diff --git a/.github/workflows/phpstan.yml b/.github/workflows/phpstan.yml
index 538868318..60b9afaf2 100644
--- a/.github/workflows/phpstan.yml
+++ b/.github/workflows/phpstan.yml
@@ -52,5 +52,5 @@ jobs:
       - name: Run PHPStan
         # Locked to phpunit 7.5 here as newer ones have void return types which break inheritance
         run: |
-          bin/composer require --dev phpstan/phpstan:^0.12.42 phpunit/phpunit:^7.5.20 --with-all-dependencies ${{ env.COMPOSER_FLAGS }}
+          bin/composer require --dev phpstan/phpstan:^0.12.69 phpunit/phpunit:^7.5.20 --with-all-dependencies ${{ env.COMPOSER_FLAGS }}
           vendor/bin/phpstan analyse --configuration=phpstan/config.neon
diff --git a/src/Composer/Autoload/AutoloadGenerator.php b/src/Composer/Autoload/AutoloadGenerator.php
index 6ad5d94b0..21dc88df0 100644
--- a/src/Composer/Autoload/AutoloadGenerator.php
+++ b/src/Composer/Autoload/AutoloadGenerator.php
@@ -838,7 +838,7 @@ PLATFORM_CHECK;
 
         $file .= <<<CLASSLOADER_INIT
         spl_autoload_register(array('ComposerAutoloaderInit$suffix', 'loadClassLoader'), true, $prependAutoloader);
-        self::\$loader = \$loader = new \\Composer\\Autoload\\ClassLoader();
+        self::\$loader = \$loader = new \\Composer\\Autoload\\ClassLoader(\\dirname(\\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInit$suffix', 'loadClassLoader'));
 
 
diff --git a/src/Composer/Autoload/ClassLoader.php b/src/Composer/Autoload/ClassLoader.php
index 1a58957d2..4d989a212 100644
--- a/src/Composer/Autoload/ClassLoader.php
+++ b/src/Composer/Autoload/ClassLoader.php
@@ -42,6 +42,8 @@ namespace Composer\Autoload;
  */
 class ClassLoader
 {
+    private $vendorDir;
+
     // PSR-4
     private $prefixLengthsPsr4 = array();
     private $prefixDirsPsr4 = array();
@@ -57,6 +59,13 @@ class ClassLoader
     private $missingClasses = array();
     private $apcuPrefix;
 
+    private static $registeredLoaders = array();
+
+    public function __construct($vendorDir = null)
+    {
+        $this->vendorDir = $vendorDir;
+    }
+
     public function getPrefixes()
     {
         if (!empty($this->prefixesPsr0)) {
@@ -300,6 +309,15 @@ class ClassLoader
     public function register($prepend = false)
     {
         spl_autoload_register(array($this, 'loadClass'), true, $prepend);
+
+        if (null === $this->vendorDir) {
+            //no-op
+        } elseif ($prepend) {
+            self::$registeredLoaders = array($this->vendorDir => $this) + self::$registeredLoaders;
+        } else {
+            unset(self::$registeredLoaders[$this->vendorDir]);
+            self::$registeredLoaders[$this->vendorDir] = $this;
+        }
     }
 
     /**
@@ -308,6 +326,10 @@ class ClassLoader
     public function unregister()
     {
         spl_autoload_unregister(array($this, 'loadClass'));
+
+        if (null !== $this->vendorDir) {
+            unset(self::$registeredLoaders[$this->vendorDir]);
+        }
     }
 
     /**
@@ -367,6 +389,16 @@ class ClassLoader
         return $file;
     }
 
+    /**
+     * Returns the currently registered loaders indexed by their corresponding vendor directories.
+     *
+     * @return self[]
+     */
+    public static function getRegisteredLoaders()
+    {
+        return self::$registeredLoaders;
+    }
+
     private function findFileWithExtension($class, $ext)
     {
         // PSR-4 lookup
diff --git a/src/Composer/InstalledVersions.php b/src/Composer/InstalledVersions.php
index e492f988a..e6909e934 100644
--- a/src/Composer/InstalledVersions.php
+++ b/src/Composer/InstalledVersions.php
@@ -12,6 +12,7 @@
 
 namespace Composer;
 
+use Composer\Autoload\ClassLoader;
 use Composer\Semver\VersionParser;
 
 /**
@@ -22,6 +23,8 @@ use Composer\Semver\VersionParser;
 class InstalledVersions
 {
     private static $installed;
+    private static $canGetVendors;
+    private static $installedByVendor = array();
 
     /**
      * Returns a list of all package names which are present, either by being installed, replaced or provided
@@ -31,7 +34,17 @@ class InstalledVersions
      */
     public static function getInstalledPackages()
     {
-        return array_keys(self::$installed['versions']);
+        $packages = array();
+        foreach (self::getInstalled() as $installed) {
+            $packages[] = array_keys($installed['versions']);
+        }
+
+
+        if (1 === \count($packages)) {
+            return $packages[0];
+        }
+
+        return array_keys(array_flip(\call_user_func_array('array_merge', $packages)));
     }
 
     /**
@@ -44,7 +57,13 @@ class InstalledVersions
      */
     public static function isInstalled($packageName)
     {
-        return isset(self::$installed['versions'][$packageName]);
+        foreach (self::getInstalled() as $installed) {
+            if (isset($installed['versions'][$packageName])) {
+                return true;
+            }
+        }
+
+        return false;
     }
 
     /**
@@ -79,25 +98,29 @@ class InstalledVersions
      */
     public static function getVersionRanges($packageName)
     {
-        if (!isset(self::$installed['versions'][$packageName])) {
-            throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
+        foreach (self::getInstalled() as $installed) {
+            if (!isset($installed['versions'][$packageName])) {
+                continue;
+            }
+
+            $ranges = array();
+            if (isset($installed['versions'][$packageName]['pretty_version'])) {
+                $ranges[] = $installed['versions'][$packageName]['pretty_version'];
+            }
+            if (array_key_exists('aliases', $installed['versions'][$packageName])) {
+                $ranges = array_merge($ranges, $installed['versions'][$packageName]['aliases']);
+            }
+            if (array_key_exists('replaced', $installed['versions'][$packageName])) {
+                $ranges = array_merge($ranges, $installed['versions'][$packageName]['replaced']);
+            }
+            if (array_key_exists('provided', $installed['versions'][$packageName])) {
+                $ranges = array_merge($ranges, $installed['versions'][$packageName]['provided']);
+            }
+
+            return implode(' || ', $ranges);
         }
 
-        $ranges = array();
-        if (isset(self::$installed['versions'][$packageName]['pretty_version'])) {
-            $ranges[] = self::$installed['versions'][$packageName]['pretty_version'];
-        }
-        if (array_key_exists('aliases', self::$installed['versions'][$packageName])) {
-            $ranges = array_merge($ranges, self::$installed['versions'][$packageName]['aliases']);
-        }
-        if (array_key_exists('replaced', self::$installed['versions'][$packageName])) {
-            $ranges = array_merge($ranges, self::$installed['versions'][$packageName]['replaced']);
-        }
-        if (array_key_exists('provided', self::$installed['versions'][$packageName])) {
-            $ranges = array_merge($ranges, self::$installed['versions'][$packageName]['provided']);
-        }
-
-        return implode(' || ', $ranges);
+        throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
     }
 
     /**
@@ -106,15 +129,19 @@ class InstalledVersions
      */
     public static function getVersion($packageName)
     {
-        if (!isset(self::$installed['versions'][$packageName])) {
-            throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
+        foreach (self::getInstalled() as $installed) {
+            if (!isset($installed['versions'][$packageName])) {
+                continue;
+            }
+
+            if (!isset($installed['versions'][$packageName]['version'])) {
+                return null;
+            }
+
+            return $installed['versions'][$packageName]['version'];
         }
 
-        if (!isset(self::$installed['versions'][$packageName]['version'])) {
-            return null;
-        }
-
-        return self::$installed['versions'][$packageName]['version'];
+        throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
     }
 
     /**
@@ -123,15 +150,19 @@ class InstalledVersions
      */
     public static function getPrettyVersion($packageName)
     {
-        if (!isset(self::$installed['versions'][$packageName])) {
-            throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
+        foreach (self::getInstalled() as $installed) {
+            if (!isset($installed['versions'][$packageName])) {
+                continue;
+            }
+
+            if (!isset($installed['versions'][$packageName]['pretty_version'])) {
+                return null;
+            }
+
+            return $installed['versions'][$packageName]['pretty_version'];
         }
 
-        if (!isset(self::$installed['versions'][$packageName]['pretty_version'])) {
-            return null;
-        }
-
-        return self::$installed['versions'][$packageName]['pretty_version'];
+        throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
     }
 
     /**
@@ -140,15 +171,19 @@ class InstalledVersions
      */
     public static function getReference($packageName)
     {
-        if (!isset(self::$installed['versions'][$packageName])) {
-            throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
+        foreach (self::getInstalled() as $installed) {
+            if (!isset($installed['versions'][$packageName])) {
+                continue;
+            }
+
+            if (!isset($installed['versions'][$packageName]['reference'])) {
+                return null;
+            }
+
+            return $installed['versions'][$packageName]['reference'];
         }
 
-        if (!isset(self::$installed['versions'][$packageName]['reference'])) {
-            return null;
-        }
-
-        return self::$installed['versions'][$packageName]['reference'];
+        throw new \OutOfBoundsException('Package "' . $packageName . '" is not installed');
     }
 
     /**
@@ -157,7 +192,9 @@ class InstalledVersions
      */
     public static function getRootPackage()
     {
-        return self::$installed['root'];
+        $installed = self::getInstalled();
+
+        return $installed[0]['root'];
     }
 
     /**
@@ -192,5 +229,33 @@ class InstalledVersions
     public static function reload($data)
     {
         self::$installed = $data;
+        self::$installedByVendor = array();
+    }
+
+    /**
+     * @return array[]
+     */
+    private static function getInstalled()
+    {
+        if (null === self::$canGetVendors) {
+            self::$canGetVendors = method_exists('Composer\Autoload\ClassLoader', 'getRegisteredLoaders');
+        }
+
+        $installed = array();
+
+        if (self::$canGetVendors) {
+            // @phpstan-ignore-next-line
+            foreach (ClassLoader::getRegisteredLoaders() as $vendorDir => $loader) {
+                if (isset(self::$installedByVendor[$vendorDir])) {
+                    $installed[] = self::$installedByVendor[$vendorDir];
+                } elseif (is_file($vendorDir.'/composer/installed.php')) {
+                    $installed[] = self::$installedByVendor[$vendorDir] = require $vendorDir.'/composer/installed.php';
+                }
+            }
+        }
+
+        $installed[] = self::$installed;
+
+        return $installed;
     }
 }
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_real_files_by_dependency.php b/tests/Composer/Test/Autoload/Fixtures/autoload_real_files_by_dependency.php
index fdff2bd42..390bff3bd 100644
--- a/tests/Composer/Test/Autoload/Fixtures/autoload_real_files_by_dependency.php
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_real_files_by_dependency.php
@@ -23,7 +23,7 @@ class ComposerAutoloaderInitFilesAutoloadOrder
         }
 
         spl_autoload_register(array('ComposerAutoloaderInitFilesAutoloadOrder', 'loadClassLoader'), true, true);
-        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
+        self::$loader = $loader = new \Composer\Autoload\ClassLoader(\dirname(\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInitFilesAutoloadOrder', 'loadClassLoader'));
 
         $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded());
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions.php b/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions.php
index 67ce8ffe2..71397e2f5 100644
--- a/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions.php
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions.php
@@ -23,7 +23,7 @@ class ComposerAutoloaderInitFilesAutoload
         }
 
         spl_autoload_register(array('ComposerAutoloaderInitFilesAutoload', 'loadClassLoader'), true, true);
-        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
+        self::$loader = $loader = new \Composer\Autoload\ClassLoader(\dirname(\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInitFilesAutoload', 'loadClassLoader'));
 
         $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded());
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_include_paths.php b/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_include_paths.php
index 790029096..47bb48f7f 100644
--- a/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_include_paths.php
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_include_paths.php
@@ -23,7 +23,7 @@ class ComposerAutoloaderInitFilesAutoload
         }
 
         spl_autoload_register(array('ComposerAutoloaderInitFilesAutoload', 'loadClassLoader'), true, true);
-        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
+        self::$loader = $loader = new \Composer\Autoload\ClassLoader(\dirname(\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInitFilesAutoload', 'loadClassLoader'));
 
         $includePaths = require __DIR__ . '/include_paths.php';
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_removed_include_paths_and_autolad_files.php b/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_removed_include_paths_and_autolad_files.php
index 7b7898df1..965542d83 100644
--- a/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_removed_include_paths_and_autolad_files.php
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_real_functions_with_removed_include_paths_and_autolad_files.php
@@ -23,7 +23,7 @@ class ComposerAutoloaderInitFilesAutoload
         }
 
         spl_autoload_register(array('ComposerAutoloaderInitFilesAutoload', 'loadClassLoader'), true, true);
-        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
+        self::$loader = $loader = new \Composer\Autoload\ClassLoader(\dirname(\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInitFilesAutoload', 'loadClassLoader'));
 
         $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded());
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_real_include_path.php b/tests/Composer/Test/Autoload/Fixtures/autoload_real_include_path.php
index 2747dbc0a..1bbf29cb2 100644
--- a/tests/Composer/Test/Autoload/Fixtures/autoload_real_include_path.php
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_real_include_path.php
@@ -23,7 +23,7 @@ class ComposerAutoloaderInitIncludePath
         }
 
         spl_autoload_register(array('ComposerAutoloaderInitIncludePath', 'loadClassLoader'), true, true);
-        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
+        self::$loader = $loader = new \Composer\Autoload\ClassLoader(\dirname(\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInitIncludePath', 'loadClassLoader'));
 
         $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded());
diff --git a/tests/Composer/Test/Autoload/Fixtures/autoload_real_target_dir.php b/tests/Composer/Test/Autoload/Fixtures/autoload_real_target_dir.php
index 8ec8cdd47..7b6b58bf9 100644
--- a/tests/Composer/Test/Autoload/Fixtures/autoload_real_target_dir.php
+++ b/tests/Composer/Test/Autoload/Fixtures/autoload_real_target_dir.php
@@ -23,7 +23,7 @@ class ComposerAutoloaderInitTargetDir
         }
 
         spl_autoload_register(array('ComposerAutoloaderInitTargetDir', 'loadClassLoader'), true, true);
-        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
+        self::$loader = $loader = new \Composer\Autoload\ClassLoader(\dirname(\dirname(__FILE__)));
         spl_autoload_unregister(array('ComposerAutoloaderInitTargetDir', 'loadClassLoader'));
 
         $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded());