyGood()
+    public function testSystemUnzipOnlyGood(): void
     {
         $this->setPrivateProperty('isWindows', false);
         $this->setPrivateProperty('hasZipArchive', false);
@@ -229,7 +229,7 @@ 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 +268,7 @@ 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 +313,16 @@ 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;
         });
 
diff --git a/tests/Composer/Test/EventDispatcher/EventDispatcherTest.php b/tests/Composer/Test/EventDispatcher/EventDispatcherTest.php
index 3217d8bf4..7a37a92c2 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();
 
@@ -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(
@@ -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(
@@ -446,7 +446,7 @@ class EventDispatcherTest extends TestCase
         $this->assertEquals($expected, $io->getOutput());
     }
 
-    public function testDispatcherDetectInfiniteRecursion()
+    public function testDispatcherDetectInfiniteRecursion(): void
     {
         self::expectException('RuntimeException');
 
@@ -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();
     }
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..767b50747 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();
 
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilterTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreListPlatformRequirementFilterTest.php
index 3cf7469a4..685e97810 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);
 
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilterTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/IgnoreNothingPlatformRequirementFilterTest.php
index 4698e94fa..02b7da85c 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();
 
diff --git a/tests/Composer/Test/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactoryTest.php b/tests/Composer/Test/Filter/PlatformRequirementFilter/PlatformRequirementFilterFactoryTest.php
index 44bcfce9e..c7e462b92 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));
     }
@@ -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..0a46ab026 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();
@@ -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();
@@ -216,7 +216,7 @@ class ConsoleIOTest extends TestCase
         $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..232e6c5d9 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();
 
@@ -47,21 +47,21 @@ 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();
 
@@ -70,7 +70,7 @@ class NullIOTest extends TestCase
         }, 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..d1aeaefcb 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__',
@@ -74,7 +74,7 @@ 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));
     }
@@ -99,7 +99,7 @@ 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));
     }
@@ -139,7 +139,7 @@ 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));
     }
@@ -163,7 +163,7 @@ 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));
     }
@@ -187,7 +187,7 @@ 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));
     }
@@ -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());
@@ -242,7 +242,7 @@ 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));
     }
@@ -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..097b72d56 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())
diff --git a/tests/Composer/Test/Installer/InstallationManagerTest.php b/tests/Composer/Test/Installer/InstallationManagerTest.php
index 4447919a3..e7acfd6c8 100644
--- a/tests/Composer/Test/Installer/InstallationManagerTest.php
+++ b/tests/Composer/Test/Installer/InstallationManagerTest.php
@@ -43,7 +43,7 @@ class InstallationManagerTest extends TestCase
         $this->io = $this->getMockBuilder('Composer\IO\IOInterface')->getMock();
     }
 
-    public function testAddGetInstaller()
+    public function testAddGetInstaller(): void
     {
         $installer = $this->createInstallerMock();
 
@@ -63,7 +63,7 @@ class InstallationManagerTest extends TestCase
         $manager->getInstaller('unregistered');
     }
 
-    public function testAddRemoveInstaller()
+    public function testAddRemoveInstaller(): void
     {
         $installer = $this->createInstallerMock();
 
@@ -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();
@@ -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..ad933dcaa 100644
--- a/tests/Composer/Test/Installer/LibraryInstallerTest.php
+++ b/tests/Composer/Test/Installer/LibraryInstallerTest.php
@@ -102,7 +102,7 @@ class LibraryInstallerTest extends TestCase
         $this->fs->removeDirectory($this->rootDir);
     }
 
-    public function testInstallerCreationShouldNotCreateVendorDirectory()
+    public function testInstallerCreationShouldNotCreateVendorDirectory(): void
     {
         $this->fs->removeDirectory($this->vendorDir);
 
@@ -110,7 +110,7 @@ class LibraryInstallerTest extends TestCase
         $this->assertFileDoesNotExist($this->vendorDir);
     }
 
-    public function testInstallerCreationShouldNotCreateBinDirectory()
+    public function testInstallerCreationShouldNotCreateBinDirectory(): void
     {
         $this->fs->removeDirectory($this->binDir);
 
@@ -118,7 +118,7 @@ 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();
@@ -137,7 +137,7 @@ class LibraryInstallerTest extends TestCase
      * @depends testInstallerCreationShouldNotCreateVendorDirectory
      * @depends testInstallerCreationShouldNotCreateBinDirectory
      */
-    public function testInstall()
+    public function testInstall(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
         $package = $this->createPackageMock();
@@ -166,7 +166,7 @@ class LibraryInstallerTest extends TestCase
      * @depends testInstallerCreationShouldNotCreateVendorDirectory
      * @depends testInstallerCreationShouldNotCreateBinDirectory
      */
-    public function testUpdate()
+    public function testUpdate(): void
     {
         $filesystem = $this->getMockBuilder('Composer\Util\Filesystem')
           ->getMock();
@@ -228,7 +228,7 @@ 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();
@@ -265,7 +265,7 @@ class LibraryInstallerTest extends TestCase
         $library->uninstall($this->repository, $package);
     }
 
-    public function testGetInstallPath()
+    public function testGetInstallPath(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
         $package = $this->createPackageMock();
@@ -278,7 +278,7 @@ class LibraryInstallerTest extends TestCase
         $this->assertEquals($this->vendorDir.'/'.$package->getName(), $library->getInstallPath($package));
     }
 
-    public function testGetInstallPathWithTargetDir()
+    public function testGetInstallPathWithTargetDir(): void
     {
         $library = new LibraryInstaller($this->io, $this->composer);
         $package = $this->createPackageMock();
@@ -299,7 +299,7 @@ class LibraryInstallerTest extends TestCase
      * @depends testInstallerCreationShouldNotCreateVendorDirectory
      * @depends testInstallerCreationShouldNotCreateBinDirectory
      */
-    public function testEnsureBinariesInstalled()
+    public function testEnsureBinariesInstalled(): void
     {
         $binaryInstallerMock = $this->getMockBuilder('Composer\Installer\BinaryInstaller')
             ->disableOriginalConstructor()
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..fc83c7261 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))
diff --git a/tests/Composer/Test/InstallerTest.php b/tests/Composer/Test/InstallerTest.php
index c0ea9b1af..959ec7a33 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));
 
@@ -127,7 +127,7 @@ class InstallerTest extends TestCase
             }));
         $lockJsonMock->expects($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);
             }));
 
@@ -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');
 
@@ -305,7 +305,7 @@ 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.';');
@@ -353,7 +353,7 @@ class InstallerTest extends TestCase
             }));
         $lockJsonMock->expects($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;
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..883eda862 100644
--- a/tests/Composer/Test/Json/JsonManipulatorTest.php
+++ b/tests/Composer/Test/Json/JsonManipulatorTest.php
@@ -25,7 +25,7 @@ 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));
@@ -1301,7 +1301,7 @@ 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));
@@ -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);
 
@@ -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..f10109aeb 100644
--- a/tests/Composer/Test/Json/JsonValidationExceptionTest.php
+++ b/tests/Composer/Test/Json/JsonValidationExceptionTest.php
@@ -22,14 +22,14 @@ class JsonValidationExceptionTest extends TestCase
      * @param string|null $message
      * @param string[]|null $errors
      */
-    public function testGetErrors($message, $errors)
+    public function testGetErrors($message, $errors): void
     {
         $object = new JsonValidationException($message, $errors);
         $this->assertEquals($message, $object->getMessage());
         $this->assertEquals($errors, $object->getErrors());
     }
 
-    public function testGetErrorsWhenNoErrorsProvided()
+    public function testGetErrorsWhenNoErrorsProvided(): void
     {
         $object = new JsonValidationException('test message');
         $this->assertEquals(array(), $object->getErrors());
diff --git a/tests/Composer/Test/Mock/FactoryMock.php b/tests/Composer/Test/Mock/FactoryMock.php
index 79b50e19d..ad2a14b9a 100644
--- a/tests/Composer/Test/Mock/FactoryMock.php
+++ b/tests/Composer/Test/Mock/FactoryMock.php
@@ -47,7 +47,7 @@ class FactoryMock extends Factory
         return new \Composer\Package\Loader\RootPackageLoader($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);
     }
@@ -57,11 +57,11 @@ class FactoryMock extends Factory
         return new InstallationManagerMock();
     }
 
-    protected function createDefaultInstallers(Installer\InstallationManager $im, Composer $composer, IOInterface $io, ProcessExecutor $process = null)
+    protected function createDefaultInstallers(Installer\InstallationManager $im, Composer $composer, IOInterface $io, ProcessExecutor $process = null): void
     {
     }
 
-    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im)
+    protected function purgePackages(InstalledRepositoryInterface $repo, Installer\InstallationManager $im): void
     {
     }
 }
diff --git a/tests/Composer/Test/Mock/InstallationManagerMock.php b/tests/Composer/Test/Mock/InstallationManagerMock.php
index ed4b33996..95fe5ecd5 100644
--- a/tests/Composer/Test/Mock/InstallationManagerMock.php
+++ b/tests/Composer/Test/Mock/InstallationManagerMock.php
@@ -45,7 +45,7 @@ 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();
@@ -103,7 +103,7 @@ 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 +113,7 @@ 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);
@@ -145,7 +145,7 @@ class InstallationManagerMock extends InstallationManager
         return $this->uninstalled;
     }
 
-    public function notifyInstalls(IOInterface $io)
+    public function notifyInstalls(IOInterface $io): void
     {
         // noop
     }
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..cf9733ee1 100644
--- a/tests/Composer/Test/Mock/ProcessExecutorMock.php
+++ b/tests/Composer/Test/Mock/ProcessExecutorMock.php
@@ -48,7 +48,7 @@ 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);
@@ -175,12 +175,12 @@ class ProcessExecutorMock extends ProcessExecutor
 
     public function executeAsync($command, $cwd = null)
     {
-        $resolver = function ($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');
         };
 
diff --git a/tests/Composer/Test/Package/Archiver/ArchivableFilesFinderTest.php b/tests/Composer/Test/Package/Archiver/ArchivableFilesFinderTest.php
index 883dd4278..384623879 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
     {
         $excludes = array(
             'prefixB.foo',
@@ -303,7 +303,7 @@ class ArchivableFilesFinderTest extends TestCase
      *
      * @return void
      */
-    protected function assertArchivableFiles($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..af8166576 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');
 
@@ -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/GitExcludeFilterTest.php b/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php
index e8cae5853..3d058375b 100644
--- a/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php
+++ b/tests/Composer/Test/Package/Archiver/GitExcludeFilterTest.php
@@ -23,7 +23,7 @@ class GitExcludeFilterTest extends TestCase
      * @param string  $ignore
      * @param mixed[] $expected
      */
-    public function testPatternEscape($ignore, $expected)
+    public function testPatternEscape($ignore, $expected): void
     {
         $filter = new GitExcludeFilter('/');
 
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($path), 0777, true);
diff --git a/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php b/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php
index 616768c01..0f1b6e325 100644
--- a/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php
+++ b/tests/Composer/Test/Package/Archiver/ZipArchiverTest.php
@@ -23,7 +23,7 @@ class ZipArchiverTest extends ArchiverTest
      *
      * @dataProvider provideGitignoreExcludeNegationTestCases
      */
-    public function testGitignoreExcludeNegation($include)
+    public function testGitignoreExcludeNegation($include): void
     {
         $this->testZipArchive(array(
             'docs/README.md' => '# The doc',
@@ -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..0818ee358 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');
 
@@ -49,7 +49,7 @@ 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));
     }
@@ -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);
 
diff --git a/tests/Composer/Test/Package/CompletePackageTest.php b/tests/Composer/Test/Package/CompletePackageTest.php
index 848315a1d..a7c3a8806 100644
--- a/tests/Composer/Test/Package/CompletePackageTest.php
+++ b/tests/Composer/Test/Package/CompletePackageTest.php
@@ -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..624347bd6 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();
 
diff --git a/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php b/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
index 86a016e55..cfe12caa9 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',
@@ -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',
@@ -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..e963981a8 100644
--- a/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/RootPackageLoaderTest.php
@@ -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..509a7f2e2 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
@@ -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);
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..c8bdf21fb 100644
--- a/tests/Composer/Test/Package/Version/VersionParserTest.php
+++ b/tests/Composer/Test/Package/Version/VersionParserTest.php
@@ -23,7 +23,7 @@ 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();
 
@@ -48,7 +48,7 @@ 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));
     }
diff --git a/tests/Composer/Test/Package/Version/VersionSelectorTest.php b/tests/Composer/Test/Package/Version/VersionSelectorTest.php
index 027534c6e..05c2d6e0d 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);
diff --git a/tests/Composer/Test/Platform/HhvmDetectorTest.php b/tests/Composer/Test/Platform/HhvmDetectorTest.php
index 736a8ae43..d3881a74d 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');
diff --git a/tests/Composer/Test/Platform/VersionTest.php b/tests/Composer/Test/Platform/VersionTest.php
index 3ebb03724..447cbc991 100644
--- a/tests/Composer/Test/Platform/VersionTest.php
+++ b/tests/Composer/Test/Platform/VersionTest.php
@@ -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);
@@ -104,7 +104,7 @@ 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));
     }
@@ -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..0bba37da8 100644
--- a/tests/Composer/Test/Plugin/PluginInstallerTest.php
+++ b/tests/Composer/Test/Plugin/PluginInstallerTest.php
@@ -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')
@@ -310,7 +310,7 @@ class PluginInstallerTest extends TestCase
         $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';
         $capabilityImplementation = 'Composer\Test\Plugin\Mock\Capability';
@@ -437,7 +437,7 @@ class PluginInstallerTest extends TestCase
      *
      * @return void
      */
-    public function testQueryingWithInvalidCapabilityClassNameThrows($invalidImplementationClassNames, $expect = 'UnexpectedValueException')
+    public function testQueryingWithInvalidCapabilityClassNameThrows($invalidImplementationClassNames, $expect = 'UnexpectedValueException'): void
     {
         self::expectException($expect);
 
@@ -456,7 +456,7 @@ class PluginInstallerTest extends TestCase
     }
 
     /** @return void */
-    public function testQueryingNonProvidedCapabilityReturnsNullSafely()
+    public function testQueryingNonProvidedCapabilityReturnsNullSafely(): void
     {
         $capabilityApi = 'Composer\Plugin\Capability\MadeUpCapability';
 
@@ -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..ed996717d 100644
--- a/tests/Composer/Test/Question/StrictConfirmationQuestionTest.php
+++ b/tests/Composer/Test/Question/StrictConfirmationQuestionTest.php
@@ -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");
 
@@ -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');
 
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..d3075278c 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',
@@ -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..8c941cc1b 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',
@@ -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),
@@ -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..daec344d2 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'));
@@ -132,7 +132,7 @@ 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));
diff --git a/tests/Composer/Test/Repository/FilesystemRepositoryTest.php b/tests/Composer/Test/Repository/FilesystemRepositoryTest.php
index ac1d78e4d..fd2667a7f 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);
diff --git a/tests/Composer/Test/Repository/FilterRepositoryTest.php b/tests/Composer/Test/Repository/FilterRepositoryTest.php
index d18e8a13e..70678ec10 100644
--- a/tests/Composer/Test/Repository/FilterRepositoryTest.php
+++ b/tests/Composer/Test/Repository/FilterRepositoryTest.php
@@ -40,7 +40,7 @@ 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();
@@ -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..96cdaa21a 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);
@@ -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
@@ -152,7 +152,7 @@ class PlatformRepositoryTest extends TestCase
         }
     }
 
-    public function testInetPtonRegression()
+    public function testInetPtonRegression(): void
     {
         $runtime = $this->getMockBuilder('Composer\Platform\Runtime')->getMock();
 
@@ -1097,7 +1097,7 @@ Linked Version => 1.2.11',
         array $functions = array(),
         array $constants = array(),
         array $classDefinitions = array()
-    ) {
+    ): void {
         $extensions = (array) $extensions;
 
         $extensionVersion = '100.200.300';
@@ -1204,7 +1204,7 @@ Linked Version => 1.2.11',
      *
      * @return void
      */
-    private function assertPackageLinks($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 +1215,7 @@ Linked Version => 1.2.11',
         }
     }
 
-    public function testComposerPlatformVersion()
+    public function testComposerPlatformVersion(): void
     {
         $runtime = $this->getMockBuilder('Composer\Platform\Runtime')->getMock();
         $runtime
@@ -1268,7 +1268,7 @@ 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));
     }
diff --git a/tests/Composer/Test/Repository/RepositoryFactoryTest.php b/tests/Composer/Test/Repository/RepositoryFactoryTest.php
index ab725b18f..9e890a99d 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(),
@@ -59,7 +59,7 @@ class RepositoryFactoryTest extends TestCase
      *
      * @phpstan-param array{url?: string} $config
      */
-    public function testGenerateRepositoryName($index, array $config, array $existingRepos, $expected)
+    public function testGenerateRepositoryName($index, array $config, array $existingRepos, $expected): void
     {
         $this->assertSame($expected, RepositoryFactory::generateRepositoryName($index, $config, $existingRepos));
     }
diff --git a/tests/Composer/Test/Repository/RepositoryManagerTest.php b/tests/Composer/Test/Repository/RepositoryManagerTest.php
index 1596c0a40..e326f98ce 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);
@@ -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..d6afde937 100644
--- a/tests/Composer/Test/Repository/Vcs/FossilDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/FossilDriverTest.php
@@ -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..1643769ae 100644
--- a/tests/Composer/Test/Repository/Vcs/GitBitbucketDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/GitBitbucketDriverTest.php
@@ -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');
@@ -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..74576d98a 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();
 
@@ -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..e00c69a19 100644
--- a/tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php
@@ -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');
 
@@ -444,7 +444,7 @@ 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));
     }
@@ -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..a173d2802 100644
--- a/tests/Composer/Test/Repository/Vcs/HgDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/HgDriverTest.php
@@ -50,7 +50,7 @@ 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)
diff --git a/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php b/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
index a918b7ff9..058bbc4c7 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');
@@ -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..892cda1c8 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/))");
@@ -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..561d0b300 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();
diff --git a/tests/Composer/Test/TestCase.php b/tests/Composer/Test/TestCase.php
index e6a171621..380bd2550 100644
--- a/tests/Composer/Test/TestCase.php
+++ b/tests/Composer/Test/TestCase.php
@@ -152,7 +152,7 @@ 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 +175,7 @@ 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 +195,7 @@ 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();
diff --git a/tests/Composer/Test/Util/AuthHelperTest.php b/tests/Composer/Test/Util/AuthHelperTest.php
index a65d022c1..5fccfda3a 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',
@@ -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',
@@ -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',
@@ -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';
@@ -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';
@@ -473,7 +473,7 @@ class AuthHelperTest extends TestCase
         $this->authHelper->storeAuth($origin, $storeAuth);
     }
 
-    public function testStoreAuthWithPromptInvalidAnswer()
+    public function testStoreAuthWithPromptInvalidAnswer(): void
     {
         self::expectException('RuntimeException');
 
@@ -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..a39202a51 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')
@@ -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..38d2f8fd5 100644
--- a/tests/Composer/Test/Util/FilesystemTest.php
+++ b/tests/Composer/Test/Util/FilesystemTest.php
@@ -59,7 +59,7 @@ 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));
@@ -122,7 +122,7 @@ 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, $directory));
@@ -173,7 +173,7 @@ class FilesystemTest extends TestCase
     /**
      * @group GH-1339
      */
-    public function testRemoveDirectoryPhp()
+    public function testRemoveDirectoryPhp(): void
     {
         @mkdir($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');
@@ -207,7 +207,7 @@ 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));
@@ -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..6c88e1a4f 100644
--- a/tests/Composer/Test/Util/GitTest.php
+++ b/tests/Composer/Test/Util/GitTest.php
@@ -47,7 +47,7 @@ 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) {
             $this->assertSame($expectedUrl, $url);
@@ -70,7 +70,7 @@ class GitTest extends TestCase
         );
     }
 
-    public function testRunCommandPrivateGitHubRepositoryNotInitialCloneNotInteractiveWithoutAuthentication()
+    public function testRunCommandPrivateGitHubRepositoryNotInitialCloneNotInteractiveWithoutAuthentication(): void
     {
         self::expectException('RuntimeException');
 
@@ -99,7 +99,7 @@ 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 ($expectedUrl) {
             if ($url !== $expectedUrl) {
@@ -148,7 +148,7 @@ class GitTest extends TestCase
      *
      * @return void
      */
-    private function mockConfig($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..5998289fb 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;
 
@@ -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;
 
@@ -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);
 
@@ -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);
 
@@ -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';
 
@@ -162,7 +162,7 @@ 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));
     }
@@ -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);
diff --git a/tests/Composer/Test/Util/Http/ProxyManagerTest.php b/tests/Composer/Test/Util/Http/ProxyManagerTest.php
index 16d926039..6f40c08df 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();
@@ -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();
diff --git a/tests/Composer/Test/Util/Http/RequestProxyTest.php b/tests/Composer/Test/Util/Http/RequestProxyTest.php
index 33f5664a8..8355e3d4e 100644
--- a/tests/Composer/Test/Util/Http/RequestProxyTest.php
+++ b/tests/Composer/Test/Util/Http/RequestProxyTest.php
@@ -23,7 +23,7 @@ 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(), '');
 
@@ -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);
 
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..9a02d7e54 100644
--- a/tests/Composer/Test/Util/NoProxyPatternTest.php
+++ b/tests/Composer/Test/Util/NoProxyPatternTest.php
@@ -24,7 +24,7 @@ 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);
@@ -55,7 +55,7 @@ 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);
@@ -84,7 +84,7 @@ 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);
@@ -113,7 +113,7 @@ 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);
diff --git a/tests/Composer/Test/Util/PackageSorterTest.php b/tests/Composer/Test/Util/PackageSorterTest.php
index 7ee94982c..e11d1b89f 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());
@@ -108,7 +108,7 @@ 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) {
diff --git a/tests/Composer/Test/Util/PerforceTest.php b/tests/Composer/Test/Util/PerforceTest.php
index 817e4fe48..764d1a68c 100644
--- a/tests/Composer/Test/Util/PerforceTest.php
+++ b/tests/Composer/Test/Util/PerforceTest.php
@@ -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();
 
@@ -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..7e57318ae 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;
@@ -73,7 +73,7 @@ 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);
@@ -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));
diff --git a/tests/Composer/Test/Util/RemoteFilesystemTest.php b/tests/Composer/Test/Util/RemoteFilesystemTest.php
index a63e24bb3..667e6d17f 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());
 
@@ -170,7 +170,7 @@ class RemoteFilesystemTest extends TestCase
         unlink($file);
     }
 
-    public function testCopyWithNoRetryOnFailure()
+    public function testCopyWithNoRetryOnFailure(): void
     {
         self::expectException('Composer\Downloader\TransportException');
         $fs = $this->getRemoteFilesystemWithMockedMethods(array('getRemoteContents'));
@@ -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();
 
@@ -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
@@ -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..e5ae3ddca 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);
@@ -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;
 
@@ -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')) {
             $this->markTestSkipped('The curl is not available.');
diff --git a/tests/Composer/Test/Util/SvnTest.php b/tests/Composer/Test/Util/SvnTest.php
index 0ab43dd45..fce3081da 100644
--- a/tests/Composer/Test/Util/SvnTest.php
+++ b/tests/Composer/Test/Util/SvnTest.php
@@ -27,7 +27,7 @@ class SvnTest extends TestCase
      *
      * @dataProvider urlProvider
      */
-    public function testCredentials($url, $expect)
+    public function testCredentials($url, $expect): void
     {
         $svn = new Svn($url, new NullIO, new Config());
         $reflMethod = new \ReflectionMethod('Composer\\Util\\Svn', 'getCredentialString');
@@ -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..35ded5266 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) : '';
@@ -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..a935e8978 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));
@@ -70,7 +70,7 @@ 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));
     }
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.');