diff --git a/phpstan/baseline.neon b/phpstan/baseline.neon
index 763f3321949f..8508c981fb48 100644
--- a/phpstan/baseline.neon
+++ b/phpstan/baseline.neon
@@ -1095,11 +1095,6 @@ parameters:
 			count: 1
 			path: ../src/Composer/Command/ShowCommand.php
 
-		-
-			message: "#^Only booleans are allowed in &&, Composer\\\\Package\\\\PackageInterface\\|false\\|null given on the right side\\.$#"
-			count: 1
-			path: ../src/Composer/Command/ShowCommand.php
-
 		-
 			message: "#^Only booleans are allowed in &&, Composer\\\\Package\\\\PackageInterface\\|null given on the left side\\.$#"
 			count: 1
@@ -3530,11 +3525,6 @@ parameters:
 			count: 1
 			path: ../src/Composer/Package/Loader/ArrayLoader.php
 
-		-
-			message: "#^Method Composer\\\\Package\\\\Loader\\\\ArrayLoader\\:\\:createObject\\(\\) should return Composer\\\\Package\\\\CompletePackage but returns PackageClass of Composer\\\\Package\\\\CompletePackageInterface\\.$#"
-			count: 1
-			path: ../src/Composer/Package/Loader/ArrayLoader.php
-
 		-
 			message: "#^Only booleans are allowed in &&, string\\|false given on the left side\\.$#"
 			count: 1
@@ -3616,12 +3606,12 @@ parameters:
 			path: ../src/Composer/Package/Loader/RootPackageLoader.php
 
 		-
-			message: "#^Parameter \\#2 \\$class \\(class\\-string\\<PackageClass of Composer\\\\Package\\\\RootPackage\\>\\) of method Composer\\\\Package\\\\Loader\\\\RootPackageLoader\\:\\:load\\(\\) should be contravariant with parameter \\$class \\(class\\-string\\<Composer\\\\Package\\\\CompletePackageInterface\\>\\) of method Composer\\\\Package\\\\Loader\\\\ArrayLoader\\:\\:load\\(\\)$#"
+			message: "#^Parameter \\#2 \\$class \\(class\\-string\\<Composer\\\\Package\\\\RootPackage\\>\\) of method Composer\\\\Package\\\\Loader\\\\RootPackageLoader\\:\\:load\\(\\) should be contravariant with parameter \\$class \\(class\\-string\\<Composer\\\\Package\\\\CompletePackage\\>\\) of method Composer\\\\Package\\\\Loader\\\\ArrayLoader\\:\\:load\\(\\)$#"
 			count: 1
 			path: ../src/Composer/Package/Loader/RootPackageLoader.php
 
 		-
-			message: "#^Parameter \\#2 \\$class \\(class\\-string\\<PackageClass of Composer\\\\Package\\\\RootPackage\\>\\) of method Composer\\\\Package\\\\Loader\\\\RootPackageLoader\\:\\:load\\(\\) should be contravariant with parameter \\$class \\(class\\-string\\<Composer\\\\Package\\\\CompletePackageInterface\\>\\) of method Composer\\\\Package\\\\Loader\\\\LoaderInterface\\:\\:load\\(\\)$#"
+			message: "#^Parameter \\#2 \\$class \\(class\\-string\\<Composer\\\\Package\\\\RootPackage\\>\\) of method Composer\\\\Package\\\\Loader\\\\RootPackageLoader\\:\\:load\\(\\) should be contravariant with parameter \\$class \\(class\\-string\\<Composer\\\\Package\\\\CompletePackage\\>\\) of method Composer\\\\Package\\\\Loader\\\\LoaderInterface\\:\\:load\\(\\)$#"
 			count: 1
 			path: ../src/Composer/Package/Loader/RootPackageLoader.php
 
@@ -4595,11 +4585,6 @@ parameters:
 			count: 10
 			path: ../src/Composer/Repository/Vcs/GitBitbucketDriver.php
 
-		-
-			message: "#^Only booleans are allowed in an if condition, mixed given\\.$#"
-			count: 1
-			path: ../src/Composer/Repository/Vcs/GitBitbucketDriver.php
-
 		-
 			message: "#^Property Composer\\\\Repository\\\\Vcs\\\\GitBitbucketDriver\\:\\:\\$branches \\(array\\<string, string\\>\\) does not accept array\\<int\\|string, mixed\\>\\.$#"
 			count: 1
@@ -4720,11 +4705,6 @@ parameters:
 			count: 1
 			path: ../src/Composer/Repository/Vcs/GitHubDriver.php
 
-		-
-			message: "#^Only booleans are allowed in an if condition, mixed given\\.$#"
-			count: 1
-			path: ../src/Composer/Repository/Vcs/GitHubDriver.php
-
 		-
 			message: "#^Only booleans are allowed in an if condition, string\\|null given\\.$#"
 			count: 2
@@ -4797,7 +4777,7 @@ parameters:
 
 		-
 			message: "#^Only booleans are allowed in an if condition, mixed given\\.$#"
-			count: 2
+			count: 1
 			path: ../src/Composer/Repository/Vcs/GitLabDriver.php
 
 		-
@@ -5020,11 +5000,6 @@ parameters:
 			count: 1
 			path: ../src/Composer/Repository/VcsRepository.php
 
-		-
-			message: "#^Only booleans are allowed in a negated boolean, array given\\.$#"
-			count: 2
-			path: ../src/Composer/Repository/VcsRepository.php
-
 		-
 			message: "#^Only booleans are allowed in a negated boolean, array\\<Composer\\\\Package\\\\BasePackage\\> given\\.$#"
 			count: 1
diff --git a/src/Composer/Autoload/AutoloadGenerator.php b/src/Composer/Autoload/AutoloadGenerator.php
index 825dc17bc930..5485565e0dd0 100644
--- a/src/Composer/Autoload/AutoloadGenerator.php
+++ b/src/Composer/Autoload/AutoloadGenerator.php
@@ -1364,12 +1364,7 @@ protected function sortPackageMap(array $packageMap)
     }
 }
 
-/**
- * @param string $fileIdentifier
- * @param string $file
- * @return void
- */
-function composerRequire(string $fileIdentifier, string $file)
+function composerRequire(string $fileIdentifier, string $file): void
 {
     if (empty($GLOBALS['__composer_autoload_files'][$fileIdentifier])) {
         $GLOBALS['__composer_autoload_files'][$fileIdentifier] = true;
diff --git a/src/Composer/Command/InitCommand.php b/src/Composer/Command/InitCommand.php
index 4f3cebbd838f..1d4dea29bc14 100644
--- a/src/Composer/Command/InitCommand.php
+++ b/src/Composer/Command/InitCommand.php
@@ -459,7 +459,7 @@ function ($value) use ($autoload) {
      * @param  string $author
      * @return array{name: string, email: string|null}
      */
-    private function parseAuthorString(string $author)
+    private function parseAuthorString(string $author): array
     {
         if (Preg::isMatch('/^(?P<name>[- .,\p{L}\p{N}\p{Mn}\'’"()]+)(?:\s+<(?P<email>.+?)>)?$/u', $author, $match)) {
             $hasEmail = isset($match['email']) && '' !== $match['email'];
diff --git a/src/Composer/Command/ShowCommand.php b/src/Composer/Command/ShowCommand.php
index 775d858e934e..8b11a8e33156 100644
--- a/src/Composer/Command/ShowCommand.php
+++ b/src/Composer/Command/ShowCommand.php
@@ -294,7 +294,7 @@ protected function execute(InputInterface $input, OutputInterface $output)
             if (
                 $input->getOption('outdated')
                 && $input->getOption('strict')
-                && $latestPackage
+                && null !== $latestPackage
                 && $latestPackage->getFullPrettyVersion() !== $package->getFullPrettyVersion()
                 && (!$latestPackage instanceof CompletePackageInterface || !$latestPackage->isAbandoned())
             ) {
@@ -415,7 +415,7 @@ protected function execute(InputInterface $input, OutputInterface $output)
                     foreach ($packages[$type] as $package) {
                         if (is_object($package)) {
                             $latestPackage = $this->findLatestPackage($package, $composer, $platformRepo, $showMinorOnly, $platformReqFilter);
-                            if ($latestPackage === false) {
+                            if ($latestPackage === null) {
                                 continue;
                             }
 
@@ -628,7 +628,7 @@ protected function getVersionStyle(PackageInterface $latestPackage, PackageInter
      * @throws \InvalidArgumentException
      * @return array{CompletePackageInterface|null, array<string, string>}
      */
-    protected function getPackage(InstalledRepository $installedRepo, RepositoryInterface $repos, string $name, $version = null)
+    protected function getPackage(InstalledRepository $installedRepo, RepositoryInterface $repos, string $name, $version = null): array
     {
         $name = strtolower($name);
         $constraint = is_string($version) ? $this->versionParser->parseConstraints($version) : $version;
@@ -1287,10 +1287,8 @@ private function writeTreeLine(string $line): void
 
     /**
      * Given a package, this finds the latest package matching it
-     *
-     * @return PackageInterface|false
      */
-    private function findLatestPackage(PackageInterface $package, Composer $composer, PlatformRepository $platformRepo, bool $minorOnly, PlatformRequirementFilterInterface $platformReqFilter)
+    private function findLatestPackage(PackageInterface $package, Composer $composer, PlatformRepository $platformRepo, bool $minorOnly, PlatformRequirementFilterInterface $platformReqFilter): ?PackageInterface
     {
         // find the latest version allowed in this repo set
         $name = $package->getName();
@@ -1320,7 +1318,7 @@ private function findLatestPackage(PackageInterface $package, Composer $composer
             $candidate = $candidate->getAliasOf();
         }
 
-        return $candidate;
+        return $candidate !== false ? $candidate : null;
     }
 
     /**
diff --git a/src/Composer/DependencyResolver/Decisions.php b/src/Composer/DependencyResolver/Decisions.php
index 837488681f47..f9e3490244c3 100644
--- a/src/Composer/DependencyResolver/Decisions.php
+++ b/src/Composer/DependencyResolver/Decisions.php
@@ -288,7 +288,7 @@ public function toString(Pool $pool = null): string
         return $str;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->toString();
     }
diff --git a/src/Composer/DependencyResolver/Pool.php b/src/Composer/DependencyResolver/Pool.php
index 94c895c28d14..1abbc6382b89 100644
--- a/src/Composer/DependencyResolver/Pool.php
+++ b/src/Composer/DependencyResolver/Pool.php
@@ -268,7 +268,7 @@ public function getUnacceptableFixedOrLockedPackages(): array
         return $this->unacceptableFixedOrLockedPackages;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         $str = "Pool:\n";
 
diff --git a/src/Composer/DependencyResolver/Rule.php b/src/Composer/DependencyResolver/Rule.php
index c1cc491d2254..bcf32fb03fd0 100644
--- a/src/Composer/DependencyResolver/Rule.php
+++ b/src/Composer/DependencyResolver/Rule.php
@@ -78,7 +78,7 @@ abstract public function getLiterals(): array;
      */
     abstract public function getHash();
 
-    abstract public function __toString();
+    abstract public function __toString(): string;
 
     /**
      * @param Rule $rule
diff --git a/src/Composer/DependencyResolver/RuleSet.php b/src/Composer/DependencyResolver/RuleSet.php
index 9fe82e24ecd6..d430b46a9339 100644
--- a/src/Composer/DependencyResolver/RuleSet.php
+++ b/src/Composer/DependencyResolver/RuleSet.php
@@ -195,7 +195,7 @@ public function getPrettyString(RepositorySet $repositorySet = null, Request $re
         return $string;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->getPrettyString();
     }
diff --git a/src/Composer/EventDispatcher/EventDispatcher.php b/src/Composer/EventDispatcher/EventDispatcher.php
index 941fc252041e..d0cd437bd66b 100644
--- a/src/Composer/EventDispatcher/EventDispatcher.php
+++ b/src/Composer/EventDispatcher/EventDispatcher.php
@@ -81,7 +81,7 @@ public function __construct(PartialComposer $composer, IOInterface $io, ProcessE
      * @param bool $runScripts
      * @return $this
      */
-    public function setRunScripts(bool $runScripts = true)
+    public function setRunScripts(bool $runScripts = true): self
     {
         $this->runScripts = (bool) $runScripts;
 
diff --git a/src/Composer/InstalledVersions.php b/src/Composer/InstalledVersions.php
index d50e0c9fcc47..41bc143c1148 100644
--- a/src/Composer/InstalledVersions.php
+++ b/src/Composer/InstalledVersions.php
@@ -21,6 +21,8 @@
  * See also https://getcomposer.org/doc/07-runtime.md#installed-versions
  *
  * To require its presence, you can require `composer-runtime-api ^2.0`
+ *
+ * @final
  */
 class InstalledVersions
 {
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index fd3cb1417fe8..d72525b334bd 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -1030,7 +1030,7 @@ private function createPoolOptimizer(PolicyInterface $policy): ?PoolOptimizer
      * @param  Composer    $composer
      * @return Installer
      */
-    public static function create(IOInterface $io, Composer $composer): Installer
+    public static function create(IOInterface $io, Composer $composer): self
     {
         return new static(
             $io,
@@ -1049,7 +1049,7 @@ public static function create(IOInterface $io, Composer $composer): Installer
      * @param  RepositoryInterface $additionalFixedRepository
      * @return $this
      */
-    public function setAdditionalFixedRepository(RepositoryInterface $additionalFixedRepository)
+    public function setAdditionalFixedRepository(RepositoryInterface $additionalFixedRepository): self
     {
         $this->additionalFixedRepository = $additionalFixedRepository;
 
@@ -1062,7 +1062,7 @@ public function setAdditionalFixedRepository(RepositoryInterface $additionalFixe
      * @param  bool      $dryRun
      * @return Installer
      */
-    public function setDryRun(bool $dryRun = true): Installer
+    public function setDryRun(bool $dryRun = true): self
     {
         $this->dryRun = (bool) $dryRun;
 
@@ -1085,7 +1085,7 @@ public function isDryRun(): bool
      * @param  bool      $preferSource
      * @return Installer
      */
-    public function setPreferSource(bool $preferSource = true): Installer
+    public function setPreferSource(bool $preferSource = true): self
     {
         $this->preferSource = (bool) $preferSource;
 
@@ -1098,7 +1098,7 @@ public function setPreferSource(bool $preferSource = true): Installer
      * @param  bool      $preferDist
      * @return Installer
      */
-    public function setPreferDist(bool $preferDist = true): Installer
+    public function setPreferDist(bool $preferDist = true): self
     {
         $this->preferDist = (bool) $preferDist;
 
@@ -1111,7 +1111,7 @@ public function setPreferDist(bool $preferDist = true): Installer
      * @param  bool      $optimizeAutoloader
      * @return Installer
      */
-    public function setOptimizeAutoloader(bool $optimizeAutoloader): Installer
+    public function setOptimizeAutoloader(bool $optimizeAutoloader): self
     {
         $this->optimizeAutoloader = (bool) $optimizeAutoloader;
         if (!$this->optimizeAutoloader) {
@@ -1130,7 +1130,7 @@ public function setOptimizeAutoloader(bool $optimizeAutoloader): Installer
      * @param  bool      $classMapAuthoritative
      * @return Installer
      */
-    public function setClassMapAuthoritative(bool $classMapAuthoritative): Installer
+    public function setClassMapAuthoritative(bool $classMapAuthoritative): self
     {
         $this->classMapAuthoritative = (bool) $classMapAuthoritative;
         if ($this->classMapAuthoritative) {
@@ -1148,7 +1148,7 @@ public function setClassMapAuthoritative(bool $classMapAuthoritative): Installer
      * @param  string|null $apcuAutoloaderPrefix
      * @return Installer
      */
-    public function setApcuAutoloader(bool $apcuAutoloader, ?string $apcuAutoloaderPrefix = null): Installer
+    public function setApcuAutoloader(bool $apcuAutoloader, ?string $apcuAutoloaderPrefix = null): self
     {
         $this->apcuAutoloader = $apcuAutoloader;
         $this->apcuAutoloaderPrefix = $apcuAutoloaderPrefix;
@@ -1162,7 +1162,7 @@ public function setApcuAutoloader(bool $apcuAutoloader, ?string $apcuAutoloaderP
      * @param  bool      $update
      * @return Installer
      */
-    public function setUpdate(bool $update): Installer
+    public function setUpdate(bool $update): self
     {
         $this->update = (bool) $update;
 
@@ -1175,7 +1175,7 @@ public function setUpdate(bool $update): Installer
      * @param  bool      $install
      * @return Installer
      */
-    public function setInstall(bool $install): Installer
+    public function setInstall(bool $install): self
     {
         $this->install = (bool) $install;
 
@@ -1188,7 +1188,7 @@ public function setInstall(bool $install): Installer
      * @param  bool      $devMode
      * @return Installer
      */
-    public function setDevMode(bool $devMode = true): Installer
+    public function setDevMode(bool $devMode = true): self
     {
         $this->devMode = (bool) $devMode;
 
@@ -1203,7 +1203,7 @@ public function setDevMode(bool $devMode = true): Installer
      * @param  bool      $dumpAutoloader
      * @return Installer
      */
-    public function setDumpAutoloader(bool $dumpAutoloader = true): Installer
+    public function setDumpAutoloader(bool $dumpAutoloader = true): self
     {
         $this->dumpAutoloader = (bool) $dumpAutoloader;
 
@@ -1219,7 +1219,7 @@ public function setDumpAutoloader(bool $dumpAutoloader = true): Installer
      * @return Installer
      * @deprecated Use setRunScripts(false) on the EventDispatcher instance being injected instead
      */
-    public function setRunScripts(bool $runScripts = true): Installer
+    public function setRunScripts(bool $runScripts = true): self
     {
         $this->runScripts = (bool) $runScripts;
 
@@ -1232,7 +1232,7 @@ public function setRunScripts(bool $runScripts = true): Installer
      * @param  Config    $config
      * @return Installer
      */
-    public function setConfig(Config $config): Installer
+    public function setConfig(Config $config): self
     {
         $this->config = $config;
 
@@ -1245,7 +1245,7 @@ public function setConfig(Config $config): Installer
      * @param  bool      $verbose
      * @return Installer
      */
-    public function setVerbose(bool $verbose = true): Installer
+    public function setVerbose(bool $verbose = true): self
     {
         $this->verbose = (bool) $verbose;
 
@@ -1275,7 +1275,7 @@ public function isVerbose(): bool
      *
      * @deprecated use setPlatformRequirementFilter instead
      */
-    public function setIgnorePlatformRequirements($ignorePlatformReqs): Installer
+    public function setIgnorePlatformRequirements($ignorePlatformReqs): self
     {
         trigger_error('Installer::setIgnorePlatformRequirements is deprecated since Composer 2.2, use setPlatformRequirementFilter instead.', E_USER_DEPRECATED);
 
@@ -1286,7 +1286,7 @@ public function setIgnorePlatformRequirements($ignorePlatformReqs): Installer
      * @param PlatformRequirementFilterInterface $platformRequirementFilter
      * @return Installer
      */
-    public function setPlatformRequirementFilter(PlatformRequirementFilterInterface $platformRequirementFilter): Installer
+    public function setPlatformRequirementFilter(PlatformRequirementFilterInterface $platformRequirementFilter): self
     {
         $this->platformRequirementFilter = $platformRequirementFilter;
 
@@ -1299,7 +1299,7 @@ public function setPlatformRequirementFilter(PlatformRequirementFilterInterface
      * @param  bool      $updateMirrors
      * @return Installer
      */
-    public function setUpdateMirrors(bool $updateMirrors): Installer
+    public function setUpdateMirrors(bool $updateMirrors): self
     {
         $this->updateMirrors = $updateMirrors;
 
@@ -1314,7 +1314,7 @@ public function setUpdateMirrors(bool $updateMirrors): Installer
      *
      * @return Installer
      */
-    public function setUpdateAllowList(array $packages): Installer
+    public function setUpdateAllowList(array $packages): self
     {
         $this->updateAllowList = array_flip(array_map('strtolower', $packages));
 
@@ -1330,7 +1330,7 @@ public function setUpdateAllowList(array $packages): Installer
      * @param  int       $updateAllowTransitiveDependencies One of the UPDATE_ constants on the Request class
      * @return Installer
      */
-    public function setUpdateAllowTransitiveDependencies(int $updateAllowTransitiveDependencies): Installer
+    public function setUpdateAllowTransitiveDependencies(int $updateAllowTransitiveDependencies): self
     {
         if (!in_array($updateAllowTransitiveDependencies, array(Request::UPDATE_ONLY_LISTED, Request::UPDATE_LISTED_WITH_TRANSITIVE_DEPS_NO_ROOT_REQUIRE, Request::UPDATE_LISTED_WITH_TRANSITIVE_DEPS), true)) {
             throw new \RuntimeException("Invalid value for updateAllowTransitiveDependencies supplied");
@@ -1347,7 +1347,7 @@ public function setUpdateAllowTransitiveDependencies(int $updateAllowTransitiveD
      * @param  bool      $preferStable
      * @return Installer
      */
-    public function setPreferStable(bool $preferStable = true): Installer
+    public function setPreferStable(bool $preferStable = true): self
     {
         $this->preferStable = (bool) $preferStable;
 
@@ -1360,7 +1360,7 @@ public function setPreferStable(bool $preferStable = true): Installer
      * @param  bool      $preferLowest
      * @return Installer
      */
-    public function setPreferLowest(bool $preferLowest = true): Installer
+    public function setPreferLowest(bool $preferLowest = true): self
     {
         $this->preferLowest = (bool) $preferLowest;
 
@@ -1375,7 +1375,7 @@ public function setPreferLowest(bool $preferLowest = true): Installer
      * @param  bool      $writeLock
      * @return Installer
      */
-    public function setWriteLock(bool $writeLock = true): Installer
+    public function setWriteLock(bool $writeLock = true): self
     {
         $this->writeLock = (bool) $writeLock;
 
@@ -1390,7 +1390,7 @@ public function setWriteLock(bool $writeLock = true): Installer
      * @param  bool      $executeOperations
      * @return Installer
      */
-    public function setExecuteOperations(bool $executeOperations = true): Installer
+    public function setExecuteOperations(bool $executeOperations = true): self
     {
         $this->executeOperations = (bool) $executeOperations;
 
@@ -1406,7 +1406,7 @@ public function setExecuteOperations(bool $executeOperations = true): Installer
      *
      * @return Installer
      */
-    public function disablePlugins(): Installer
+    public function disablePlugins(): self
     {
         $this->installationManager->disablePlugins();
 
@@ -1417,7 +1417,7 @@ public function disablePlugins(): Installer
      * @param  SuggestedPackagesReporter $suggestedPackagesReporter
      * @return Installer
      */
-    public function setSuggestedPackagesReporter(SuggestedPackagesReporter $suggestedPackagesReporter): Installer
+    public function setSuggestedPackagesReporter(SuggestedPackagesReporter $suggestedPackagesReporter): self
     {
         $this->suggestedPackagesReporter = $suggestedPackagesReporter;
 
diff --git a/src/Composer/Package/Archiver/ArchiveManager.php b/src/Composer/Package/Archiver/ArchiveManager.php
index 236304e69750..c7f5def613b1 100644
--- a/src/Composer/Package/Archiver/ArchiveManager.php
+++ b/src/Composer/Package/Archiver/ArchiveManager.php
@@ -68,7 +68,7 @@ public function addArchiver(ArchiverInterface $archiver): void
      *
      * @return $this
      */
-    public function setOverwriteFiles(bool $overwriteFiles)
+    public function setOverwriteFiles(bool $overwriteFiles): self
     {
         $this->overwriteFiles = $overwriteFiles;
 
diff --git a/src/Composer/Package/Loader/ArrayLoader.php b/src/Composer/Package/Loader/ArrayLoader.php
index b7fe52207be4..cd1f9e2bac73 100644
--- a/src/Composer/Package/Loader/ArrayLoader.php
+++ b/src/Composer/Package/Loader/ArrayLoader.php
@@ -49,7 +49,7 @@ public function __construct(VersionParser $parser = null, bool $loadOptions = fa
     /**
      * @inheritDoc
      */
-    public function load(array $config, $class = 'Composer\Package\CompletePackage')
+    public function load(array $config, string $class = 'Composer\Package\CompletePackage'): BasePackage
     {
         if ($class !== 'Composer\Package\CompletePackage' && $class !== 'Composer\Package\RootPackage') {
             trigger_error('The $class arg is deprecated, please reach out to Composer maintainers ASAP if you still need this.', E_USER_DEPRECATED);
@@ -81,7 +81,7 @@ public function load(array $config, $class = 'Composer\Package\CompletePackage')
      *
      * @return list<CompletePackage|CompleteAliasPackage>
      */
-    public function loadPackages(array $versions)
+    public function loadPackages(array $versions): array
     {
         $packages = array();
         $linkCache = array();
@@ -99,7 +99,7 @@ public function loadPackages(array $versions)
     }
 
     /**
-     * @template PackageClass of CompletePackageInterface
+     * @template PackageClass of CompletePackage
      *
      * @param mixed[] $config package data
      * @param string  $class  FQCN to be instantiated
@@ -108,7 +108,7 @@ public function loadPackages(array $versions)
      *
      * @phpstan-param class-string<PackageClass> $class
      */
-    private function createObject(array $config, string $class)
+    private function createObject(array $config, string $class): CompletePackage
     {
         if (!isset($config['name'])) {
             throw new \UnexpectedValueException('Unknown package has no name defined ('.json_encode($config).').');
@@ -141,7 +141,7 @@ private function createObject(array $config, string $class)
      *
      * @return RootPackage|RootAliasPackage|CompletePackage|CompleteAliasPackage
      */
-    private function configureObject(PackageInterface $package, array $config): PackageInterface
+    private function configureObject(PackageInterface $package, array $config): BasePackage
     {
         if (!$package instanceof CompletePackage) {
             throw new \LogicException('ArrayLoader expects instances of the Composer\Package\CompletePackage class to function correctly');
diff --git a/src/Composer/Package/Loader/JsonLoader.php b/src/Composer/Package/Loader/JsonLoader.php
index 9e9ae689efeb..4bfeaff8b8ba 100644
--- a/src/Composer/Package/Loader/JsonLoader.php
+++ b/src/Composer/Package/Loader/JsonLoader.php
@@ -13,8 +13,11 @@
 namespace Composer\Package\Loader;
 
 use Composer\Json\JsonFile;
+use Composer\Package\BasePackage;
 use Composer\Package\CompletePackage;
 use Composer\Package\CompleteAliasPackage;
+use Composer\Package\RootPackage;
+use Composer\Package\RootAliasPackage;
 
 /**
  * @author Konstantin Kudryashiv <ever.zet@gmail.com>
@@ -31,9 +34,9 @@ public function __construct(LoaderInterface $loader)
 
     /**
      * @param  string|JsonFile                      $json A filename, json string or JsonFile instance to load the package from
-     * @return CompletePackage|CompleteAliasPackage
+     * @return CompletePackage|CompleteAliasPackage|RootPackage|RootAliasPackage
      */
-    public function load($json)
+    public function load($json): BasePackage
     {
         if ($json instanceof JsonFile) {
             $config = $json->read();
diff --git a/src/Composer/Package/Loader/LoaderInterface.php b/src/Composer/Package/Loader/LoaderInterface.php
index 01c1910daa14..c81176ded91d 100644
--- a/src/Composer/Package/Loader/LoaderInterface.php
+++ b/src/Composer/Package/Loader/LoaderInterface.php
@@ -17,6 +17,7 @@
 use Composer\Package\CompleteAliasPackage;
 use Composer\Package\RootAliasPackage;
 use Composer\Package\RootPackage;
+use Composer\Package\BasePackage;
 
 /**
  * Defines a loader that takes an array to create package instances
@@ -28,14 +29,12 @@ interface LoaderInterface
     /**
      * Converts a package from an array to a real instance
      *
-     * @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
+     * @phpstan-param class-string<CompletePackage|RootPackage> $class
      */
-    public function load(array $config, string $class = 'Composer\Package\CompletePackage');
+    public function load(array $config, string $class = 'Composer\Package\CompletePackage'): BasePackage;
 }
diff --git a/src/Composer/Package/Loader/RootPackageLoader.php b/src/Composer/Package/Loader/RootPackageLoader.php
index 0ab5cefecca7..89eb276368e3 100644
--- a/src/Composer/Package/Loader/RootPackageLoader.php
+++ b/src/Composer/Package/Loader/RootPackageLoader.php
@@ -61,15 +61,13 @@ public function __construct(RepositoryManager $manager, Config $config, VersionP
     /**
      * @inheritDoc
      *
-     * @template PackageClass of RootPackage
-     *
      * @param string|null $cwd
      *
      * @return RootPackage|RootAliasPackage
      *
-     * @phpstan-param class-string<PackageClass> $class
+     * @phpstan-param class-string<RootPackage> $class
      */
-    public function load(array $config, $class = 'Composer\Package\RootPackage', ?string $cwd = null)
+    public function load(array $config, string $class = 'Composer\Package\RootPackage', ?string $cwd = null): BasePackage
     {
         if ($class !== 'Composer\Package\RootPackage') {
             trigger_error('The $class arg is deprecated, please reach out to Composer maintainers ASAP if you still need this.', E_USER_DEPRECATED);
diff --git a/src/Composer/Package/Loader/ValidatingArrayLoader.php b/src/Composer/Package/Loader/ValidatingArrayLoader.php
index 85f4cb67e88d..997287f4424c 100644
--- a/src/Composer/Package/Loader/ValidatingArrayLoader.php
+++ b/src/Composer/Package/Loader/ValidatingArrayLoader.php
@@ -59,7 +59,7 @@ public function __construct(LoaderInterface $loader, bool $strictName = true, Ve
     /**
      * @inheritDoc
      */
-    public function load(array $config, $class = 'Composer\Package\CompletePackage')
+    public function load(array $config, string $class = 'Composer\Package\CompletePackage'): BasePackage
     {
         $this->errors = array();
         $this->warnings = array();
diff --git a/src/Composer/Repository/ArrayRepository.php b/src/Composer/Repository/ArrayRepository.php
index 096ab9820928..d1a10e3ea287 100644
--- a/src/Composer/Repository/ArrayRepository.php
+++ b/src/Composer/Repository/ArrayRepository.php
@@ -97,7 +97,7 @@ public function loadPackages(array $packageMap, array $acceptableStabilities, ar
     /**
      * @inheritDoc
      */
-    public function findPackage($name, $constraint)
+    public function findPackage(string $name, $constraint)
     {
         $name = strtolower($name);
 
@@ -121,7 +121,7 @@ public function findPackage($name, $constraint)
     /**
      * @inheritDoc
      */
-    public function findPackages($name, $constraint = null)
+    public function findPackages(string $name, $constraint = null)
     {
         // normalize name
         $name = strtolower($name);
@@ -237,7 +237,7 @@ public function addPackage(PackageInterface $package)
     /**
      * @inheritDoc
      */
-    public function getProviders($packageName)
+    public function getProviders(string $packageName)
     {
         $result = array();
 
diff --git a/src/Composer/Repository/ComposerRepository.php b/src/Composer/Repository/ComposerRepository.php
index edb382264b81..22f8c5548ff9 100644
--- a/src/Composer/Repository/ComposerRepository.php
+++ b/src/Composer/Repository/ComposerRepository.php
@@ -187,7 +187,7 @@ public function getRepoConfig()
     /**
      * @inheritDoc
      */
-    public function findPackage($name, $constraint)
+    public function findPackage(string $name, $constraint)
     {
         // this call initializes loadRootServerFile which is needed for the rest below to work
         $hasProviders = $this->hasProviders();
@@ -227,7 +227,7 @@ public function findPackage($name, $constraint)
     /**
      * @inheritDoc
      */
-    public function findPackages($name, $constraint = null)
+    public function findPackages(string $name, $constraint = null)
     {
         // this call initializes loadRootServerFile which is needed for the rest below to work
         $hasProviders = $this->hasProviders();
@@ -598,7 +598,7 @@ public function search(string $query, int $mode = 0, ?string $type = null)
         return parent::search($query, $mode);
     }
 
-    public function getProviders($packageName)
+    public function getProviders(string $packageName)
     {
         $this->loadRootServerFile();
         $result = array();
diff --git a/src/Composer/Repository/FilesystemRepository.php b/src/Composer/Repository/FilesystemRepository.php
index 990df2910e53..81745ed7bc86 100644
--- a/src/Composer/Repository/FilesystemRepository.php
+++ b/src/Composer/Repository/FilesystemRepository.php
@@ -116,7 +116,7 @@ public function reload()
     /**
      * Writes writable repository.
      */
-    public function write($devMode, InstallationManager $installationManager)
+    public function write(bool $devMode, InstallationManager $installationManager)
     {
         $data = array('packages' => array(), 'dev' => $devMode, 'dev-package-names' => array());
         $dumper = new ArrayDumper();
diff --git a/src/Composer/Repository/VcsRepository.php b/src/Composer/Repository/VcsRepository.php
index 8bbf65730944..9978413c748d 100644
--- a/src/Composer/Repository/VcsRepository.php
+++ b/src/Composer/Repository/VcsRepository.php
@@ -436,7 +436,7 @@ protected function initialize()
      *
      * @return array{name: string|null, dist: array{type: string, url: string, reference: string, shasum: string}|null, source: array{type: string, url: string, reference: string}}
      */
-    protected function preProcess(VcsDriverInterface $driver, array $data, string $identifier)
+    protected function preProcess(VcsDriverInterface $driver, array $data, string $identifier): array
     {
         // keep the name of the main identifier for all packages
         // this ensures that a package can be renamed in one place and that all old tags
diff --git a/src/Composer/Repository/WritableArrayRepository.php b/src/Composer/Repository/WritableArrayRepository.php
index 722cc68f1156..6f98ad18dde6 100644
--- a/src/Composer/Repository/WritableArrayRepository.php
+++ b/src/Composer/Repository/WritableArrayRepository.php
@@ -57,7 +57,7 @@ public function getDevPackageNames()
     /**
      * @inheritDoc
      */
-    public function write($devMode, InstallationManager $installationManager)
+    public function write(bool $devMode, InstallationManager $installationManager)
     {
         $this->devMode = $devMode;
     }
diff --git a/src/Composer/Script/Event.php b/src/Composer/Script/Event.php
index ed6b7f0ba50d..0f05a72912f1 100644
--- a/src/Composer/Script/Event.php
+++ b/src/Composer/Script/Event.php
@@ -40,7 +40,7 @@ class Event extends BaseEvent
     private $devMode;
 
     /**
-     * @var BaseEvent
+     * @var BaseEvent|null
      */
     private $originatingEvent;
 
@@ -97,7 +97,7 @@ public function isDevMode(): bool
      *
      * @return ?BaseEvent
      */
-    public function getOriginatingEvent()
+    public function getOriginatingEvent(): ?BaseEvent
     {
         return $this->originatingEvent;
     }
@@ -108,7 +108,7 @@ public function getOriginatingEvent()
      * @param  BaseEvent $event
      * @return $this
      */
-    public function setOriginatingEvent(BaseEvent $event)
+    public function setOriginatingEvent(BaseEvent $event): self
     {
         $this->originatingEvent = $this->calculateOriginatingEvent($event);
 
diff --git a/src/Composer/Util/Git.php b/src/Composer/Util/Git.php
index be86b3dd01d6..f28271c74e10 100644
--- a/src/Composer/Util/Git.php
+++ b/src/Composer/Util/Git.php
@@ -422,7 +422,7 @@ public static function cleanEnv(): void
     /**
      * @return non-empty-string
      */
-    public static function getGitHubDomainsRegex(Config $config)
+    public static function getGitHubDomainsRegex(Config $config): string
     {
         return '(' . implode('|', array_map('preg_quote', $config->get('github-domains'))) . ')';
     }
@@ -430,7 +430,7 @@ public static function getGitHubDomainsRegex(Config $config)
     /**
      * @return non-empty-string
      */
-    public static function getGitLabDomainsRegex(Config $config)
+    public static function getGitLabDomainsRegex(Config $config): string
     {
         return '(' . implode('|', array_map('preg_quote', $config->get('gitlab-domains'))) . ')';
     }
diff --git a/src/Composer/Util/Perforce.php b/src/Composer/Util/Perforce.php
index f10f4b4f0858..ee21169e107c 100644
--- a/src/Composer/Util/Perforce.php
+++ b/src/Composer/Util/Perforce.php
@@ -151,7 +151,7 @@ public function initializeDepotAndBranch(?string $depot, ?string $branch): void
     /**
      * @return non-empty-string
      */
-    public function generateUniquePerforceClientName()
+    public function generateUniquePerforceClientName(): string
     {
         return gethostname() . "_" . time();
     }
@@ -281,7 +281,7 @@ public function getStreamWithoutLabel(string $stream): string
     /**
      * @return non-empty-string
      */
-    public function getP4ClientSpec()
+    public function getP4ClientSpec(): string
     {
         return $this->path . '/' . $this->getClient() . '.p4.spec';
     }
@@ -330,7 +330,7 @@ public function queryP4User(): void
      * @param  string  $name
      * @return ?string
      */
-    protected function getP4variable(string $name)
+    protected function getP4variable(string $name): ?string
     {
         if ($this->windowsFlag) {
             $command = 'p4 set';
@@ -385,7 +385,7 @@ public function queryP4Password(): ?string
      *
      * @return non-empty-string
      */
-    public function generateP4Command(string $command, bool $useClient = true)
+    public function generateP4Command(string $command, bool $useClient = true): string
     {
         $p4Command = 'p4 ';
         $p4Command .= '-u ' . $this->getUser() . ' ';
@@ -614,7 +614,7 @@ public function getFilePath(string $file, string $identifier): ?string
     /**
      * @return array{master: string}
      */
-    public function getBranches()
+    public function getBranches(): array
     {
         $possibleBranches = array();
         if (!$this->isStream()) {
diff --git a/src/bootstrap.php b/src/bootstrap.php
index 39eba006502e..f61063a4a0f8 100644
--- a/src/bootstrap.php
+++ b/src/bootstrap.php
@@ -10,11 +10,7 @@
  * file that was distributed with this source code.
  */
 
-/**
- * @param string $file
- * @return ?\Composer\Autoload\ClassLoader
- */
-function includeIfExists(string $file)
+function includeIfExists(string $file): ?\Composer\Autoload\ClassLoader
 {
     return file_exists($file) ? include $file : null;
 }
diff --git a/tests/Composer/Test/TestCase.php b/tests/Composer/Test/TestCase.php
index e901822ba47c..0e61bb87ab29 100644
--- a/tests/Composer/Test/TestCase.php
+++ b/tests/Composer/Test/TestCase.php
@@ -113,7 +113,7 @@ protected function getVersionConstraint($operator, string $version): Constraint
     }
 
     /**
-     * @template PackageClass of PackageInterface
+     * @template PackageClass of CompletePackage|CompleteAliasPackage
      *
      * @param  string $class  FQCN to be instantiated
      * @param  string $name
@@ -124,7 +124,7 @@ protected function getVersionConstraint($operator, string $version): Constraint
      * @phpstan-param class-string<PackageClass> $class
      * @phpstan-return PackageClass
      */
-    protected function getPackage(string $name = 'dummy/pkg', string $version = '1.0.0', string $class = 'Composer\Package\CompletePackage')
+    protected function getPackage(string $name = 'dummy/pkg', string $version = '1.0.0', string $class = 'Composer\Package\CompletePackage'): BasePackage
     {
         $normVersion = self::getVersionParser()->normalize($version);