diff --git a/src/Composer/Autoload/ClassMapGenerator.php b/src/Composer/Autoload/ClassMapGenerator.php
index 1bb204362..0451077fb 100644
--- a/src/Composer/Autoload/ClassMapGenerator.php
+++ b/src/Composer/Autoload/ClassMapGenerator.php
@@ -33,7 +33,7 @@ class ClassMapGenerator
     /**
      * Generate a class map file
      *
-     * @param \Traversable|array<string> $dirs Directories or a single path to search in
+     * @param \Traversable<string>|array<string> $dirs Directories or a single path to search in
      * @param string                     $file The name of the class map file
      */
     public static function dump($dirs, $file)
@@ -58,6 +58,8 @@ class ClassMapGenerator
      *
      * @throws \RuntimeException When the path is neither an existing file nor directory
      * @return array             A class map array
+     *
+     * @phpstan-param \Traversable<\SplFileInfo>|string|array<string> $path
      */
     public static function createMap($path, $excluded = null, IOInterface $io = null, $namespace = null, $autoloadType = null, &$scannedFiles = array())
     {
diff --git a/src/Composer/Cache.php b/src/Composer/Cache.php
index a3aaebf57..c02a636a2 100644
--- a/src/Composer/Cache.php
+++ b/src/Composer/Cache.php
@@ -24,12 +24,19 @@ use Symfony\Component\Finder\Finder;
  */
 class Cache
 {
+    /** @var bool|null */
     private static $cacheCollected = null;
+    /** @var IOInterface */
     private $io;
+    /** @var string */
     private $root;
+    /** @var bool */
     private $enabled = true;
+    /** @var string */
     private $allowlist;
+    /** @var Filesystem */
     private $filesystem;
+    /** @var bool */
     private $readOnly;
 
     /**
diff --git a/src/Composer/Command/BaseDependencyCommand.php b/src/Composer/Command/BaseDependencyCommand.php
index 962c1b2ce..e116b79ed 100644
--- a/src/Composer/Command/BaseDependencyCommand.php
+++ b/src/Composer/Command/BaseDependencyCommand.php
@@ -203,7 +203,7 @@ class BaseDependencyCommand extends BaseCommand
             /**
              * @var PackageInterface $package
              * @var Link             $link
-             * @var array|bool     $children
+             * @var mixed[]|bool     $children
              */
             list($package, $link, $children) = $result;
 
diff --git a/src/Composer/Compiler.php b/src/Composer/Compiler.php
index 8476a529f..da5e1d842 100644
--- a/src/Composer/Compiler.php
+++ b/src/Composer/Compiler.php
@@ -27,8 +27,11 @@ use Seld\PharUtils\Linter;
  */
 class Compiler
 {
+    /** @var string */
     private $version;
+    /** @var string */
     private $branchAliasVersion = '';
+    /** @var \DateTime */
     private $versionDate;
 
     /**
@@ -313,7 +316,7 @@ EOF;
 
         // add warning once the phar is older than 60 days
         if (preg_match('{^[a-f0-9]+$}', $this->version)) {
-            $warningTime = $this->versionDate->format('U') + 60 * 86400;
+            $warningTime = ((int) $this->versionDate->format('U')) + 60 * 86400;
             $stub .= "define('COMPOSER_DEV_WARNING_TIME', $warningTime);\n";
         }
 
diff --git a/src/Composer/Config.php b/src/Composer/Config.php
index 4cb7ab8ae..30110331f 100644
--- a/src/Composer/Config.php
+++ b/src/Composer/Config.php
@@ -25,6 +25,7 @@ class Config
 {
     const RELATIVE_PATHS = 1;
 
+    /** @var array<string, mixed> */
     public static $defaultConfig = array(
         'process-timeout' => 300,
         'use-include-path' => false,
@@ -77,6 +78,7 @@ class Config
         // bearer
     );
 
+    /** @var array<string, mixed> */
     public static $defaultRepositories = array(
         'packagist.org' => array(
             'type' => 'composer',
@@ -84,14 +86,19 @@ class Config
         ),
     );
 
+    /** @var array<string, mixed> */
     private $config;
+    /** @var ?string */
     private $baseDir;
+    /** @var array<int|string, mixed> */
     private $repositories;
     /** @var ConfigSourceInterface */
     private $configSource;
     /** @var ConfigSourceInterface */
     private $authConfigSource;
+    /** @var bool */
     private $useEnvironment;
+    /** @var array<string, true> */
     private $warnedHosts = array();
 
     /**
diff --git a/src/Composer/Config/ConfigSourceInterface.php b/src/Composer/Config/ConfigSourceInterface.php
index e1ef38de0..111d5d53c 100644
--- a/src/Composer/Config/ConfigSourceInterface.php
+++ b/src/Composer/Config/ConfigSourceInterface.php
@@ -23,8 +23,11 @@ interface ConfigSourceInterface
     /**
      * Add a repository
      *
-     * @param string      $name   Name
-     * @param array|false $config Configuration
+     * @param string        $name   Name
+     * @param mixed[]|false $config Configuration
+     * @param bool          $append Whether the repo should be appended (true) or prepended (false)
+     *
+     * @return void
      */
     public function addRepository($name, $config, $append = true);
 
@@ -32,14 +35,18 @@ interface ConfigSourceInterface
      * Remove a repository
      *
      * @param string $name
+     *
+     * @return void
      */
     public function removeRepository($name);
 
     /**
      * Add a config setting
      *
-     * @param string       $name  Name
-     * @param string|array $value Value
+     * @param string $name  Name
+     * @param mixed  $value Value
+     *
+     * @return void
      */
     public function addConfigSetting($name, $value);
 
@@ -47,6 +54,8 @@ interface ConfigSourceInterface
      * Remove a config setting
      *
      * @param string $name
+     *
+     * @return void
      */
     public function removeConfigSetting($name);
 
@@ -55,6 +64,8 @@ interface ConfigSourceInterface
      *
      * @param string $name  Name
      * @param string $value Value
+     *
+     * @return void
      */
     public function addProperty($name, $value);
 
@@ -62,6 +73,8 @@ interface ConfigSourceInterface
      * Remove a property
      *
      * @param string $name
+     *
+     * @return void
      */
     public function removeProperty($name);
 
@@ -71,6 +84,8 @@ interface ConfigSourceInterface
      * @param string $type  Type (require, require-dev, provide, suggest, replace, conflict)
      * @param string $name  Name
      * @param string $value Value
+     *
+     * @return void
      */
     public function addLink($type, $name, $value);
 
@@ -79,6 +94,8 @@ interface ConfigSourceInterface
      *
      * @param string $type Type (require, require-dev, provide, suggest, replace, conflict)
      * @param string $name Name
+     *
+     * @return void
      */
     public function removeLink($type, $name);
 
diff --git a/src/Composer/DependencyResolver/PolicyInterface.php b/src/Composer/DependencyResolver/PolicyInterface.php
index b10a43e9f..48ce24e16 100644
--- a/src/Composer/DependencyResolver/PolicyInterface.php
+++ b/src/Composer/DependencyResolver/PolicyInterface.php
@@ -13,13 +13,25 @@
 namespace Composer\DependencyResolver;
 
 use Composer\Package\PackageInterface;
+use Composer\Semver\Constraint\Constraint;
 
 /**
  * @author Nils Adermann <naderman@naderman.de>
  */
 interface PolicyInterface
 {
+    /**
+     * @param  string $operator
+     * @return bool
+     *
+     * @phpstan-param Constraint::STR_OP_* $operator
+     */
     public function versionCompare(PackageInterface $a, PackageInterface $b, $operator);
 
+    /**
+     * @param  int[]   $literals
+     * @param  ?string $requiredPackage
+     * @return int[]
+     */
     public function selectPreferredPackages(Pool $pool, array $literals, $requiredPackage = null);
 }
diff --git a/src/Composer/Downloader/DownloaderInterface.php b/src/Composer/Downloader/DownloaderInterface.php
index ef2357958..b19f902a8 100644
--- a/src/Composer/Downloader/DownloaderInterface.php
+++ b/src/Composer/Downloader/DownloaderInterface.php
@@ -33,6 +33,7 @@ interface DownloaderInterface
     /**
      * This should do any network-related tasks to prepare for an upcoming install/update
      *
+     * @param  string $path download path
      * @return PromiseInterface|null
      */
     public function download(PackageInterface $package, $path, PackageInterface $prevPackage = null);
diff --git a/src/Composer/EventDispatcher/EventSubscriberInterface.php b/src/Composer/EventDispatcher/EventSubscriberInterface.php
index 6b0c4ca06..4de66c80b 100644
--- a/src/Composer/EventDispatcher/EventSubscriberInterface.php
+++ b/src/Composer/EventDispatcher/EventSubscriberInterface.php
@@ -42,7 +42,7 @@ interface EventSubscriberInterface
      * * array('eventName' => array('methodName', $priority))
      * * array('eventName' => array(array('methodName1', $priority), array('methodName2'))
      *
-     * @return array The event names to listen to
+     * @return array<string, string|array{0: string, 1?: int}|array<array{0: string, 1?: int}>> The event names to listen to
      */
     public static function getSubscribedEvents();
 }
diff --git a/src/Composer/IO/IOInterface.php b/src/Composer/IO/IOInterface.php
index fd2a2d503..cf19321c2 100644
--- a/src/Composer/IO/IOInterface.php
+++ b/src/Composer/IO/IOInterface.php
@@ -66,56 +66,68 @@ interface IOInterface extends LoggerInterface
     /**
      * Writes a message to the output.
      *
-     * @param string|array $messages  The message as an array of lines or a single string
-     * @param bool         $newline   Whether to add a newline or not
-     * @param int          $verbosity Verbosity level from the VERBOSITY_* constants
+     * @param string|string[] $messages  The message as an array of lines or a single string
+     * @param bool            $newline   Whether to add a newline or not
+     * @param int             $verbosity Verbosity level from the VERBOSITY_* constants
+     *
+     * @return void
      */
     public function write($messages, $newline = true, $verbosity = self::NORMAL);
 
     /**
      * Writes a message to the error output.
      *
-     * @param string|array $messages  The message as an array of lines or a single string
-     * @param bool         $newline   Whether to add a newline or not
-     * @param int          $verbosity Verbosity level from the VERBOSITY_* constants
+     * @param string|string[] $messages  The message as an array of lines or a single string
+     * @param bool            $newline   Whether to add a newline or not
+     * @param int             $verbosity Verbosity level from the VERBOSITY_* constants
+     *
+     * @return void
      */
     public function writeError($messages, $newline = true, $verbosity = self::NORMAL);
 
     /**
      * Writes a message to the output, without formatting it.
      *
-     * @param string|array $messages  The message as an array of lines or a single string
-     * @param bool         $newline   Whether to add a newline or not
-     * @param int          $verbosity Verbosity level from the VERBOSITY_* constants
+     * @param string|string[] $messages  The message as an array of lines or a single string
+     * @param bool            $newline   Whether to add a newline or not
+     * @param int             $verbosity Verbosity level from the VERBOSITY_* constants
+     *
+     * @return void
      */
     public function writeRaw($messages, $newline = true, $verbosity = self::NORMAL);
 
     /**
      * Writes a message to the error output, without formatting it.
      *
-     * @param string|array $messages  The message as an array of lines or a single string
-     * @param bool         $newline   Whether to add a newline or not
-     * @param int          $verbosity Verbosity level from the VERBOSITY_* constants
+     * @param string|string[] $messages  The message as an array of lines or a single string
+     * @param bool            $newline   Whether to add a newline or not
+     * @param int             $verbosity Verbosity level from the VERBOSITY_* constants
+     *
+     * @return void
      */
     public function writeErrorRaw($messages, $newline = true, $verbosity = self::NORMAL);
 
     /**
      * Overwrites a previous message to the output.
      *
-     * @param string|array $messages  The message as an array of lines or a single string
-     * @param bool         $newline   Whether to add a newline or not
-     * @param int          $size      The size of line
-     * @param int          $verbosity Verbosity level from the VERBOSITY_* constants
+     * @param string|string[] $messages  The message as an array of lines or a single string
+     * @param bool            $newline   Whether to add a newline or not
+     * @param int             $size      The size of line
+     * @param int             $verbosity Verbosity level from the VERBOSITY_* constants
+     *
+     * @return void
      */
     public function overwrite($messages, $newline = true, $size = null, $verbosity = self::NORMAL);
 
     /**
      * Overwrites a previous message to the error output.
      *
-     * @param string|array $messages  The message as an array of lines or a single string
-     * @param bool         $newline   Whether to add a newline or not
-     * @param int          $size      The size of line
-     * @param int          $verbosity Verbosity level from the VERBOSITY_* constants
+     * @param string|string[] $messages  The message as an array of lines or a single string
+     * @param bool            $newline   Whether to add a newline or not
+     * @param int             $size      The size of line
+     * @param int             $verbosity Verbosity level from the VERBOSITY_* constants
+     *
+     * @return void
      */
     public function overwriteError($messages, $newline = true, $size = null, $verbosity = self::NORMAL);
 
@@ -172,21 +184,21 @@ interface IOInterface extends LoggerInterface
      * Asks the user to select a value.
      *
      * @param string      $question     The question to ask
-     * @param array       $choices      List of choices to pick from
+     * @param string[]    $choices      List of choices to pick from
      * @param bool|string $default      The default answer if the user enters nothing
      * @param bool|int    $attempts     Max number of times to ask before giving up (false by default, which means infinite)
      * @param string      $errorMessage Message which will be shown if invalid value from choice list would be picked
      * @param bool        $multiselect  Select more than one value separated by comma
      *
      * @throws \InvalidArgumentException
-     * @return int|string|array|bool     The selected value or values (the key of the choices array)
+     * @return int|string|string[]|bool     The selected value or values (the key of the choices array)
      */
     public function select($question, $choices, $default, $attempts = false, $errorMessage = 'Value "%s" is invalid', $multiselect = false);
 
     /**
      * Get all authentication information entered.
      *
-     * @return array The map of authentication data
+     * @return array<string, array{username: string, password: string}> The map of authentication data
      */
     public function getAuthentications();
 
@@ -204,16 +216,18 @@ interface IOInterface extends LoggerInterface
      *
      * @param string $repositoryName The unique name of repository
      *
-     * @return array The 'username' and 'password'
+     * @return array{username: string|null, password: string|null}
      */
     public function getAuthentication($repositoryName);
 
     /**
      * Set the authentication information for the repository.
      *
-     * @param string $repositoryName The unique name of repository
-     * @param string $username       The username
-     * @param string $password       The password
+     * @param string  $repositoryName The unique name of repository
+     * @param string  $username       The username
+     * @param ?string $password       The password
+     *
+     * @return void
      */
     public function setAuthentication($repositoryName, $username, $password = null);
 
@@ -221,6 +235,8 @@ interface IOInterface extends LoggerInterface
      * Loads authentications from a config instance
      *
      * @param Config $config
+     *
+     * @return void
      */
     public function loadConfiguration(Config $config);
 }
diff --git a/src/Composer/InstalledVersions.php b/src/Composer/InstalledVersions.php
index 7c5502ca4..d50e0c9fc 100644
--- a/src/Composer/InstalledVersions.php
+++ b/src/Composer/InstalledVersions.php
@@ -24,8 +24,21 @@ use Composer\Semver\VersionParser;
  */
 class InstalledVersions
 {
+    /**
+     * @var mixed[]|null
+     * @psalm-var array{root: array{name: string, version: string, reference: string, pretty_version: string, aliases: string[], dev: bool, install_path: string, type: string}, versions: array<string, array{dev_requirement: bool, pretty_version?: string, version?: string, aliases?: string[], reference?: string, replaced?: string[], provided?: string[], install_path?: string, type?: string}>}|array{}|null
+     */
     private static $installed;
+
+    /**
+     * @var bool|null
+     */
     private static $canGetVendors;
+
+    /**
+     * @var array[]
+     * @psalm-var array<string, array{root: array{name: string, version: string, reference: string, pretty_version: string, aliases: string[], dev: bool, install_path: string, type: string}, versions: array<string, array{dev_requirement: bool, pretty_version?: string, version?: string, aliases?: string[], reference?: string, replaced?: string[], provided?: string[], install_path?: string, type?: string}>}>
+     */
     private static $installedByVendor = array();
 
     /**
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index d04005132..fab317822 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -117,23 +117,41 @@ class Installer
      */
     protected $autoloadGenerator;
 
+    /** @var bool */
     protected $preferSource = false;
+    /** @var bool */
     protected $preferDist = false;
+    /** @var bool */
     protected $optimizeAutoloader = false;
+    /** @var bool */
     protected $classMapAuthoritative = false;
+    /** @var bool */
     protected $apcuAutoloader = false;
-    protected $apcuAutoloaderPrefix;
+    /** @var string|null */
+    protected $apcuAutoloaderPrefix = null;
+    /** @var bool */
     protected $devMode = false;
+    /** @var bool */
     protected $dryRun = false;
+    /** @var bool */
     protected $verbose = false;
+    /** @var bool */
     protected $update = false;
+    /** @var bool */
     protected $install = true;
+    /** @var bool */
     protected $dumpAutoloader = true;
+    /** @var bool */
     protected $runScripts = true;
+    /** @var bool|string[] */
     protected $ignorePlatformReqs = false;
+    /** @var bool */
     protected $preferStable = false;
+    /** @var bool */
     protected $preferLowest = false;
+    /** @var bool */
     protected $writeLock;
+    /** @var bool */
     protected $executeOperations = true;
 
     /** @var bool */
@@ -141,9 +159,10 @@ class Installer
     /**
      * Array of package names/globs flagged for update
      *
-     * @var array|null
+     * @var string[]|null
      */
     protected $updateAllowList = null;
+    /** @var Request::UPDATE_* */
     protected $updateAllowTransitiveDependencies = Request::UPDATE_ONLY_LISTED;
 
     /**
diff --git a/src/Composer/Installer/BinaryPresenceInterface.php b/src/Composer/Installer/BinaryPresenceInterface.php
index 2749ffafd..61907f778 100644
--- a/src/Composer/Installer/BinaryPresenceInterface.php
+++ b/src/Composer/Installer/BinaryPresenceInterface.php
@@ -25,6 +25,8 @@ interface BinaryPresenceInterface
      * Make sure binaries are installed for a given package.
      *
      * @param PackageInterface $package package instance
+     *
+     * @return void
      */
     public function ensureBinariesPresence(PackageInterface $package);
 }
diff --git a/src/Composer/Package/AliasPackage.php b/src/Composer/Package/AliasPackage.php
index c993230c6..7b9ab2c14 100644
--- a/src/Composer/Package/AliasPackage.php
+++ b/src/Composer/Package/AliasPackage.php
@@ -20,11 +20,17 @@ use Composer\Package\Version\VersionParser;
  */
 class AliasPackage extends BasePackage
 {
+    /** @var string */
     protected $version;
+    /** @var string */
     protected $prettyVersion;
+    /** @var bool */
     protected $dev;
+    /** @var bool */
     protected $rootPackageAlias = false;
+    /** @var string */
     protected $stability;
+    /** @var bool */
     protected $hasSelfVersionRequires = false;
 
     /** @var BasePackage */
diff --git a/src/Composer/Package/Archiver/ArchivableFilesFilter.php b/src/Composer/Package/Archiver/ArchivableFilesFilter.php
index f49486a04..863b6e99b 100644
--- a/src/Composer/Package/Archiver/ArchivableFilesFilter.php
+++ b/src/Composer/Package/Archiver/ArchivableFilesFilter.php
@@ -17,6 +17,7 @@ use PharData;
 
 class ArchivableFilesFilter extends FilterIterator
 {
+    /** @var string[] */
     private $dirs = array();
 
     /**
diff --git a/src/Composer/Package/Archiver/ArchiverInterface.php b/src/Composer/Package/Archiver/ArchiverInterface.php
index 4e5fa54f9..4ea8dc485 100644
--- a/src/Composer/Package/Archiver/ArchiverInterface.php
+++ b/src/Composer/Package/Archiver/ArchiverInterface.php
@@ -22,10 +22,11 @@ interface ArchiverInterface
     /**
      * Create an archive from the sources.
      *
-     * @param string $sources  The sources directory
-     * @param string $target   The target file
-     * @param string $format   The format used for archive
-     * @param array  $excludes A list of patterns for files to exclude
+     * @param string   $sources       The sources directory
+     * @param string   $target        The target file
+     * @param string   $format        The format used for archive
+     * @param string[] $excludes      A list of patterns for files to exclude
+     * @param bool     $ignoreFilters Whether to ignore filters when looking for files
      *
      * @return string The path to the written archive file
      */
diff --git a/src/Composer/Package/Archiver/BaseExcludeFilter.php b/src/Composer/Package/Archiver/BaseExcludeFilter.php
index a67ad272a..0d9fda409 100644
--- a/src/Composer/Package/Archiver/BaseExcludeFilter.php
+++ b/src/Composer/Package/Archiver/BaseExcludeFilter.php
@@ -25,7 +25,7 @@ abstract class BaseExcludeFilter
     protected $sourcePath;
 
     /**
-     * @var array
+     * @var array<array{0: string, 1: bool, 2: bool}> array of [$pattern, $negate, $stripLeadingSlash] arrays
      */
     protected $excludePatterns;
 
diff --git a/src/Composer/Package/Archiver/PharArchiver.php b/src/Composer/Package/Archiver/PharArchiver.php
index 955d1b3c9..2767642ff 100644
--- a/src/Composer/Package/Archiver/PharArchiver.php
+++ b/src/Composer/Package/Archiver/PharArchiver.php
@@ -19,6 +19,7 @@ namespace Composer\Package\Archiver;
  */
 class PharArchiver implements ArchiverInterface
 {
+    /** @var array<string, int> */
     protected static $formats = array(
         'zip' => \Phar::ZIP,
         'tar' => \Phar::TAR,
@@ -26,6 +27,7 @@ class PharArchiver implements ArchiverInterface
         'tar.bz2' => \Phar::TAR,
     );
 
+    /** @var array<string, int> */
     protected static $compressFormats = array(
         'tar.gz' => \Phar::GZ,
         'tar.bz2' => \Phar::BZ2,
diff --git a/src/Composer/Package/Archiver/ZipArchiver.php b/src/Composer/Package/Archiver/ZipArchiver.php
index c0ce41d65..3be7ccc71 100644
--- a/src/Composer/Package/Archiver/ZipArchiver.php
+++ b/src/Composer/Package/Archiver/ZipArchiver.php
@@ -20,8 +20,9 @@ use Composer\Util\Filesystem;
  */
 class ZipArchiver implements ArchiverInterface
 {
+    /** @var array<string, bool> */
     protected static $formats = array(
-        'zip' => 1,
+        'zip' => true,
     );
 
     /**
diff --git a/src/Composer/Package/BasePackage.php b/src/Composer/Package/BasePackage.php
index 04a57f5bc..a104f4726 100644
--- a/src/Composer/Package/BasePackage.php
+++ b/src/Composer/Package/BasePackage.php
@@ -40,6 +40,7 @@ abstract class BasePackage implements PackageInterface
     const STABILITY_ALPHA = 15;
     const STABILITY_DEV = 20;
 
+    /** @var array<string, self::STABILITY_*> */
     public static $stabilities = array(
         'stable' => self::STABILITY_STABLE,
         'RC' => self::STABILITY_RC,
diff --git a/src/Composer/Package/Comparer/Comparer.php b/src/Composer/Package/Comparer/Comparer.php
index 652a94d28..be23bedf6 100644
--- a/src/Composer/Package/Comparer/Comparer.php
+++ b/src/Composer/Package/Comparer/Comparer.php
@@ -19,8 +19,11 @@ namespace Composer\Package\Comparer;
  */
 class Comparer
 {
+    /** @var string Source directory */
     private $source;
+    /** @var string Target directory */
     private $update;
+    /** @var array{changed?: string[], removed?: string[], added?: string[]} */
     private $changed;
 
     public function setSource($source)
diff --git a/src/Composer/Package/CompletePackage.php b/src/Composer/Package/CompletePackage.php
index dc0b115c3..b125fc142 100644
--- a/src/Composer/Package/CompletePackage.php
+++ b/src/Composer/Package/CompletePackage.php
@@ -19,17 +19,29 @@ namespace Composer\Package;
  */
 class CompletePackage extends Package implements CompletePackageInterface
 {
+    /** @var mixed[] */
     protected $repositories = array();
+    /** @var string[] */
     protected $license = array();
+    /** @var string[] */
     protected $keywords = array();
+    /** @var array<array{name?: string, homepage?: string, email?: string, role?: string}> */
     protected $authors = array();
-    protected $description;
-    protected $homepage;
+    /** @var ?string */
+    protected $description = null;
+    /** @var ?string */
+    protected $homepage = null;
+    /** @var array<string, string[]> Map of script name to array of handlers */
     protected $scripts = array();
+    /** @var array{issues?: string, forum?: string, wiki?: string, source?: string, email?: string, irc?: string, docs?: string, rss?: string, chat?: string} */
     protected $support = array();
+    /** @var array<array{url?: string, type?: string}> */
     protected $funding = array();
+    /** @var bool|string */
     protected $abandoned = false;
-    protected $archiveName;
+    /** @var ?string */
+    protected $archiveName = null;
+    /** @var string[] */
     protected $archiveExcludes = array();
 
     /**
diff --git a/src/Composer/Package/CompletePackageInterface.php b/src/Composer/Package/CompletePackageInterface.php
index 0d9425496..00e4a088e 100644
--- a/src/Composer/Package/CompletePackageInterface.php
+++ b/src/Composer/Package/CompletePackageInterface.php
@@ -22,7 +22,7 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns the scripts of this package
      *
-     * @return array<string, string[]> array('script name' => array('listeners'))
+     * @return array<string, string[]> Map of script name to array of handlers
      */
     public function getScripts();
 
@@ -35,14 +35,14 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns an array of repositories
      *
-     * @return array<array{type: string, url?: string}> Repositories
+     * @return mixed[] Repositories
      */
     public function getRepositories();
 
     /**
      * Set the repositories
      *
-     * @param  array<array{type: string, url?: string}> $repositories
+     * @param  mixed[] $repositories
      * @return void
      */
     public function setRepositories(array $repositories);
@@ -80,7 +80,7 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns the package description
      *
-     * @return string
+     * @return ?string
      */
     public function getDescription();
 
@@ -95,7 +95,7 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns the package homepage
      *
-     * @return string
+     * @return ?string
      */
     public function getHomepage();
 
@@ -127,14 +127,14 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns the support information
      *
-     * @return array<string, string>
+     * @return array{issues?: string, forum?: string, wiki?: string, source?: string, email?: string, irc?: string, docs?: string, rss?: string, chat?: string}
      */
     public function getSupport();
 
     /**
      * Set the support information
      *
-     * @param  array<string, string> $support
+     * @param  array{issues?: string, forum?: string, wiki?: string, source?: string, email?: string, irc?: string, docs?: string, rss?: string, chat?: string} $support
      * @return void
      */
     public function setSupport(array $support);
@@ -144,14 +144,14 @@ interface CompletePackageInterface extends PackageInterface
      *
      * Each item will contain type and url keys
      *
-     * @return array<array{type: string, url: string}>
+     * @return array<array{type?: string, url?: string}>
      */
     public function getFunding();
 
     /**
      * Set the funding
      *
-     * @param  array<array{type: string, url: string}> $funding
+     * @param  array<array{type?: string, url?: string}> $funding
      * @return void
      */
     public function setFunding(array $funding);
@@ -179,7 +179,7 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns default base filename for archive
      *
-     * @return array
+     * @return ?string
      */
     public function getArchiveName();
 
@@ -194,7 +194,7 @@ interface CompletePackageInterface extends PackageInterface
     /**
      * Returns a list of patterns to exclude from package archives
      *
-     * @return array
+     * @return string[]
      */
     public function getArchiveExcludes();
 
diff --git a/src/Composer/Package/Loader/ArrayLoader.php b/src/Composer/Package/Loader/ArrayLoader.php
index 4367b9a3a..0e164b865 100644
--- a/src/Composer/Package/Loader/ArrayLoader.php
+++ b/src/Composer/Package/Loader/ArrayLoader.php
@@ -31,7 +31,9 @@ use Composer\Package\Version\VersionParser;
  */
 class ArrayLoader implements LoaderInterface
 {
+    /** @var VersionParser */
     protected $versionParser;
+    /** @var bool */
     protected $loadOptions;
 
     public function __construct(VersionParser $parser = null, $loadOptions = false)
diff --git a/src/Composer/Package/Loader/InvalidPackageException.php b/src/Composer/Package/Loader/InvalidPackageException.php
index 2f0c845c6..1e9164dd6 100644
--- a/src/Composer/Package/Loader/InvalidPackageException.php
+++ b/src/Composer/Package/Loader/InvalidPackageException.php
@@ -17,8 +17,11 @@ namespace Composer\Package\Loader;
  */
 class InvalidPackageException extends \Exception
 {
+    /** @var string[] */
     private $errors;
+    /** @var string[] */
     private $warnings;
+    /** @var mixed[] package config */
     private $data;
 
     public function __construct(array $errors, array $warnings, array $data)
diff --git a/src/Composer/Package/Loader/JsonLoader.php b/src/Composer/Package/Loader/JsonLoader.php
index 66f75c034..9e9ae689e 100644
--- a/src/Composer/Package/Loader/JsonLoader.php
+++ b/src/Composer/Package/Loader/JsonLoader.php
@@ -21,6 +21,7 @@ use Composer\Package\CompleteAliasPackage;
  */
 class JsonLoader
 {
+    /** @var LoaderInterface */
     private $loader;
 
     public function __construct(LoaderInterface $loader)
diff --git a/src/Composer/Package/Loader/LoaderInterface.php b/src/Composer/Package/Loader/LoaderInterface.php
index 295ddf95b..9a89d68aa 100644
--- a/src/Composer/Package/Loader/LoaderInterface.php
+++ b/src/Composer/Package/Loader/LoaderInterface.php
@@ -12,6 +12,12 @@
 
 namespace Composer\Package\Loader;
 
+use Composer\Package\CompletePackageInterface;
+use Composer\Package\CompletePackage;
+use Composer\Package\CompleteAliasPackage;
+use Composer\Package\RootAliasPackage;
+use Composer\Package\RootPackage;
+
 /**
  * Defines a loader that takes an array to create package instances
  *
@@ -22,9 +28,14 @@ interface LoaderInterface
     /**
      * Converts a package from an array to a real instance
      *
-     * @param  array                              $package Package config
-     * @param  string                             $class   Package class to use
-     * @return \Composer\Package\PackageInterface
+     * @template PackageClass of CompletePackageInterface
+     *
+     * @param  mixed[] $config package data
+     * @param  string  $class  FQCN to be instantiated
+     *
+     * @return CompletePackage|CompleteAliasPackage|RootPackage|RootAliasPackage
+     *
+     * @phpstan-param class-string<PackageClass> $class
      */
-    public function load(array $package, $class = 'Composer\Package\CompletePackage');
+    public function load(array $config, $class = 'Composer\Package\CompletePackage');
 }
diff --git a/src/Composer/Package/Loader/RootPackageLoader.php b/src/Composer/Package/Loader/RootPackageLoader.php
index f2e7ebf86..2089df17f 100644
--- a/src/Composer/Package/Loader/RootPackageLoader.php
+++ b/src/Composer/Package/Loader/RootPackageLoader.php
@@ -59,10 +59,11 @@ class RootPackageLoader extends ArrayLoader
 
     /**
      * @template PackageClass of RootPackage
-     * @param  array                        $config package data
-     * @param  class-string<PackageClass>   $class  FQCN to be instantiated
+     *
      * @param  string                       $cwd    cwd of the root package to be used to guess the version if it is not provided
      * @return RootPackage|RootAliasPackage
+     *
+     * @phpstan-param class-string<PackageClass> $class
      */
     public function load(array $config, $class = 'Composer\Package\RootPackage', $cwd = null)
     {
@@ -183,6 +184,9 @@ class RootPackageLoader extends ArrayLoader
         return $package;
     }
 
+    /**
+     * @return array<array{package: string, version: string, alias: string, alias_normalized: string}>
+     */
     private function extractAliases(array $requires, array $aliases)
     {
         foreach ($requires as $reqName => $reqVersion) {
@@ -203,6 +207,7 @@ class RootPackageLoader extends ArrayLoader
 
     /**
      * @internal
+     * @return array<string, BasePackage::STABILITY_*>
      */
     public static function extractStabilityFlags(array $requires, $minimumStability, array $stabilityFlags)
     {
@@ -259,6 +264,7 @@ class RootPackageLoader extends ArrayLoader
 
     /**
      * @internal
+     * @return array<string, string>
      */
     public static function extractReferences(array $requires, array $references)
     {
diff --git a/src/Composer/Package/Loader/ValidatingArrayLoader.php b/src/Composer/Package/Loader/ValidatingArrayLoader.php
index 62fa84943..8b04a4df5 100644
--- a/src/Composer/Package/Loader/ValidatingArrayLoader.php
+++ b/src/Composer/Package/Loader/ValidatingArrayLoader.php
@@ -27,12 +27,19 @@ class ValidatingArrayLoader implements LoaderInterface
     const CHECK_UNBOUND_CONSTRAINTS = 1;
     const CHECK_STRICT_CONSTRAINTS = 2;
 
+    /** @var LoaderInterface */
     private $loader;
+    /** @var VersionParser */
     private $versionParser;
+    /** @var string[] */
     private $errors;
+    /** @var string[] */
     private $warnings;
+    /** @var mixed[] */
     private $config;
+    /** @var bool */
     private $strictName;
+    /** @var int One or more of self::CHECK_* constants */
     private $flags;
 
     public function __construct(LoaderInterface $loader, $strictName = true, VersionParser $parser = null, $flags = 0)
@@ -388,7 +395,7 @@ class ValidatingArrayLoader implements LoaderInterface
         }
 
         $package = $this->loader->load($this->config, $class);
-        $this->config = null;
+        $this->config = array();
 
         return $package;
     }
diff --git a/src/Composer/Package/Locker.php b/src/Composer/Package/Locker.php
index 2ab1e2052..49894ffc3 100644
--- a/src/Composer/Package/Locker.php
+++ b/src/Composer/Package/Locker.php
@@ -46,8 +46,10 @@ class Locker
     private $dumper;
     /** @var ProcessExecutor */
     private $process;
-    private $lockDataCache;
-    private $virtualFileWritten;
+    /** @var mixed[]|null */
+    private $lockDataCache = null;
+    /** @var bool */
+    private $virtualFileWritten = false;
 
     /**
      * Initializes packages locker.
diff --git a/src/Composer/Package/Package.php b/src/Composer/Package/Package.php
index 2b3ccb1aa..7881978c1 100644
--- a/src/Composer/Package/Package.php
+++ b/src/Composer/Package/Package.php
@@ -34,7 +34,7 @@ class Package extends BasePackage
     protected $sourceUrl;
     /** @var ?string */
     protected $sourceReference;
-    /** @var ?array */
+    /** @var ?array<int, array{url: string, preferred: bool}> */
     protected $sourceMirrors;
     /** @var ?string */
     protected $distType;
@@ -44,7 +44,7 @@ class Package extends BasePackage
     protected $distReference;
     /** @var ?string */
     protected $distSha1Checksum;
-    /** @var ?array */
+    /** @var ?array<int, array{url: string, preferred: bool}> */
     protected $distMirrors;
     /** @var string */
     protected $version;
diff --git a/src/Composer/Package/PackageInterface.php b/src/Composer/Package/PackageInterface.php
index c3df0fd63..cad79a8e0 100644
--- a/src/Composer/Package/PackageInterface.php
+++ b/src/Composer/Package/PackageInterface.php
@@ -55,6 +55,8 @@ interface PackageInterface
      * Allows the solver to set an id for this package to refer to it.
      *
      * @param int $id
+     *
+     * @return void
      */
     public function setId($id);
 
@@ -98,6 +100,8 @@ interface PackageInterface
      *
      * @param string $type source/dist
      * @phpstan-param 'source'|'dist'|null $type
+     *
+     * @return void
      */
     public function setInstallationSource($type);
 
@@ -140,12 +144,12 @@ interface PackageInterface
     /**
      * Returns the source mirrors of this package
      *
-     * @return ?array
+     * @return ?array<int, array{url: string, preferred: bool}>
      */
     public function getSourceMirrors();
 
     /**
-     * @param  ?array $mirrors
+     * @param  ?array<int, array{url: string, preferred: bool}> $mirrors
      * @return void
      */
     public function setSourceMirrors($mirrors);
@@ -188,12 +192,12 @@ interface PackageInterface
     /**
      * Returns the dist mirrors of this package
      *
-     * @return ?array
+     * @return ?array<int, array{url: string, preferred: bool}>
      */
     public function getDistMirrors();
 
     /**
-     * @param  ?array $mirrors
+     * @param  ?array<int, array{url: string, preferred: bool}> $mirrors
      * @return void
      */
     public function setDistMirrors($mirrors);
@@ -326,6 +330,8 @@ interface PackageInterface
      * Stores a reference to the repository that owns the package
      *
      * @param RepositoryInterface $repository
+     *
+     * @return void
      */
     public function setRepository(RepositoryInterface $repository);
 
@@ -386,6 +392,8 @@ interface PackageInterface
     /**
      * Configures the list of options to download package dist files
      *
+     * @param mixed[] $options
+     *
      * @return void
      */
     public function setTransportOptions(array $options);
diff --git a/src/Composer/Package/RootPackage.php b/src/Composer/Package/RootPackage.php
index 945bfd17e..25f062056 100644
--- a/src/Composer/Package/RootPackage.php
+++ b/src/Composer/Package/RootPackage.php
@@ -21,11 +21,17 @@ class RootPackage extends CompletePackage implements RootPackageInterface
 {
     const DEFAULT_PRETTY_VERSION = '1.0.0+no-version-set';
 
+    /** @var string */
     protected $minimumStability = 'stable';
+    /** @var bool */
     protected $preferStable = false;
+    /** @var array<string, BasePackage::STABILITY_*> Map of package name to stability constant */
     protected $stabilityFlags = array();
+    /** @var mixed[] */
     protected $config = array();
+    /** @var array<string, string> Map of package name to reference/commit hash */
     protected $references = array();
+    /** @var array<array{package: string, version: string, alias: string, alias_normalized: string}> */
     protected $aliases = array();
 
     /**
diff --git a/src/Composer/Package/RootPackageInterface.php b/src/Composer/Package/RootPackageInterface.php
index ecd72e444..1092832dc 100644
--- a/src/Composer/Package/RootPackageInterface.php
+++ b/src/Composer/Package/RootPackageInterface.php
@@ -22,7 +22,7 @@ interface RootPackageInterface extends CompletePackageInterface
     /**
      * Returns a set of package names and their aliases
      *
-     * @return array
+     * @return array<array{package: string, version: string, alias: string, alias_normalized: string}>
      */
     public function getAliases();
 
@@ -38,7 +38,7 @@ interface RootPackageInterface extends CompletePackageInterface
      *
      * array('foo/bar' => 'dev')
      *
-     * @return array
+     * @return array<string, BasePackage::STABILITY_*>
      */
     public function getStabilityFlags();
 
@@ -47,7 +47,7 @@ interface RootPackageInterface extends CompletePackageInterface
      *
      * array('foo/bar' => 'abcd1234')
      *
-     * @return array
+     * @return array<string, string>
      */
     public function getReferences();
 
@@ -61,7 +61,7 @@ interface RootPackageInterface extends CompletePackageInterface
     /**
      * Returns the root package's configuration
      *
-     * @return array
+     * @return mixed[]
      */
     public function getConfig();
 
@@ -69,6 +69,8 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the required packages
      *
      * @param Link[] $requires A set of package links
+     *
+     * @return void
      */
     public function setRequires(array $requires);
 
@@ -76,6 +78,8 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the recommended packages
      *
      * @param Link[] $devRequires A set of package links
+     *
+     * @return void
      */
     public function setDevRequires(array $devRequires);
 
@@ -83,6 +87,8 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the conflicting packages
      *
      * @param Link[] $conflicts A set of package links
+     *
+     * @return void
      */
     public function setConflicts(array $conflicts);
 
@@ -90,6 +96,8 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the provided virtual packages
      *
      * @param Link[] $provides A set of package links
+     *
+     * @return void
      */
     public function setProvides(array $provides);
 
@@ -97,34 +105,44 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the packages this one replaces
      *
      * @param Link[] $replaces A set of package links
+     *
+     * @return void
      */
     public function setReplaces(array $replaces);
 
     /**
      * Set the repositories
      *
-     * @param array $repositories
+     * @param mixed[] $repositories
+     *
+     * @return void
      */
     public function setRepositories(array $repositories);
 
     /**
      * Set the autoload mapping
      *
-     * @param array $autoload Mapping of autoloading rules
+     * @param array{psr-0?: array<string, string|string[]>, psr-4?: array<string, string|string[]>, classmap?: list<string>, files?: list<string>} $autoload Mapping of autoloading rules
+     *
+     * @return void
      */
     public function setAutoload(array $autoload);
 
     /**
      * Set the dev autoload mapping
      *
-     * @param array $devAutoload Mapping of dev autoloading rules
+     * @param array{psr-0?: array<string, string|string[]>, psr-4?: array<string, string|string[]>, classmap?: list<string>, files?: list<string>} $devAutoload Mapping of dev autoloading rules
+     *
+     * @return void
      */
     public function setDevAutoload(array $devAutoload);
 
     /**
      * Set the stabilityFlags
      *
-     * @param array $stabilityFlags
+     * @param array<string, BasePackage::STABILITY_*> $stabilityFlags
+     *
+     * @return void
      */
     public function setStabilityFlags(array $stabilityFlags);
 
@@ -132,6 +150,8 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the minimumStability
      *
      * @param string $minimumStability
+     *
+     * @return void
      */
     public function setMinimumStability($minimumStability);
 
@@ -139,39 +159,51 @@ interface RootPackageInterface extends CompletePackageInterface
      * Set the preferStable
      *
      * @param bool $preferStable
+     *
+     * @return void
      */
     public function setPreferStable($preferStable);
 
     /**
      * Set the config
      *
-     * @param array $config
+     * @param mixed[] $config
+     *
+     * @return void
      */
     public function setConfig(array $config);
 
     /**
      * Set the references
      *
-     * @param array $references
+     * @param array<string, string> $references
+     *
+     * @return void
      */
     public function setReferences(array $references);
 
     /**
      * Set the aliases
      *
-     * @param array $aliases
+     * @param array<array{package: string, version: string, alias: string, alias_normalized: string}> $aliases
+     *
+     * @return void
      */
     public function setAliases(array $aliases);
 
     /**
      * Set the suggested packages
      *
-     * @param array $suggests A set of package names/comments
+     * @param array<string, string> $suggests A set of package names/comments
+     *
+     * @return void
      */
     public function setSuggests(array $suggests);
 
     /**
-     * @param array $extra
+     * @param mixed[] $extra
+     *
+     * @return void
      */
     public function setExtra(array $extra);
 }
diff --git a/src/Composer/Package/Version/VersionParser.php b/src/Composer/Package/Version/VersionParser.php
index df3e9feb2..455162726 100644
--- a/src/Composer/Package/Version/VersionParser.php
+++ b/src/Composer/Package/Version/VersionParser.php
@@ -15,11 +15,13 @@ namespace Composer\Package\Version;
 use Composer\Repository\PlatformRepository;
 use Composer\Semver\VersionParser as SemverVersionParser;
 use Composer\Semver\Semver;
+use Composer\Semver\Constraint\ConstraintInterface;
 
 class VersionParser extends SemverVersionParser
 {
     const DEFAULT_BRANCH_ALIAS = '9999999-dev';
 
+    /** @var array<string, ConstraintInterface> Constraint parsing cache */
     private static $constraints = array();
 
     /**
diff --git a/src/Composer/Package/Version/VersionSelector.php b/src/Composer/Package/Version/VersionSelector.php
index ab5355d1a..641575488 100644
--- a/src/Composer/Package/Version/VersionSelector.php
+++ b/src/Composer/Package/Version/VersionSelector.php
@@ -21,6 +21,7 @@ use Composer\Package\Dumper\ArrayDumper;
 use Composer\Repository\RepositorySet;
 use Composer\Repository\PlatformRepository;
 use Composer\Semver\Constraint\Constraint;
+use Composer\Semver\Constraint\ConstraintInterface;
 
 /**
  * Selects the best possible version for a package
@@ -30,10 +31,13 @@ use Composer\Semver\Constraint\Constraint;
  */
 class VersionSelector
 {
+    /** @var RepositorySet */
     private $repositorySet;
 
+    /** @var array<string, ConstraintInterface[]> */
     private $platformConstraints = array();
 
+    /** @var VersionParser */
     private $parser;
 
     /**
diff --git a/src/Composer/Plugin/PluginInterface.php b/src/Composer/Plugin/PluginInterface.php
index 305a784bc..c7c42c5f9 100644
--- a/src/Composer/Plugin/PluginInterface.php
+++ b/src/Composer/Plugin/PluginInterface.php
@@ -39,6 +39,8 @@ interface PluginInterface
      *
      * @param Composer    $composer
      * @param IOInterface $io
+     *
+     * @return void
      */
     public function activate(Composer $composer, IOInterface $io);
 
@@ -51,6 +53,8 @@ interface PluginInterface
      *
      * @param Composer    $composer
      * @param IOInterface $io
+     *
+     * @return void
      */
     public function deactivate(Composer $composer, IOInterface $io);
 
@@ -61,6 +65,8 @@ interface PluginInterface
      *
      * @param Composer    $composer
      * @param IOInterface $io
+     *
+     * @return void
      */
     public function uninstall(Composer $composer, IOInterface $io);
 }
diff --git a/src/Composer/Repository/ComposerRepository.php b/src/Composer/Repository/ComposerRepository.php
index 75b3aabec..3310da516 100644
--- a/src/Composer/Repository/ComposerRepository.php
+++ b/src/Composer/Repository/ComposerRepository.php
@@ -91,9 +91,9 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     private $allowSslDowngrade = false;
     /** @var ?EventDispatcher */
     private $eventDispatcher;
-    /** @var ?array<string, array{url: string, preferred: bool}> */
+    /** @var ?array<string, array<int, array{url: string, preferred: bool}>> */
     private $sourceMirrors;
-    /** @var ?array<string, array{url: string, preferred: bool}> */
+    /** @var ?array<int, array{url: string, preferred: bool}> */
     private $distMirrors;
     /** @var bool */
     private $degradedMode = false;
diff --git a/src/Composer/Repository/ConfigurableRepositoryInterface.php b/src/Composer/Repository/ConfigurableRepositoryInterface.php
index ff202dc88..46a535830 100644
--- a/src/Composer/Repository/ConfigurableRepositoryInterface.php
+++ b/src/Composer/Repository/ConfigurableRepositoryInterface.php
@@ -19,5 +19,8 @@ namespace Composer\Repository;
  */
 interface ConfigurableRepositoryInterface
 {
+    /**
+     * @return mixed[]
+     */
     public function getRepoConfig();
 }
diff --git a/src/Composer/Repository/Vcs/PerforceDriver.php b/src/Composer/Repository/Vcs/PerforceDriver.php
index 76249fd54..c6f9fc816 100644
--- a/src/Composer/Repository/Vcs/PerforceDriver.php
+++ b/src/Composer/Repository/Vcs/PerforceDriver.php
@@ -47,8 +47,6 @@ class PerforceDriver extends VcsDriver
 
         $this->perforce->writeP4ClientSpec();
         $this->perforce->connectClient();
-
-        return true;
     }
 
     private function initPerforce($repoConfig)
diff --git a/src/Composer/Repository/Vcs/VcsDriverInterface.php b/src/Composer/Repository/Vcs/VcsDriverInterface.php
index 22cb33007..cdddd022b 100644
--- a/src/Composer/Repository/Vcs/VcsDriverInterface.php
+++ b/src/Composer/Repository/Vcs/VcsDriverInterface.php
@@ -22,14 +22,16 @@ interface VcsDriverInterface
 {
     /**
      * Initializes the driver (git clone, svn checkout, fetch info etc)
+     *
+     * @return void
      */
     public function initialize();
 
     /**
      * Return the composer.json file information
      *
-     * @param  string $identifier Any identifier to a specific branch/tag/commit
-     * @return array  containing all infos from the composer.json file
+     * @param  string  $identifier Any identifier to a specific branch/tag/commit
+     * @return mixed[] containing all infos from the composer.json file
      */
     public function getComposerInformation($identifier);
 
@@ -60,26 +62,28 @@ interface VcsDriverInterface
     /**
      * Return list of branches in the repository
      *
-     * @return array Branch names as keys, identifiers as values
+     * @return array<string, string> Branch names as keys, identifiers as values
      */
     public function getBranches();
 
     /**
      * Return list of tags in the repository
      *
-     * @return array Tag names as keys, identifiers as values
+     * @return array<string, string> Tag names as keys, identifiers as values
      */
     public function getTags();
 
     /**
-     * @param  string     $identifier Any identifier to a specific branch/tag/commit
-     * @return array|null With type, url reference and shasum keys.
+     * @param string $identifier Any identifier to a specific branch/tag/commit
+     *
+     * @return array{type: string, url: string, reference: string, shasum: string}|null
      */
     public function getDist($identifier);
 
     /**
-     * @param  string $identifier Any identifier to a specific branch/tag/commit
-     * @return array  With type, url and reference keys.
+     * @param string $identifier Any identifier to a specific branch/tag/commit
+     *
+     * @return array{type: string, url: string, reference: string}
      */
     public function getSource($identifier);
 
@@ -101,6 +105,8 @@ interface VcsDriverInterface
 
     /**
      * Performs any cleanup necessary as the driver is not longer needed
+     *
+     * @return void
      */
     public function cleanup();
 
diff --git a/src/Composer/Repository/VersionCacheInterface.php b/src/Composer/Repository/VersionCacheInterface.php
index 3b05fb8ff..80de9833c 100644
--- a/src/Composer/Repository/VersionCacheInterface.php
+++ b/src/Composer/Repository/VersionCacheInterface.php
@@ -15,9 +15,9 @@ namespace Composer\Repository;
 interface VersionCacheInterface
 {
     /**
-     * @param  string           $version
-     * @param  string           $identifier
-     * @return array|null|false Package version data if found, false to indicate the identifier is known but has no package, null for an unknown identifier
+     * @param  string             $version
+     * @param  string             $identifier
+     * @return mixed[]|null|false Package version data if found, false to indicate the identifier is known but has no package, null for an unknown identifier
      */
     public function getVersionPackage($version, $identifier);
 }
diff --git a/src/Composer/Repository/WritableRepositoryInterface.php b/src/Composer/Repository/WritableRepositoryInterface.php
index f1d8b9e95..376af2108 100644
--- a/src/Composer/Repository/WritableRepositoryInterface.php
+++ b/src/Composer/Repository/WritableRepositoryInterface.php
@@ -26,6 +26,7 @@ interface WritableRepositoryInterface extends RepositoryInterface
      * Writes repository (f.e. to the disc).
      *
      * @param bool $devMode Whether dev requirements were included or not in this installation
+     * @return void
      */
     public function write($devMode, InstallationManager $installationManager);
 
@@ -33,6 +34,7 @@ interface WritableRepositoryInterface extends RepositoryInterface
      * Adds package to the repository.
      *
      * @param PackageInterface $package package instance
+     * @return void
      */
     public function addPackage(PackageInterface $package);
 
@@ -40,6 +42,7 @@ interface WritableRepositoryInterface extends RepositoryInterface
      * Removes package from the repository.
      *
      * @param PackageInterface $package package instance
+     * @return void
      */
     public function removePackage(PackageInterface $package);
 
@@ -52,11 +55,14 @@ interface WritableRepositoryInterface extends RepositoryInterface
 
     /**
      * Forces a reload of all packages.
+     *
+     * @return void
      */
     public function reload();
 
     /**
      * @param string[] $devPackageNames
+     * @return void
      */
     public function setDevPackageNames(array $devPackageNames);
 
diff --git a/src/Composer/Util/AuthHelper.php b/src/Composer/Util/AuthHelper.php
index b8dc44407..4c012ba92 100644
--- a/src/Composer/Util/AuthHelper.php
+++ b/src/Composer/Util/AuthHelper.php
@@ -120,8 +120,11 @@ class AuthHelper
             $message = "\n".'Could not fetch '.$url.', enter your ' . $origin . ' credentials ' .($statusCode === 401 ? 'to access private repos' : 'to go over the API rate limit');
             $gitLabUtil = new GitLab($this->io, $this->config, null);
 
-            if ($this->io->hasAuthentication($origin) && ($auth = $this->io->getAuthentication($origin)) && in_array($auth['password'], array('gitlab-ci-token', 'private-token', 'oauth2'), true)) {
-                throw new TransportException("Invalid credentials for '" . $url . "', aborting.", $statusCode);
+            if ($this->io->hasAuthentication($origin)) {
+                $auth = $this->io->getAuthentication($origin);
+                if (in_array($auth['password'], array('gitlab-ci-token', 'private-token', 'oauth2'), true)) {
+                    throw new TransportException("Invalid credentials for '" . $url . "', aborting.", $statusCode);
+                }
             }
 
             if (!$gitLabUtil->authorizeOAuth($origin)
diff --git a/src/Composer/Util/Http/ProxyManager.php b/src/Composer/Util/Http/ProxyManager.php
index 586b08e83..659c715c8 100644
--- a/src/Composer/Util/Http/ProxyManager.php
+++ b/src/Composer/Util/Http/ProxyManager.php
@@ -22,13 +22,20 @@ use Composer\Util\Url;
  */
 class ProxyManager
 {
-    private $error;
+    /** @var ?string */
+    private $error = null;
+    /** @var array{http: ?string, https: ?string} */
     private $fullProxy;
+    /** @var array{http: ?string, https: ?string} */
     private $safeProxy;
+    /** @var array{http: array{options: mixed[]|null}, https: array{options: mixed[]|null}} */
     private $streams;
+    /** @var bool */
     private $hasProxy;
-    private $info;
-    private $lastProxy;
+    /** @var ?string */
+    private $info = null;
+    /** @var ?string */
+    private $lastProxy = null;
     /** @var ?NoProxyPattern */
     private $noProxyHandler = null;
 
@@ -154,7 +161,7 @@ class ProxyManager
      * Sets initial data
      *
      * @param string $url    Proxy url
-     * @param string $scheme Environment variable scheme
+     * @param 'http'|'https' $scheme Environment variable scheme
      */
     private function setData($url, $scheme)
     {
diff --git a/src/Composer/Util/Perforce.php b/src/Composer/Util/Perforce.php
index ea2b1152c..e316661fd 100644
--- a/src/Composer/Util/Perforce.php
+++ b/src/Composer/Util/Perforce.php
@@ -20,23 +20,39 @@ use Symfony\Component\Process\Process;
  */
 class Perforce
 {
+    /** @var string */
     protected $path;
+    /** @var ?string */
     protected $p4Depot;
+    /** @var string */
     protected $p4Client;
+    /** @var ?string */
     protected $p4User;
+    /** @var ?string */
     protected $p4Password;
+    /** @var int */
     protected $p4Port;
+    /** @var string */
     protected $p4Stream;
+    /** @var string */
     protected $p4ClientSpec;
+    /** @var ?string */
     protected $p4DepotType;
+    /** @var ?string */
     protected $p4Branch;
+    /** @var ProcessExecutor */
     protected $process;
+    /** @var string */
     protected $uniquePerforceClientName;
+    /** @var bool */
     protected $windowsFlag;
+    /** @var string */
     protected $commandResult;
 
+    /** @var IOInterface */
     protected $io;
 
+    /** @var Filesystem */
     protected $filesystem;
 
     public function __construct($repoConfig, $port, $path, ProcessExecutor $process, $isWindows, IOInterface $io)