diff --git a/composer/composer/ClassLoader.php b/composer/composer/ClassLoader.php
index a72151c77c..7824d8f7ea 100644
--- a/composer/composer/ClassLoader.php
+++ b/composer/composer/ClassLoader.php
@@ -45,35 +45,34 @@ class ClassLoader
     /** @var \Closure(string):void */
     private static $includeFile;
 
-    /** @var ?string */
+    /** @var string|null */
     private $vendorDir;
 
     // PSR-4
     /**
-     * @var array[]
-     * @psalm-var array<string, array<string, int>>
+     * @var array<string, array<string, int>>
      */
     private $prefixLengthsPsr4 = array();
     /**
-     * @var array[]
-     * @psalm-var array<string, array<int, string>>
+     * @var array<string, list<string>>
      */
     private $prefixDirsPsr4 = array();
     /**
-     * @var array[]
-     * @psalm-var array<string, string>
+     * @var list<string>
      */
     private $fallbackDirsPsr4 = array();
 
     // PSR-0
     /**
-     * @var array[]
-     * @psalm-var array<string, array<string, string[]>>
+     * List of PSR-0 prefixes
+     *
+     * Structured as array('F (first letter)' => array('Foo\Bar (full prefix)' => array('path', 'path2')))
+     *
+     * @var array<string, array<string, list<string>>>
      */
     private $prefixesPsr0 = array();
     /**
-     * @var array[]
-     * @psalm-var array<string, string>
+     * @var list<string>
      */
     private $fallbackDirsPsr0 = array();
 
@@ -81,8 +80,7 @@ class ClassLoader
     private $useIncludePath = false;
 
     /**
-     * @var string[]
-     * @psalm-var array<string, string>
+     * @var array<string, string>
      */
     private $classMap = array();
 
@@ -90,21 +88,20 @@ class ClassLoader
     private $classMapAuthoritative = false;
 
     /**
-     * @var bool[]
-     * @psalm-var array<string, bool>
+     * @var array<string, bool>
      */
     private $missingClasses = array();
 
-    /** @var ?string */
+    /** @var string|null */
     private $apcuPrefix;
 
     /**
-     * @var self[]
+     * @var array<string, self>
      */
     private static $registeredLoaders = array();
 
     /**
-     * @param ?string $vendorDir
+     * @param string|null $vendorDir
      */
     public function __construct($vendorDir = null)
     {
@@ -113,7 +110,7 @@ public function __construct($vendorDir = null)
     }
 
     /**
-     * @return string[]
+     * @return array<string, list<string>>
      */
     public function getPrefixes()
     {
@@ -125,8 +122,7 @@ public function getPrefixes()
     }
 
     /**
-     * @return array[]
-     * @psalm-return array<string, array<int, string>>
+     * @return array<string, list<string>>
      */
     public function getPrefixesPsr4()
     {
@@ -134,8 +130,7 @@ public function getPrefixesPsr4()
     }
 
     /**
-     * @return array[]
-     * @psalm-return array<string, string>
+     * @return list<string>
      */
     public function getFallbackDirs()
     {
@@ -143,8 +138,7 @@ public function getFallbackDirs()
     }
 
     /**
-     * @return array[]
-     * @psalm-return array<string, string>
+     * @return list<string>
      */
     public function getFallbackDirsPsr4()
     {
@@ -152,8 +146,7 @@ public function getFallbackDirsPsr4()
     }
 
     /**
-     * @return string[] Array of classname => path
-     * @psalm-return array<string, string>
+     * @return array<string, string> Array of classname => path
      */
     public function getClassMap()
     {
@@ -161,8 +154,7 @@ public function getClassMap()
     }
 
     /**
-     * @param string[] $classMap Class to filename map
-     * @psalm-param array<string, string> $classMap
+     * @param array<string, string> $classMap Class to filename map
      *
      * @return void
      */
@@ -179,24 +171,25 @@ public function addClassMap(array $classMap)
      * Registers a set of PSR-0 directories for a given prefix, either
      * appending or prepending to the ones previously set for this prefix.
      *
-     * @param string          $prefix  The prefix
-     * @param string[]|string $paths   The PSR-0 root directories
-     * @param bool            $prepend Whether to prepend the directories
+     * @param string              $prefix  The prefix
+     * @param list<string>|string $paths   The PSR-0 root directories
+     * @param bool                $prepend Whether to prepend the directories
      *
      * @return void
      */
     public function add($prefix, $paths, $prepend = false)
     {
+        $paths = (array) $paths;
         if (!$prefix) {
             if ($prepend) {
                 $this->fallbackDirsPsr0 = array_merge(
-                    (array) $paths,
+                    $paths,
                     $this->fallbackDirsPsr0
                 );
             } else {
                 $this->fallbackDirsPsr0 = array_merge(
                     $this->fallbackDirsPsr0,
-                    (array) $paths
+                    $paths
                 );
             }
 
@@ -205,19 +198,19 @@ public function add($prefix, $paths, $prepend = false)
 
         $first = $prefix[0];
         if (!isset($this->prefixesPsr0[$first][$prefix])) {
-            $this->prefixesPsr0[$first][$prefix] = (array) $paths;
+            $this->prefixesPsr0[$first][$prefix] = $paths;
 
             return;
         }
         if ($prepend) {
             $this->prefixesPsr0[$first][$prefix] = array_merge(
-                (array) $paths,
+                $paths,
                 $this->prefixesPsr0[$first][$prefix]
             );
         } else {
             $this->prefixesPsr0[$first][$prefix] = array_merge(
                 $this->prefixesPsr0[$first][$prefix],
-                (array) $paths
+                $paths
             );
         }
     }
@@ -226,9 +219,9 @@ public function add($prefix, $paths, $prepend = false)
      * Registers a set of PSR-4 directories for a given namespace, either
      * appending or prepending to the ones previously set for this namespace.
      *
-     * @param string          $prefix  The prefix/namespace, with trailing '\\'
-     * @param string[]|string $paths   The PSR-4 base directories
-     * @param bool            $prepend Whether to prepend the directories
+     * @param string              $prefix  The prefix/namespace, with trailing '\\'
+     * @param list<string>|string $paths   The PSR-4 base directories
+     * @param bool                $prepend Whether to prepend the directories
      *
      * @throws \InvalidArgumentException
      *
@@ -236,17 +229,18 @@ public function add($prefix, $paths, $prepend = false)
      */
     public function addPsr4($prefix, $paths, $prepend = false)
     {
+        $paths = (array) $paths;
         if (!$prefix) {
             // Register directories for the root namespace.
             if ($prepend) {
                 $this->fallbackDirsPsr4 = array_merge(
-                    (array) $paths,
+                    $paths,
                     $this->fallbackDirsPsr4
                 );
             } else {
                 $this->fallbackDirsPsr4 = array_merge(
                     $this->fallbackDirsPsr4,
-                    (array) $paths
+                    $paths
                 );
             }
         } elseif (!isset($this->prefixDirsPsr4[$prefix])) {
@@ -256,18 +250,18 @@ public function addPsr4($prefix, $paths, $prepend = false)
                 throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
             }
             $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
-            $this->prefixDirsPsr4[$prefix] = (array) $paths;
+            $this->prefixDirsPsr4[$prefix] = $paths;
         } elseif ($prepend) {
             // Prepend directories for an already registered namespace.
             $this->prefixDirsPsr4[$prefix] = array_merge(
-                (array) $paths,
+                $paths,
                 $this->prefixDirsPsr4[$prefix]
             );
         } else {
             // Append directories for an already registered namespace.
             $this->prefixDirsPsr4[$prefix] = array_merge(
                 $this->prefixDirsPsr4[$prefix],
-                (array) $paths
+                $paths
             );
         }
     }
@@ -276,8 +270,8 @@ public function addPsr4($prefix, $paths, $prepend = false)
      * Registers a set of PSR-0 directories for a given prefix,
      * replacing any others previously set for this prefix.
      *
-     * @param string          $prefix The prefix
-     * @param string[]|string $paths  The PSR-0 base directories
+     * @param string              $prefix The prefix
+     * @param list<string>|string $paths  The PSR-0 base directories
      *
      * @return void
      */
@@ -294,8 +288,8 @@ public function set($prefix, $paths)
      * Registers a set of PSR-4 directories for a given namespace,
      * replacing any others previously set for this namespace.
      *
-     * @param string          $prefix The prefix/namespace, with trailing '\\'
-     * @param string[]|string $paths  The PSR-4 base directories
+     * @param string              $prefix The prefix/namespace, with trailing '\\'
+     * @param list<string>|string $paths  The PSR-4 base directories
      *
      * @throws \InvalidArgumentException
      *
@@ -481,9 +475,9 @@ public function findFile($class)
     }
 
     /**
-     * Returns the currently registered loaders indexed by their corresponding vendor directories.
+     * Returns the currently registered loaders keyed by their corresponding vendor directories.
      *
-     * @return self[]
+     * @return array<string, self>
      */
     public static function getRegisteredLoaders()
     {