diff --git a/src/Composer/Command/DiagnoseCommand.php b/src/Composer/Command/DiagnoseCommand.php
index dc4166e058d9..8ed6ed0f46cc 100644
--- a/src/Composer/Command/DiagnoseCommand.php
+++ b/src/Composer/Command/DiagnoseCommand.php
@@ -52,7 +52,7 @@ class DiagnoseCommand extends BaseCommand
     /**
      * @return void
      */
-    protected function configure()
+    protected function configure(): void
     {
         $this
             ->setName('diagnose')
diff --git a/src/Composer/Command/InitCommand.php b/src/Composer/Command/InitCommand.php
index 896e45dca12e..4f3cebbd838f 100644
--- a/src/Composer/Command/InitCommand.php
+++ b/src/Composer/Command/InitCommand.php
@@ -456,11 +456,10 @@ function ($value) use ($autoload) {
     }
 
     /**
-     * @private
      * @param  string $author
      * @return array{name: string, email: string|null}
      */
-    public function parseAuthorString(string $author)
+    private function parseAuthorString(string $author)
     {
         if (Preg::isMatch('/^(?P<name>[- .,\p{L}\p{N}\p{Mn}\'’"()]+)(?:\s+<(?P<email>.+?)>)?$/u', $author, $match)) {
             $hasEmail = isset($match['email']) && '' !== $match['email'];
@@ -485,7 +484,7 @@ public function parseAuthorString(string $author)
      *
      * @return array<int, array{name: string, email?: string}>
      */
-    protected function formatAuthors(string $author)
+    protected function formatAuthors(string $author): array
     {
         $author = $this->parseAuthorString($author);
         if (null === $author['email']) {
@@ -504,7 +503,7 @@ protected function formatAuthors(string $author)
      *
      * @return string|null
      */
-    public function namespaceFromPackageName(string $packageName)
+    public function namespaceFromPackageName(string $packageName): ?string
     {
         if (!$packageName || strpos($packageName, '/') === false) {
             return null;
@@ -526,7 +525,7 @@ function ($part): string {
     /**
      * @return array<string, string>
      */
-    protected function getGitConfig()
+    protected function getGitConfig(): array
     {
         if (null !== $this->gitConfig) {
             return $this->gitConfig;
@@ -567,7 +566,7 @@ protected function getGitConfig()
      *
      * @return bool
      */
-    protected function hasVendorIgnore(string $ignoreFile, string $vendor = 'vendor')
+    protected function hasVendorIgnore(string $ignoreFile, string $vendor = 'vendor'): bool
     {
         if (!file_exists($ignoreFile)) {
             return false;
@@ -591,7 +590,7 @@ protected function hasVendorIgnore(string $ignoreFile, string $vendor = 'vendor'
      *
      * @return void
      */
-    protected function addVendorIgnore(string $ignoreFile, string $vendor = '/vendor/')
+    protected function addVendorIgnore(string $ignoreFile, string $vendor = '/vendor/'): void
     {
         $contents = "";
         if (file_exists($ignoreFile)) {
@@ -610,7 +609,7 @@ protected function addVendorIgnore(string $ignoreFile, string $vendor = '/vendor
      *
      * @return bool
      */
-    protected function isValidEmail(string $email)
+    protected function isValidEmail(string $email): bool
     {
         // assume it's valid if we can't validate it
         if (!function_exists('filter_var')) {
diff --git a/src/Composer/Command/RequireCommand.php b/src/Composer/Command/RequireCommand.php
index 5aa890f0267b..5a65ccec91eb 100644
--- a/src/Composer/Command/RequireCommand.php
+++ b/src/Composer/Command/RequireCommand.php
@@ -335,15 +335,6 @@ private function getPackagesByRequireKey(): array
         );
     }
 
-    /**
-     * @private
-     * @return void
-     */
-    public function markSolverComplete()
-    {
-        $this->dependencyResolutionCompleted = true;
-    }
-
     /**
      * @param array<string, string> $requirements
      * @param string $requireKey
@@ -358,7 +349,9 @@ private function doUpdate(InputInterface $input, OutputInterface $output, IOInte
         $composer = $this->requireComposer();
 
         $this->dependencyResolutionCompleted = false;
-        $composer->getEventDispatcher()->addListener(InstallerEvents::PRE_OPERATIONS_EXEC, array($this, 'markSolverComplete'), 10000);
+        $composer->getEventDispatcher()->addListener(InstallerEvents::PRE_OPERATIONS_EXEC, function (): void {
+            $this->dependencyResolutionCompleted = true;
+        }, 10000);
 
         if ($input->getOption('dry-run')) {
             $rootPackage = $composer->getPackage();
@@ -480,7 +473,7 @@ protected function interact(InputInterface $input, OutputInterface $output): voi
      * @param bool $hardExit
      * @return void
      */
-    public function revertComposerFile(bool $hardExit = true)
+    public function revertComposerFile(bool $hardExit = true): void
     {
         $io = $this->getIO();
 
diff --git a/src/Composer/Command/ShowCommand.php b/src/Composer/Command/ShowCommand.php
index 36fb376dd853..775d858e934e 100644
--- a/src/Composer/Command/ShowCommand.php
+++ b/src/Composer/Command/ShowCommand.php
@@ -602,7 +602,7 @@ protected function execute(InputInterface $input, OutputInterface $output)
     /**
      * @return string[]
      */
-    protected function getRootRequires()
+    protected function getRootRequires(): array
     {
         $rootPackage = $this->requireComposer()->getPackage();
 
@@ -677,7 +677,7 @@ protected function getPackage(InstalledRepository $installedRepo, RepositoryInte
      *
      * @return void
      */
-    protected function printPackageInfo(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo, PackageInterface $latestPackage = null)
+    protected function printPackageInfo(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo, PackageInterface $latestPackage = null): void
     {
         $io = $this->getIO();
 
@@ -705,7 +705,7 @@ protected function printPackageInfo(CompletePackageInterface $package, array $ve
      *
      * @return void
      */
-    protected function printMeta(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo, PackageInterface $latestPackage = null)
+    protected function printMeta(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo, PackageInterface $latestPackage = null): void
     {
         $io = $this->getIO();
         $io->write('<info>name</info>     : ' . $package->getPrettyName());
@@ -773,7 +773,7 @@ protected function printMeta(CompletePackageInterface $package, array $versions,
      *
      * @return void
      */
-    protected function printVersions(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo)
+    protected function printVersions(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo): void
     {
         $versions = array_keys($versions);
         $versions = Semver::rsort($versions);
@@ -802,7 +802,7 @@ protected function printVersions(CompletePackageInterface $package, array $versi
      *
      * @return void
      */
-    protected function printLinks(CompletePackageInterface $package, string $linkType, string $title = null)
+    protected function printLinks(CompletePackageInterface $package, string $linkType, string $title = null): void
     {
         $title = $title ?: $linkType;
         $io = $this->getIO();
@@ -820,7 +820,7 @@ protected function printLinks(CompletePackageInterface $package, string $linkTyp
      *
      * @return void
      */
-    protected function printLicenses(CompletePackageInterface $package)
+    protected function printLicenses(CompletePackageInterface $package): void
     {
         $spdxLicenses = new SpdxLicenses();
 
@@ -852,7 +852,7 @@ protected function printLicenses(CompletePackageInterface $package)
      *
      * @return void
      */
-    protected function printPackageInfoAsJson(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo, PackageInterface $latestPackage = null)
+    protected function printPackageInfoAsJson(CompletePackageInterface $package, array $versions, InstalledRepository $installedRepo, PackageInterface $latestPackage = null): void
     {
         $json = array(
             'name' => $package->getPrettyName(),
@@ -1030,7 +1030,7 @@ private function appendLink(array $json, CompletePackageInterface $package, stri
      *
      * @return void
      */
-    protected function initStyles(OutputInterface $output)
+    protected function initStyles(OutputInterface $output): void
     {
         $this->colors = array(
             'green',
@@ -1052,7 +1052,7 @@ protected function initStyles(OutputInterface $output)
      * @param array<int, array<string, string|mixed[]>> $arrayTree
      * @return void
      */
-    protected function displayPackageTree(array $arrayTree)
+    protected function displayPackageTree(array $arrayTree): void
     {
         $io = $this->getIO();
         foreach ($arrayTree as $package) {
@@ -1101,7 +1101,7 @@ protected function generatePackageTree(
         PackageInterface $package,
         InstalledRepository $installedRepo,
         RepositoryInterface $remoteRepos
-    ) {
+    ): array {
         $requires = $package->getRequires();
         ksort($requires);
         $children = array();
@@ -1149,7 +1149,7 @@ protected function displayTree(
         array $packagesInTree,
         string $previousTreeBar = '├',
         int $level = 1
-    ) {
+    ): void {
         $previousTreeBar = str_replace('├', '│', $previousTreeBar);
         if (is_array($package) && isset($package['requires'])) {
             $requires = $package['requires'];
@@ -1202,7 +1202,7 @@ protected function addTree(
         InstalledRepository $installedRepo,
         RepositoryInterface $remoteRepos,
         array $packagesInTree
-    ) {
+    ): array {
         $children = array();
         list($package) = $this->getPackage(
             $installedRepo,
diff --git a/src/Composer/EventDispatcher/Event.php b/src/Composer/EventDispatcher/Event.php
index c7af569d2c80..a3f2294ef057 100644
--- a/src/Composer/EventDispatcher/Event.php
+++ b/src/Composer/EventDispatcher/Event.php
@@ -58,7 +58,7 @@ public function __construct(string $name, array $args = array(), array $flags =
      *
      * @return string The event name
      */
-    public function getName()
+    public function getName(): string
     {
         return $this->name;
     }
@@ -68,7 +68,7 @@ public function getName()
      *
      * @return string[] The event arguments
      */
-    public function getArguments()
+    public function getArguments(): array
     {
         return $this->args;
     }
@@ -78,7 +78,7 @@ public function getArguments()
      *
      * @return mixed[] The event flags
      */
-    public function getFlags()
+    public function getFlags(): array
     {
         return $this->flags;
     }
@@ -88,7 +88,7 @@ public function getFlags()
      *
      * @return bool Whether propagation has been stopped
      */
-    public function isPropagationStopped()
+    public function isPropagationStopped(): bool
     {
         return $this->propagationStopped;
     }
@@ -98,7 +98,7 @@ public function isPropagationStopped()
      *
      * @return void
      */
-    public function stopPropagation()
+    public function stopPropagation(): void
     {
         $this->propagationStopped = true;
     }
diff --git a/src/Composer/EventDispatcher/EventDispatcher.php b/src/Composer/EventDispatcher/EventDispatcher.php
index 8cfc8b1945e3..941fc252041e 100644
--- a/src/Composer/EventDispatcher/EventDispatcher.php
+++ b/src/Composer/EventDispatcher/EventDispatcher.php
@@ -96,7 +96,7 @@ public function setRunScripts(bool $runScripts = true)
      * @return int    return code of the executed script if any, for php scripts a false return
      *                          value is changed to 1, anything else to 0
      */
-    public function dispatch(string $eventName, Event $event = null)
+    public function dispatch(string $eventName, Event $event = null): int
     {
         if (null === $event) {
             $event = new Event($eventName);
@@ -115,7 +115,7 @@ public function dispatch(string $eventName, Event $event = null)
      * @return int                                  return code of the executed script if any, for php scripts a false return
      *                                              value is changed to 1, anything else to 0
      */
-    public function dispatchScript(string $eventName, bool $devMode = false, array $additionalArgs = array(), array $flags = array())
+    public function dispatchScript(string $eventName, bool $devMode = false, array $additionalArgs = array(), array $flags = array()): int
     {
         assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
 
@@ -134,7 +134,7 @@ public function dispatchScript(string $eventName, bool $devMode = false, array $
      * @return int return code of the executed script if any, for php scripts a false return
      *             value is changed to 1, anything else to 0
      */
-    public function dispatchPackageEvent(string $eventName, bool $devMode, RepositoryInterface $localRepo, array $operations, OperationInterface $operation)
+    public function dispatchPackageEvent(string $eventName, bool $devMode, RepositoryInterface $localRepo, array $operations, OperationInterface $operation): int
     {
         assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
 
@@ -152,7 +152,7 @@ public function dispatchPackageEvent(string $eventName, bool $devMode, Repositor
      * @return int return code of the executed script if any, for php scripts a false return
      *             value is changed to 1, anything else to 0
      */
-    public function dispatchInstallerEvent(string $eventName, bool $devMode, bool $executeOperations, Transaction $transaction)
+    public function dispatchInstallerEvent(string $eventName, bool $devMode, bool $executeOperations, Transaction $transaction): int
     {
         assert($this->composer instanceof Composer, new \LogicException('This should only be reached with a fully loaded Composer'));
 
@@ -342,7 +342,7 @@ protected function doDispatch(Event $event)
      *
      * @return int
      */
-    protected function executeTty(string $exec)
+    protected function executeTty(string $exec): int
     {
         if ($this->io->isInteractive()) {
             return $this->process->executeTty($exec);
@@ -354,7 +354,7 @@ protected function executeTty(string $exec)
     /**
      * @return string
      */
-    protected function getPhpExecCommand()
+    protected function getPhpExecCommand(): string
     {
         $finder = new PhpExecutableFinder();
         $phpPath = $finder->find(false);
@@ -397,7 +397,7 @@ protected function executeEventPhpScript(string $className, string $methodName,
      *
      * @return void
      */
-    public function addListener(string $eventName, callable $listener, int $priority = 0)
+    public function addListener(string $eventName, callable $listener, int $priority = 0): void
     {
         $this->listeners[$eventName][$priority][] = $listener;
     }
@@ -407,7 +407,7 @@ public function addListener(string $eventName, callable $listener, int $priority
      *
      * @return void
      */
-    public function removeListener($listener)
+    public function removeListener($listener): void
     {
         foreach ($this->listeners as $eventName => $priorities) {
             foreach ($priorities as $priority => $listeners) {
@@ -429,7 +429,7 @@ public function removeListener($listener)
      *
      * @return void
      */
-    public function addSubscriber(EventSubscriberInterface $subscriber)
+    public function addSubscriber(EventSubscriberInterface $subscriber): void
     {
         foreach ($subscriber->getSubscribedEvents() as $eventName => $params) {
             if (is_string($params)) {
@@ -450,7 +450,7 @@ public function addSubscriber(EventSubscriberInterface $subscriber)
      * @param  Event $event
      * @return array<callable|string> All listeners: callables and scripts
      */
-    protected function getListeners(Event $event)
+    protected function getListeners(Event $event): array
     {
         $scriptListeners = $this->runScripts ? $this->getScriptListeners($event) : array();
 
@@ -471,7 +471,7 @@ protected function getListeners(Event $event)
      * @param  Event $event
      * @return bool
      */
-    public function hasEventListeners(Event $event)
+    public function hasEventListeners(Event $event): bool
     {
         $listeners = $this->getListeners($event);
 
@@ -484,7 +484,7 @@ public function hasEventListeners(Event $event)
      * @param  Event $event Event object
      * @return string[] Listeners
      */
-    protected function getScriptListeners(Event $event)
+    protected function getScriptListeners(Event $event): array
     {
         $package = $this->composer->getPackage();
         $scripts = $package->getScripts();
@@ -519,7 +519,7 @@ protected function getScriptListeners(Event $event)
      * @param  string $callable
      * @return bool
      */
-    protected function isPhpScript(string $callable)
+    protected function isPhpScript(string $callable): bool
     {
         return false === strpos($callable, ' ') && false !== strpos($callable, '::');
     }
@@ -530,7 +530,7 @@ protected function isPhpScript(string $callable)
      * @param  string $callable
      * @return bool
      */
-    protected function isComposerScript(string $callable)
+    protected function isComposerScript(string $callable): bool
     {
         return strpos($callable, '@') === 0 && strpos($callable, '@php ') !== 0 && strpos($callable, '@putenv ') !== 0;
     }
@@ -542,7 +542,7 @@ protected function isComposerScript(string $callable)
      * @throws \RuntimeException
      * @return int
      */
-    protected function pushEvent(Event $event)
+    protected function pushEvent(Event $event): int
     {
         $eventName = $event->getName();
         if (in_array($eventName, $this->eventStack)) {
@@ -557,7 +557,7 @@ protected function pushEvent(Event $event)
      *
      * @return string|null
      */
-    protected function popEvent()
+    protected function popEvent(): ?string
     {
         return array_pop($this->eventStack);
     }
diff --git a/src/Composer/Installer/InstallationManager.php b/src/Composer/Installer/InstallationManager.php
index 7d3150d60bec..8200080b3163 100644
--- a/src/Composer/Installer/InstallationManager.php
+++ b/src/Composer/Installer/InstallationManager.php
@@ -63,7 +63,7 @@ public function __construct(Loop $loop, IOInterface $io, EventDispatcher $eventD
     /**
      * @return void
      */
-    public function reset()
+    public function reset(): void
     {
         $this->notifiablePackages = array();
         FileDownloader::$downloadMetadata = array();
@@ -76,7 +76,7 @@ public function reset()
      *
      * @return void
      */
-    public function addInstaller(InstallerInterface $installer)
+    public function addInstaller(InstallerInterface $installer): void
     {
         array_unshift($this->installers, $installer);
         $this->cache = array();
@@ -89,7 +89,7 @@ public function addInstaller(InstallerInterface $installer)
      *
      * @return void
      */
-    public function removeInstaller(InstallerInterface $installer)
+    public function removeInstaller(InstallerInterface $installer): void
     {
         if (false !== ($key = array_search($installer, $this->installers, true))) {
             array_splice($this->installers, $key, 1);
@@ -106,7 +106,7 @@ public function removeInstaller(InstallerInterface $installer)
      *
      * @return void
      */
-    public function disablePlugins()
+    public function disablePlugins(): void
     {
         foreach ($this->installers as $i => $installer) {
             if (!$installer instanceof PluginInstaller) {
@@ -125,7 +125,7 @@ public function disablePlugins()
      * @throws \InvalidArgumentException if installer for provided type is not registered
      * @return InstallerInterface
      */
-    public function getInstaller(string $type)
+    public function getInstaller(string $type): InstallerInterface
     {
         $type = strtolower($type);
 
@@ -150,7 +150,7 @@ public function getInstaller(string $type)
      *
      * @return bool
      */
-    public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageInterface $package)
+    public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageInterface $package): bool
     {
         if ($package instanceof AliasPackage) {
             return $repo->hasPackage($package) && $this->isPackageInstalled($repo, $package->getAliasOf());
@@ -167,7 +167,7 @@ public function isPackageInstalled(InstalledRepositoryInterface $repo, PackageIn
      *
      * @return void
      */
-    public function ensureBinariesPresence(PackageInterface $package)
+    public function ensureBinariesPresence(PackageInterface $package): void
     {
         try {
             $installer = $this->getInstaller($package->getType());
@@ -192,7 +192,7 @@ public function ensureBinariesPresence(PackageInterface $package)
      *
      * @return void
      */
-    public function execute(InstalledRepositoryInterface $repo, array $operations, bool $devMode = true, bool $runScripts = true)
+    public function execute(InstalledRepositoryInterface $repo, array $operations, bool $devMode = true, bool $runScripts = true): void
     {
         /** @var PromiseInterface[] */
         $cleanupPromises = array();
@@ -511,7 +511,7 @@ private function waitOnPromises(array $promises): void
      *
      * @return PromiseInterface|null
      */
-    public function install(InstalledRepositoryInterface $repo, InstallOperation $operation)
+    public function install(InstalledRepositoryInterface $repo, InstallOperation $operation): ?PromiseInterface
     {
         $package = $operation->getPackage();
         $installer = $this->getInstaller($package->getType());
@@ -529,7 +529,7 @@ public function install(InstalledRepositoryInterface $repo, InstallOperation $op
      *
      * @return PromiseInterface|null
      */
-    public function update(InstalledRepositoryInterface $repo, UpdateOperation $operation)
+    public function update(InstalledRepositoryInterface $repo, UpdateOperation $operation): ?PromiseInterface
     {
         $initial = $operation->getInitialPackage();
         $target = $operation->getTargetPackage();
@@ -569,7 +569,7 @@ public function update(InstalledRepositoryInterface $repo, UpdateOperation $oper
      *
      * @return PromiseInterface|null
      */
-    public function uninstall(InstalledRepositoryInterface $repo, UninstallOperation $operation)
+    public function uninstall(InstalledRepositoryInterface $repo, UninstallOperation $operation): ?PromiseInterface
     {
         $package = $operation->getPackage();
         $installer = $this->getInstaller($package->getType());
@@ -585,7 +585,7 @@ public function uninstall(InstalledRepositoryInterface $repo, UninstallOperation
      *
      * @return void
      */
-    public function markAliasInstalled(InstalledRepositoryInterface $repo, MarkAliasInstalledOperation $operation)
+    public function markAliasInstalled(InstalledRepositoryInterface $repo, MarkAliasInstalledOperation $operation): void
     {
         $package = $operation->getPackage();
 
@@ -602,7 +602,7 @@ public function markAliasInstalled(InstalledRepositoryInterface $repo, MarkAlias
      *
      * @return void
      */
-    public function markAliasUninstalled(InstalledRepositoryInterface $repo, MarkAliasUninstalledOperation $operation)
+    public function markAliasUninstalled(InstalledRepositoryInterface $repo, MarkAliasUninstalledOperation $operation): void
     {
         $package = $operation->getPackage();
 
@@ -615,7 +615,7 @@ public function markAliasUninstalled(InstalledRepositoryInterface $repo, MarkAli
      * @param  PackageInterface $package
      * @return string           path
      */
-    public function getInstallPath(PackageInterface $package)
+    public function getInstallPath(PackageInterface $package): string
     {
         $installer = $this->getInstaller($package->getType());
 
@@ -627,7 +627,7 @@ public function getInstallPath(PackageInterface $package)
      *
      * @return void
      */
-    public function setOutputProgress(bool $outputProgress)
+    public function setOutputProgress(bool $outputProgress): void
     {
         $this->outputProgress = $outputProgress;
     }
@@ -635,7 +635,7 @@ public function setOutputProgress(bool $outputProgress)
     /**
      * @return void
      */
-    public function notifyInstalls(IOInterface $io)
+    public function notifyInstalls(IOInterface $io): void
     {
         $promises = array();
 
diff --git a/src/Composer/Installer/InstallerEvent.php b/src/Composer/Installer/InstallerEvent.php
index 88dc7b737526..cafd0dc1b31a 100644
--- a/src/Composer/Installer/InstallerEvent.php
+++ b/src/Composer/Installer/InstallerEvent.php
@@ -68,7 +68,7 @@ public function __construct(string $eventName, Composer $composer, IOInterface $
     /**
      * @return Composer
      */
-    public function getComposer()
+    public function getComposer(): Composer
     {
         return $this->composer;
     }
@@ -76,7 +76,7 @@ public function getComposer()
     /**
      * @return IOInterface
      */
-    public function getIO()
+    public function getIO(): IOInterface
     {
         return $this->io;
     }
@@ -84,7 +84,7 @@ public function getIO()
     /**
      * @return bool
      */
-    public function isDevMode()
+    public function isDevMode(): bool
     {
         return $this->devMode;
     }
@@ -92,7 +92,7 @@ public function isDevMode()
     /**
      * @return bool
      */
-    public function isExecutingOperations()
+    public function isExecutingOperations(): bool
     {
         return $this->executeOperations;
     }
@@ -100,7 +100,7 @@ public function isExecutingOperations()
     /**
      * @return Transaction|null
      */
-    public function getTransaction()
+    public function getTransaction(): ?Transaction
     {
         return $this->transaction;
     }
diff --git a/src/Composer/Installer/PackageEvent.php b/src/Composer/Installer/PackageEvent.php
index 177869fb9dde..55029fb9319f 100644
--- a/src/Composer/Installer/PackageEvent.php
+++ b/src/Composer/Installer/PackageEvent.php
@@ -81,7 +81,7 @@ public function __construct(string $eventName, Composer $composer, IOInterface $
     /**
      * @return Composer
      */
-    public function getComposer()
+    public function getComposer(): Composer
     {
         return $this->composer;
     }
@@ -89,7 +89,7 @@ public function getComposer()
     /**
      * @return IOInterface
      */
-    public function getIO()
+    public function getIO(): IOInterface
     {
         return $this->io;
     }
@@ -97,7 +97,7 @@ public function getIO()
     /**
      * @return bool
      */
-    public function isDevMode()
+    public function isDevMode(): bool
     {
         return $this->devMode;
     }
@@ -105,7 +105,7 @@ public function isDevMode()
     /**
      * @return RepositoryInterface
      */
-    public function getLocalRepo()
+    public function getLocalRepo(): RepositoryInterface
     {
         return $this->localRepo;
     }
@@ -113,7 +113,7 @@ public function getLocalRepo()
     /**
      * @return OperationInterface[]
      */
-    public function getOperations()
+    public function getOperations(): array
     {
         return $this->operations;
     }
@@ -123,7 +123,7 @@ public function getOperations()
      *
      * @return OperationInterface
      */
-    public function getOperation()
+    public function getOperation(): OperationInterface
     {
         return $this->operation;
     }
diff --git a/src/Composer/Json/JsonFile.php b/src/Composer/Json/JsonFile.php
index b16af75ac01c..3ec407f8a7e5 100644
--- a/src/Composer/Json/JsonFile.php
+++ b/src/Composer/Json/JsonFile.php
@@ -69,7 +69,7 @@ public function __construct(string $path, HttpDownloader $httpDownloader = null,
     /**
      * @return string
      */
-    public function getPath()
+    public function getPath(): string
     {
         return $this->path;
     }
@@ -79,7 +79,7 @@ public function getPath()
      *
      * @return bool
      */
-    public function exists()
+    public function exists(): bool
     {
         return is_file($this->path);
     }
@@ -188,7 +188,7 @@ private function filePutContentsIfModified(string $path, string $content)
      * @throws ParsingException
      * @return bool                    true on success
      */
-    public function validateSchema(int $schema = self::STRICT_SCHEMA, ?string $schemaFile = null)
+    public function validateSchema(int $schema = self::STRICT_SCHEMA, ?string $schemaFile = null): bool
     {
         $content = file_get_contents($this->path);
         $data = json_decode($content);
@@ -309,7 +309,7 @@ public static function parseJson(?string $json, string $file = null)
      * @throws ParsingException
      * @return bool                      true on success
      */
-    protected static function validateSyntax(string $json, string $file = null)
+    protected static function validateSyntax(string $json, string $file = null): bool
     {
         $parser = new JsonParser();
         $result = $parser->lint($json);
diff --git a/src/Composer/Json/JsonManipulator.php b/src/Composer/Json/JsonManipulator.php
index 68de5dd4a08d..4a8781c648ef 100644
--- a/src/Composer/Json/JsonManipulator.php
+++ b/src/Composer/Json/JsonManipulator.php
@@ -58,7 +58,7 @@ public function __construct(string $contents)
     /**
      * @return string
      */
-    public function getContents()
+    public function getContents(): string
     {
         return $this->contents . $this->newline;
     }
@@ -70,7 +70,7 @@ public function getContents()
      * @param bool   $sortPackages
      * @return bool
      */
-    public function addLink(string $type, string $package, string $constraint, bool $sortPackages = false)
+    public function addLink(string $type, string $package, string $constraint, bool $sortPackages = false): bool
     {
         $decoded = JsonFile::parseJson($this->contents);
 
@@ -170,7 +170,7 @@ private function sortPackages(array &$packages = array()): void
      * @param bool                       $append
      * @return bool
      */
-    public function addRepository(string $name, $config, bool $append = true)
+    public function addRepository(string $name, $config, bool $append = true): bool
     {
         return $this->addSubNode('repositories', $name, $config, $append);
     }
@@ -179,7 +179,7 @@ public function addRepository(string $name, $config, bool $append = true)
      * @param string $name
      * @return bool
      */
-    public function removeRepository(string $name)
+    public function removeRepository(string $name): bool
     {
         return $this->removeSubNode('repositories', $name);
     }
@@ -189,7 +189,7 @@ public function removeRepository(string $name)
      * @param mixed  $value
      * @return bool
      */
-    public function addConfigSetting(string $name, $value)
+    public function addConfigSetting(string $name, $value): bool
     {
         return $this->addSubNode('config', $name, $value);
     }
@@ -198,7 +198,7 @@ public function addConfigSetting(string $name, $value)
      * @param string $name
      * @return bool
      */
-    public function removeConfigSetting(string $name)
+    public function removeConfigSetting(string $name): bool
     {
         return $this->removeSubNode('config', $name);
     }
@@ -208,7 +208,7 @@ public function removeConfigSetting(string $name)
      * @param mixed $value
      * @return bool
      */
-    public function addProperty(string $name, $value)
+    public function addProperty(string $name, $value): bool
     {
         if (strpos($name, 'suggest.') === 0) {
             return $this->addSubNode('suggest', substr($name, 8), $value);
@@ -229,7 +229,7 @@ public function addProperty(string $name, $value)
      * @param string $name
      * @return bool
      */
-    public function removeProperty(string $name)
+    public function removeProperty(string $name): bool
     {
         if (strpos($name, 'suggest.') === 0) {
             return $this->removeSubNode('suggest', substr($name, 8));
@@ -253,7 +253,7 @@ public function removeProperty(string $name)
      * @param bool   $append
      * @return bool
      */
-    public function addSubNode(string $mainNode, string $name, $value, bool $append = true)
+    public function addSubNode(string $mainNode, string $name, $value, bool $append = true): bool
     {
         $decoded = JsonFile::parseJson($this->contents);
 
@@ -363,7 +363,7 @@ public function addSubNode(string $mainNode, string $name, $value, bool $append
      * @param string $name
      * @return bool
      */
-    public function removeSubNode(string $mainNode, string $name)
+    public function removeSubNode(string $mainNode, string $name): bool
     {
         $decoded = JsonFile::parseJson($this->contents);
 
@@ -468,7 +468,7 @@ public function removeSubNode(string $mainNode, string $name)
      * @param mixed  $content
      * @return bool
      */
-    public function addMainKey(string $key, $content)
+    public function addMainKey(string $key, $content): bool
     {
         $decoded = JsonFile::parseJson($this->contents);
         $content = $this->format($content);
@@ -512,7 +512,7 @@ public function addMainKey(string $key, $content)
      * @param string $key
      * @return bool
      */
-    public function removeMainKey(string $key)
+    public function removeMainKey(string $key): bool
     {
         $decoded = JsonFile::parseJson($this->contents);
 
@@ -549,7 +549,7 @@ public function removeMainKey(string $key)
      * @param string $key
      * @return bool
      */
-    public function removeMainKeyIfEmpty(string $key)
+    public function removeMainKeyIfEmpty(string $key): bool
     {
         $decoded = JsonFile::parseJson($this->contents);
 
@@ -569,7 +569,7 @@ public function removeMainKeyIfEmpty(string $key)
      * @param int   $depth
      * @return string
      */
-    public function format($data, int $depth = 0)
+    public function format($data, int $depth = 0): string
     {
         if (is_array($data)) {
             reset($data);
@@ -597,7 +597,7 @@ public function format($data, int $depth = 0)
     /**
      * @return void
      */
-    protected function detectIndenting()
+    protected function detectIndenting(): void
     {
         if (Preg::isMatch('{^([ \t]+)"}m', $this->contents, $match)) {
             $this->indent = $match[1];
diff --git a/src/Composer/Package/AliasPackage.php b/src/Composer/Package/AliasPackage.php
index 3d82debb6e09..332c1fe05ac5 100644
--- a/src/Composer/Package/AliasPackage.php
+++ b/src/Composer/Package/AliasPackage.php
@@ -159,14 +159,10 @@ public function getDevRequires(): array
      * Stores whether this is an alias created by an aliasing in the requirements of the root package or not
      *
      * Use by the policy for sorting manually aliased packages first, see #576
-     *
-     * @param bool $value
-     *
-     * @return mixed
      */
-    public function setRootPackageAlias(bool $value)
+    public function setRootPackageAlias(bool $value): void
     {
-        return $this->rootPackageAlias = $value;
+        $this->rootPackageAlias = $value;
     }
 
     /**
@@ -225,7 +221,7 @@ public function hasSelfVersionRequires(): bool
         return $this->hasSelfVersionRequires;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return parent::__toString().' ('.($this->rootPackageAlias ? 'root ' : ''). 'alias of '.$this->aliasOf->getVersion().')';
     }
diff --git a/src/Composer/Package/BasePackage.php b/src/Composer/Package/BasePackage.php
index f29bee056fbe..35be9e75ada5 100644
--- a/src/Composer/Package/BasePackage.php
+++ b/src/Composer/Package/BasePackage.php
@@ -78,7 +78,7 @@ public function __construct(string $name)
     /**
      * @inheritDoc
      */
-    public function getName()
+    public function getName(): string
     {
         return $this->name;
     }
@@ -86,7 +86,7 @@ public function getName()
     /**
      * @inheritDoc
      */
-    public function getPrettyName()
+    public function getPrettyName(): string
     {
         return $this->prettyName;
     }
@@ -94,7 +94,7 @@ public function getPrettyName()
     /**
      * @inheritDoc
      */
-    public function getNames($provides = true)
+    public function getNames($provides = true): array
     {
         $names = array(
             $this->getName() => true,
@@ -116,7 +116,7 @@ public function getNames($provides = true)
     /**
      * @inheritDoc
      */
-    public function setId($id)
+    public function setId(int $id): void
     {
         $this->id = $id;
     }
@@ -124,7 +124,7 @@ public function setId($id)
     /**
      * @inheritDoc
      */
-    public function getId()
+    public function getId(): int
     {
         return $this->id;
     }
@@ -132,7 +132,7 @@ public function getId()
     /**
      * @inheritDoc
      */
-    public function setRepository(RepositoryInterface $repository)
+    public function setRepository(RepositoryInterface $repository): void
     {
         if ($this->repository && $repository !== $this->repository) {
             throw new \LogicException('A package can only be added to one repository');
@@ -143,7 +143,7 @@ public function setRepository(RepositoryInterface $repository)
     /**
      * @inheritDoc
      */
-    public function getRepository()
+    public function getRepository(): ?RepositoryInterface
     {
         return $this->repository;
     }
@@ -153,7 +153,7 @@ public function getRepository()
      *
      * @return bool
      */
-    public function isPlatform()
+    public function isPlatform(): bool
     {
         return $this->getRepository() instanceof PlatformRepository;
     }
@@ -163,7 +163,7 @@ public function isPlatform()
      *
      * @return string
      */
-    public function getUniqueName()
+    public function getUniqueName(): string
     {
         return $this->getName().'-'.$this->getVersion();
     }
@@ -171,7 +171,7 @@ public function getUniqueName()
     /**
      * @return bool
      */
-    public function equals(PackageInterface $package)
+    public function equals(PackageInterface $package): bool
     {
         $self = $this;
         if ($this instanceof AliasPackage) {
@@ -189,12 +189,12 @@ public function equals(PackageInterface $package)
      *
      * @return string
      */
-    public function __toString()
+    public function __toString(): string
     {
         return $this->getUniqueName();
     }
 
-    public function getPrettyString()
+    public function getPrettyString(): string
     {
         return $this->getPrettyName().' '.$this->getPrettyVersion();
     }
@@ -202,7 +202,7 @@ public function getPrettyString()
     /**
      * @inheritDoc
      */
-    public function getFullPrettyVersion($truncate = true, $displayMode = PackageInterface::DISPLAY_SOURCE_REF_IF_DEV)
+    public function getFullPrettyVersion(bool $truncate = true, int $displayMode = PackageInterface::DISPLAY_SOURCE_REF_IF_DEV): string
     {
         if ($displayMode === PackageInterface::DISPLAY_SOURCE_REF_IF_DEV &&
             (!$this->isDev() || !\in_array($this->getSourceType(), array('hg', 'git')))
@@ -239,7 +239,7 @@ public function getFullPrettyVersion($truncate = true, $displayMode = PackageInt
      *
      * @phpstan-return self::STABILITY_*
      */
-    public function getStabilityPriority()
+    public function getStabilityPriority(): int
     {
         return self::$stabilities[$this->getStability()];
     }
@@ -257,7 +257,7 @@ public function __clone()
      * @param  non-empty-string $wrap         Wrap the cleaned string by the given string
      * @return non-empty-string
      */
-    public static function packageNameToRegexp(string $allowPattern, string $wrap = '{^%s$}i')
+    public static function packageNameToRegexp(string $allowPattern, string $wrap = '{^%s$}i'): string
     {
         $cleanedAllowPattern = str_replace('\\*', '.*', preg_quote($allowPattern));
 
@@ -271,7 +271,7 @@ public static function packageNameToRegexp(string $allowPattern, string $wrap =
      * @param non-empty-string $wrap
      * @return non-empty-string
      */
-    public static function packageNamesToRegexp(array $packageNames, string $wrap = '{^(?:%s)$}iD')
+    public static function packageNamesToRegexp(array $packageNames, string $wrap = '{^(?:%s)$}iD'): string
     {
         $packageNames = array_map(
             function ($packageName): string {
diff --git a/src/Composer/Package/CompletePackage.php b/src/Composer/Package/CompletePackage.php
index 1234ddf0c912..666f3b0c749f 100644
--- a/src/Composer/Package/CompletePackage.php
+++ b/src/Composer/Package/CompletePackage.php
@@ -47,7 +47,7 @@ class CompletePackage extends Package implements CompletePackageInterface
     /**
      * @inheritDoc
      */
-    public function setScripts(array $scripts)
+    public function setScripts(array $scripts): void
     {
         $this->scripts = $scripts;
     }
@@ -55,7 +55,7 @@ public function setScripts(array $scripts)
     /**
      * @inheritDoc
      */
-    public function getScripts()
+    public function getScripts(): array
     {
         return $this->scripts;
     }
@@ -63,7 +63,7 @@ public function getScripts()
     /**
      * @inheritDoc
      */
-    public function setRepositories(array $repositories)
+    public function setRepositories(array $repositories): void
     {
         $this->repositories = $repositories;
     }
@@ -71,7 +71,7 @@ public function setRepositories(array $repositories)
     /**
      * @inheritDoc
      */
-    public function getRepositories()
+    public function getRepositories(): array
     {
         return $this->repositories;
     }
@@ -79,7 +79,7 @@ public function getRepositories()
     /**
      * @inheritDoc
      */
-    public function setLicense(array $license)
+    public function setLicense(array $license): void
     {
         $this->license = $license;
     }
@@ -87,7 +87,7 @@ public function setLicense(array $license)
     /**
      * @inheritDoc
      */
-    public function getLicense()
+    public function getLicense(): array
     {
         return $this->license;
     }
@@ -95,7 +95,7 @@ public function getLicense()
     /**
      * @inheritDoc
      */
-    public function setKeywords(array $keywords)
+    public function setKeywords(array $keywords): void
     {
         $this->keywords = $keywords;
     }
@@ -103,7 +103,7 @@ public function setKeywords(array $keywords)
     /**
      * @inheritDoc
      */
-    public function getKeywords()
+    public function getKeywords(): array
     {
         return $this->keywords;
     }
@@ -111,7 +111,7 @@ public function getKeywords()
     /**
      * @inheritDoc
      */
-    public function setAuthors(array $authors)
+    public function setAuthors(array $authors): void
     {
         $this->authors = $authors;
     }
@@ -119,7 +119,7 @@ public function setAuthors(array $authors)
     /**
      * @inheritDoc
      */
-    public function getAuthors()
+    public function getAuthors(): array
     {
         return $this->authors;
     }
@@ -127,7 +127,7 @@ public function getAuthors()
     /**
      * @inheritDoc
      */
-    public function setDescription(?string $description)
+    public function setDescription(?string $description): void
     {
         $this->description = $description;
     }
@@ -135,7 +135,7 @@ public function setDescription(?string $description)
     /**
      * @inheritDoc
      */
-    public function getDescription()
+    public function getDescription(): ?string
     {
         return $this->description;
     }
@@ -143,7 +143,7 @@ public function getDescription()
     /**
      * @inheritDoc
      */
-    public function setHomepage(?string $homepage)
+    public function setHomepage(?string $homepage): void
     {
         $this->homepage = $homepage;
     }
@@ -151,7 +151,7 @@ public function setHomepage(?string $homepage)
     /**
      * @inheritDoc
      */
-    public function getHomepage()
+    public function getHomepage(): ?string
     {
         return $this->homepage;
     }
@@ -159,7 +159,7 @@ public function getHomepage()
     /**
      * @inheritDoc
      */
-    public function setSupport(array $support)
+    public function setSupport(array $support): void
     {
         $this->support = $support;
     }
@@ -167,7 +167,7 @@ public function setSupport(array $support)
     /**
      * @inheritDoc
      */
-    public function getSupport()
+    public function getSupport(): array
     {
         return $this->support;
     }
@@ -175,7 +175,7 @@ public function getSupport()
     /**
      * @inheritDoc
      */
-    public function setFunding(array $funding)
+    public function setFunding(array $funding): void
     {
         $this->funding = $funding;
     }
@@ -183,7 +183,7 @@ public function setFunding(array $funding)
     /**
      * @inheritDoc
      */
-    public function getFunding()
+    public function getFunding(): array
     {
         return $this->funding;
     }
@@ -191,7 +191,7 @@ public function getFunding()
     /**
      * @inheritDoc
      */
-    public function isAbandoned()
+    public function isAbandoned(): bool
     {
         return (bool) $this->abandoned;
     }
@@ -199,7 +199,7 @@ public function isAbandoned()
     /**
      * @inheritDoc
      */
-    public function setAbandoned($abandoned)
+    public function setAbandoned($abandoned): void
     {
         $this->abandoned = $abandoned;
     }
@@ -207,7 +207,7 @@ public function setAbandoned($abandoned)
     /**
      * @inheritDoc
      */
-    public function getReplacementPackage()
+    public function getReplacementPackage(): ?string
     {
         return \is_string($this->abandoned) ? $this->abandoned : null;
     }
@@ -215,7 +215,7 @@ public function getReplacementPackage()
     /**
      * @inheritDoc
      */
-    public function setArchiveName(?string $name)
+    public function setArchiveName(?string $name): void
     {
         $this->archiveName = $name;
     }
@@ -223,7 +223,7 @@ public function setArchiveName(?string $name)
     /**
      * @inheritDoc
      */
-    public function getArchiveName()
+    public function getArchiveName(): ?string
     {
         return $this->archiveName;
     }
@@ -231,7 +231,7 @@ public function getArchiveName()
     /**
      * @inheritDoc
      */
-    public function setArchiveExcludes(array $excludes)
+    public function setArchiveExcludes(array $excludes): void
     {
         $this->archiveExcludes = $excludes;
     }
@@ -239,7 +239,7 @@ public function setArchiveExcludes(array $excludes)
     /**
      * @inheritDoc
      */
-    public function getArchiveExcludes()
+    public function getArchiveExcludes(): array
     {
         return $this->archiveExcludes;
     }
diff --git a/src/Composer/Package/CompletePackageInterface.php b/src/Composer/Package/CompletePackageInterface.php
index e65cc0c4c694..35db9e3cb2fc 100644
--- a/src/Composer/Package/CompletePackageInterface.php
+++ b/src/Composer/Package/CompletePackageInterface.php
@@ -15,6 +15,8 @@
 /**
  * Defines package metadata that is not necessarily needed for solving and installing packages
  *
+ * PackageInterface & derivatives are considered internal, you may use them in type hints but extending/implementing them is not recommended and not supported. Things may change without notice.
+ *
  * @author Nils Adermann <naderman@naderman.de>
  */
 interface CompletePackageInterface extends PackageInterface
@@ -24,20 +26,20 @@ interface CompletePackageInterface extends PackageInterface
      *
      * @return array<string, string[]> Map of script name to array of handlers
      */
-    public function getScripts();
+    public function getScripts(): array;
 
     /**
      * @param  array<string, string[]> $scripts
      * @return void
      */
-    public function setScripts(array $scripts);
+    public function setScripts(array $scripts): void;
 
     /**
      * Returns an array of repositories
      *
      * @return mixed[] Repositories
      */
-    public function getRepositories();
+    public function getRepositories(): array;
 
     /**
      * Set the repositories
@@ -45,14 +47,14 @@ public function getRepositories();
      * @param  mixed[] $repositories
      * @return void
      */
-    public function setRepositories(array $repositories);
+    public function setRepositories(array $repositories): void;
 
     /**
      * Returns the package license, e.g. MIT, BSD, GPL
      *
      * @return string[] The package licenses
      */
-    public function getLicense();
+    public function getLicense(): array;
 
     /**
      * Set the license
@@ -60,14 +62,14 @@ public function getLicense();
      * @param  string[] $license
      * @return void
      */
-    public function setLicense(array $license);
+    public function setLicense(array $license): void;
 
     /**
      * Returns an array of keywords relating to the package
      *
      * @return string[]
      */
-    public function getKeywords();
+    public function getKeywords(): array;
 
     /**
      * Set the keywords
@@ -75,14 +77,14 @@ public function getKeywords();
      * @param  string[] $keywords
      * @return void
      */
-    public function setKeywords(array $keywords);
+    public function setKeywords(array $keywords): void;
 
     /**
      * Returns the package description
      *
      * @return ?string
      */
-    public function getDescription();
+    public function getDescription(): ?string;
 
     /**
      * Set the description
@@ -90,14 +92,14 @@ public function getDescription();
      * @param  string $description
      * @return void
      */
-    public function setDescription(string $description);
+    public function setDescription(string $description): void;
 
     /**
      * Returns the package homepage
      *
      * @return ?string
      */
-    public function getHomepage();
+    public function getHomepage(): ?string;
 
     /**
      * Set the homepage
@@ -105,7 +107,7 @@ public function getHomepage();
      * @param  string $homepage
      * @return void
      */
-    public function setHomepage(string $homepage);
+    public function setHomepage(string $homepage): void;
 
     /**
      * Returns an array of authors of the package
@@ -114,7 +116,7 @@ public function setHomepage(string $homepage);
      *
      * @return array<array{name?: string, homepage?: string, email?: string, role?: string}>
      */
-    public function getAuthors();
+    public function getAuthors(): array;
 
     /**
      * Set the authors
@@ -122,14 +124,14 @@ public function getAuthors();
      * @param  array<array{name?: string, homepage?: string, email?: string, role?: string}> $authors
      * @return void
      */
-    public function setAuthors(array $authors);
+    public function setAuthors(array $authors): void;
 
     /**
      * Returns the support information
      *
      * @return array{issues?: string, forum?: string, wiki?: string, source?: string, email?: string, irc?: string, docs?: string, rss?: string, chat?: string}
      */
-    public function getSupport();
+    public function getSupport(): array;
 
     /**
      * Set the support information
@@ -137,7 +139,7 @@ public function getSupport();
      * @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);
+    public function setSupport(array $support): void;
 
     /**
      * Returns an array of funding options for the package
@@ -146,7 +148,7 @@ public function setSupport(array $support);
      *
      * @return array<array{type?: string, url?: string}>
      */
-    public function getFunding();
+    public function getFunding(): array;
 
     /**
      * Set the funding
@@ -154,34 +156,34 @@ public function getFunding();
      * @param  array<array{type?: string, url?: string}> $funding
      * @return void
      */
-    public function setFunding(array $funding);
+    public function setFunding(array $funding): void;
 
     /**
      * Returns if the package is abandoned or not
      *
      * @return bool
      */
-    public function isAbandoned();
+    public function isAbandoned(): bool;
 
     /**
      * If the package is abandoned and has a suggested replacement, this method returns it
      *
      * @return string|null
      */
-    public function getReplacementPackage();
+    public function getReplacementPackage(): ?string;
 
     /**
      * @param  bool|string $abandoned
      * @return void
      */
-    public function setAbandoned($abandoned);
+    public function setAbandoned($abandoned): void;
 
     /**
      * Returns default base filename for archive
      *
      * @return ?string
      */
-    public function getArchiveName();
+    public function getArchiveName(): ?string;
 
     /**
      * Sets default base filename for archive
@@ -189,14 +191,14 @@ public function getArchiveName();
      * @param  string $name
      * @return void
      */
-    public function setArchiveName(string $name);
+    public function setArchiveName(string $name): void;
 
     /**
      * Returns a list of patterns to exclude from package archives
      *
      * @return string[]
      */
-    public function getArchiveExcludes();
+    public function getArchiveExcludes(): array;
 
     /**
      * Sets a list of patterns to be excluded from archives
@@ -204,5 +206,5 @@ public function getArchiveExcludes();
      * @param  string[] $excludes
      * @return void
      */
-    public function setArchiveExcludes(array $excludes);
+    public function setArchiveExcludes(array $excludes): void;
 }
diff --git a/src/Composer/Package/Link.php b/src/Composer/Package/Link.php
index a7628a720ef8..481988c154f1 100644
--- a/src/Composer/Package/Link.php
+++ b/src/Composer/Package/Link.php
@@ -21,22 +21,22 @@
  */
 class Link
 {
-    const TYPE_REQUIRE = 'requires';
-    const TYPE_DEV_REQUIRE = 'devRequires';
-    const TYPE_PROVIDE = 'provides';
-    const TYPE_CONFLICT = 'conflicts';
-    const TYPE_REPLACE = 'replaces';
+    public const TYPE_REQUIRE = 'requires';
+    public const TYPE_DEV_REQUIRE = 'devRequires';
+    public const TYPE_PROVIDE = 'provides';
+    public const TYPE_CONFLICT = 'conflicts';
+    public const TYPE_REPLACE = 'replaces';
 
     /**
      * Special type
      * @internal
      */
-    const TYPE_DOES_NOT_REQUIRE = 'does not require';
+    public const TYPE_DOES_NOT_REQUIRE = 'does not require';
     /**
      * TODO should be marked private once 5.3 is dropped
      * @private
      */
-    const TYPE_UNKNOWN = 'relates to';
+    private const TYPE_UNKNOWN = 'relates to';
 
     /**
      * Will be converted into a constant once the min PHP version allows this
diff --git a/src/Composer/Package/Package.php b/src/Composer/Package/Package.php
index ed5d52ee8f6b..90dc0688fdb6 100644
--- a/src/Composer/Package/Package.php
+++ b/src/Composer/Package/Package.php
@@ -120,7 +120,7 @@ public function __construct(string $name, string $version, string $prettyVersion
     /**
      * @inheritDoc
      */
-    public function isDev()
+    public function isDev(): bool
     {
         return $this->dev;
     }
@@ -130,7 +130,7 @@ public function isDev()
      *
      * @return void
      */
-    public function setType(string $type)
+    public function setType(string $type): void
     {
         $this->type = $type;
     }
@@ -138,7 +138,7 @@ public function setType(string $type)
     /**
      * @inheritDoc
      */
-    public function getType()
+    public function getType(): string
     {
         return $this->type ?: 'library';
     }
@@ -146,7 +146,7 @@ public function getType()
     /**
      * @inheritDoc
      */
-    public function getStability()
+    public function getStability(): string
     {
         return $this->stability;
     }
@@ -154,7 +154,7 @@ public function getStability()
     /**
      * @return void
      */
-    public function setTargetDir(?string $targetDir)
+    public function setTargetDir(?string $targetDir): void
     {
         $this->targetDir = $targetDir;
     }
@@ -162,7 +162,7 @@ public function setTargetDir(?string $targetDir)
     /**
      * @inheritDoc
      */
-    public function getTargetDir()
+    public function getTargetDir(): ?string
     {
         if (null === $this->targetDir) {
             return null;
@@ -176,7 +176,7 @@ public function getTargetDir()
      *
      * @return void
      */
-    public function setExtra(array $extra)
+    public function setExtra(array $extra): void
     {
         $this->extra = $extra;
     }
@@ -184,7 +184,7 @@ public function setExtra(array $extra)
     /**
      * @inheritDoc
      */
-    public function getExtra()
+    public function getExtra(): array
     {
         return $this->extra;
     }
@@ -194,7 +194,7 @@ public function getExtra()
      *
      * @return void
      */
-    public function setBinaries(array $binaries)
+    public function setBinaries(array $binaries): void
     {
         $this->binaries = $binaries;
     }
@@ -202,7 +202,7 @@ public function setBinaries(array $binaries)
     /**
      * @inheritDoc
      */
-    public function getBinaries()
+    public function getBinaries(): array
     {
         return $this->binaries;
     }
@@ -212,7 +212,7 @@ public function getBinaries()
      *
      * @return void
      */
-    public function setInstallationSource(?string $type)
+    public function setInstallationSource(?string $type): void
     {
         $this->installationSource = $type;
     }
@@ -220,7 +220,7 @@ public function setInstallationSource(?string $type)
     /**
      * @inheritDoc
      */
-    public function getInstallationSource()
+    public function getInstallationSource(): ?string
     {
         return $this->installationSource;
     }
@@ -228,7 +228,7 @@ public function getInstallationSource()
     /**
      * @return void
      */
-    public function setSourceType(?string $type)
+    public function setSourceType(?string $type): void
     {
         $this->sourceType = $type;
     }
@@ -236,7 +236,7 @@ public function setSourceType(?string $type)
     /**
      * @inheritDoc
      */
-    public function getSourceType()
+    public function getSourceType(): ?string
     {
         return $this->sourceType;
     }
@@ -244,7 +244,7 @@ public function getSourceType()
     /**
      * @return void
      */
-    public function setSourceUrl(?string $url)
+    public function setSourceUrl(?string $url): void
     {
         $this->sourceUrl = $url;
     }
@@ -252,7 +252,7 @@ public function setSourceUrl(?string $url)
     /**
      * @inheritDoc
      */
-    public function getSourceUrl()
+    public function getSourceUrl(): ?string
     {
         return $this->sourceUrl;
     }
@@ -260,7 +260,7 @@ public function getSourceUrl()
     /**
      * @return void
      */
-    public function setSourceReference(?string $reference)
+    public function setSourceReference(?string $reference): void
     {
         $this->sourceReference = $reference;
     }
@@ -268,7 +268,7 @@ public function setSourceReference(?string $reference)
     /**
      * @inheritDoc
      */
-    public function getSourceReference()
+    public function getSourceReference(): ?string
     {
         return $this->sourceReference;
     }
@@ -276,7 +276,7 @@ public function getSourceReference()
     /**
      * @return void
      */
-    public function setSourceMirrors(?array $mirrors)
+    public function setSourceMirrors(?array $mirrors): void
     {
         $this->sourceMirrors = $mirrors;
     }
@@ -284,7 +284,7 @@ public function setSourceMirrors(?array $mirrors)
     /**
      * @inheritDoc
      */
-    public function getSourceMirrors()
+    public function getSourceMirrors(): ?array
     {
         return $this->sourceMirrors;
     }
@@ -292,7 +292,7 @@ public function getSourceMirrors()
     /**
      * @inheritDoc
      */
-    public function getSourceUrls()
+    public function getSourceUrls(): array
     {
         return $this->getUrls($this->sourceUrl, $this->sourceMirrors, $this->sourceReference, $this->sourceType, 'source');
     }
@@ -302,7 +302,7 @@ public function getSourceUrls()
      *
      * @return void
      */
-    public function setDistType(?string $type)
+    public function setDistType(?string $type): void
     {
         $this->distType = $type;
     }
@@ -310,7 +310,7 @@ public function setDistType(?string $type)
     /**
      * @inheritDoc
      */
-    public function getDistType()
+    public function getDistType(): ?string
     {
         return $this->distType;
     }
@@ -320,7 +320,7 @@ public function getDistType()
      *
      * @return void
      */
-    public function setDistUrl(?string $url)
+    public function setDistUrl(?string $url): void
     {
         $this->distUrl = $url;
     }
@@ -328,7 +328,7 @@ public function setDistUrl(?string $url)
     /**
      * @inheritDoc
      */
-    public function getDistUrl()
+    public function getDistUrl(): ?string
     {
         return $this->distUrl;
     }
@@ -338,7 +338,7 @@ public function getDistUrl()
      *
      * @return void
      */
-    public function setDistReference(?string $reference)
+    public function setDistReference(?string $reference): void
     {
         $this->distReference = $reference;
     }
@@ -346,7 +346,7 @@ public function setDistReference(?string $reference)
     /**
      * @inheritDoc
      */
-    public function getDistReference()
+    public function getDistReference(): ?string
     {
         return $this->distReference;
     }
@@ -356,7 +356,7 @@ public function getDistReference()
      *
      * @return void
      */
-    public function setDistSha1Checksum(?string $sha1checksum)
+    public function setDistSha1Checksum(?string $sha1checksum): void
     {
         $this->distSha1Checksum = $sha1checksum;
     }
@@ -364,7 +364,7 @@ public function setDistSha1Checksum(?string $sha1checksum)
     /**
      * @inheritDoc
      */
-    public function getDistSha1Checksum()
+    public function getDistSha1Checksum(): ?string
     {
         return $this->distSha1Checksum;
     }
@@ -372,7 +372,7 @@ public function getDistSha1Checksum()
     /**
      * @return void
      */
-    public function setDistMirrors(?array $mirrors)
+    public function setDistMirrors(?array $mirrors): void
     {
         $this->distMirrors = $mirrors;
     }
@@ -380,7 +380,7 @@ public function setDistMirrors(?array $mirrors)
     /**
      * @inheritDoc
      */
-    public function getDistMirrors()
+    public function getDistMirrors(): ?array
     {
         return $this->distMirrors;
     }
@@ -388,7 +388,7 @@ public function getDistMirrors()
     /**
      * @inheritDoc
      */
-    public function getDistUrls()
+    public function getDistUrls(): array
     {
         return $this->getUrls($this->distUrl, $this->distMirrors, $this->distReference, $this->distType, 'dist');
     }
@@ -396,7 +396,7 @@ public function getDistUrls()
     /**
      * @inheritDoc
      */
-    public function getTransportOptions()
+    public function getTransportOptions(): array
     {
         return $this->transportOptions;
     }
@@ -404,7 +404,7 @@ public function getTransportOptions()
     /**
      * @inheritDoc
      */
-    public function setTransportOptions(array $options)
+    public function setTransportOptions(array $options): void
     {
         $this->transportOptions = $options;
     }
@@ -412,7 +412,7 @@ public function setTransportOptions(array $options)
     /**
      * @inheritDoc
      */
-    public function getVersion()
+    public function getVersion(): string
     {
         return $this->version;
     }
@@ -420,7 +420,7 @@ public function getVersion()
     /**
      * @inheritDoc
      */
-    public function getPrettyVersion()
+    public function getPrettyVersion(): string
     {
         return $this->prettyVersion;
     }
@@ -428,7 +428,7 @@ public function getPrettyVersion()
     /**
      * @return void
      */
-    public function setReleaseDate(?\DateTimeInterface $releaseDate)
+    public function setReleaseDate(?\DateTimeInterface $releaseDate): void
     {
         $this->releaseDate = $releaseDate;
     }
@@ -436,7 +436,7 @@ public function setReleaseDate(?\DateTimeInterface $releaseDate)
     /**
      * @inheritDoc
      */
-    public function getReleaseDate()
+    public function getReleaseDate(): ?\DateTimeInterface
     {
         return $this->releaseDate;
     }
@@ -448,7 +448,7 @@ public function getReleaseDate()
      *
      * @return void
      */
-    public function setRequires(array $requires)
+    public function setRequires(array $requires): void
     {
         if (isset($requires[0])) { // @phpstan-ignore-line
             $requires = $this->convertLinksToMap($requires, 'setRequires');
@@ -460,7 +460,7 @@ public function setRequires(array $requires)
     /**
      * @inheritDoc
      */
-    public function getRequires()
+    public function getRequires(): array
     {
         return $this->requires;
     }
@@ -472,7 +472,7 @@ public function getRequires()
      *
      * @return void
      */
-    public function setConflicts(array $conflicts)
+    public function setConflicts(array $conflicts): void
     {
         if (isset($conflicts[0])) { // @phpstan-ignore-line
             $conflicts = $this->convertLinksToMap($conflicts, 'setConflicts');
@@ -485,7 +485,7 @@ public function setConflicts(array $conflicts)
      * @inheritDoc
      * @return array<string, Link>
      */
-    public function getConflicts()
+    public function getConflicts(): array
     {
         return $this->conflicts;
     }
@@ -497,7 +497,7 @@ public function getConflicts()
      *
      * @return void
      */
-    public function setProvides(array $provides)
+    public function setProvides(array $provides): void
     {
         if (isset($provides[0])) { // @phpstan-ignore-line
             $provides = $this->convertLinksToMap($provides, 'setProvides');
@@ -510,7 +510,7 @@ public function setProvides(array $provides)
      * @inheritDoc
      * @return array<string, Link>
      */
-    public function getProvides()
+    public function getProvides(): array
     {
         return $this->provides;
     }
@@ -522,7 +522,7 @@ public function getProvides()
      *
      * @return void
      */
-    public function setReplaces(array $replaces)
+    public function setReplaces(array $replaces): void
     {
         if (isset($replaces[0])) { // @phpstan-ignore-line
             $replaces = $this->convertLinksToMap($replaces, 'setReplaces');
@@ -535,7 +535,7 @@ public function setReplaces(array $replaces)
      * @inheritDoc
      * @return array<string, Link>
      */
-    public function getReplaces()
+    public function getReplaces(): array
     {
         return $this->replaces;
     }
@@ -547,7 +547,7 @@ public function getReplaces()
      *
      * @return void
      */
-    public function setDevRequires(array $devRequires)
+    public function setDevRequires(array $devRequires): void
     {
         if (isset($devRequires[0])) { // @phpstan-ignore-line
             $devRequires = $this->convertLinksToMap($devRequires, 'setDevRequires');
@@ -559,7 +559,7 @@ public function setDevRequires(array $devRequires)
     /**
      * @inheritDoc
      */
-    public function getDevRequires()
+    public function getDevRequires(): array
     {
         return $this->devRequires;
     }
@@ -571,7 +571,7 @@ public function getDevRequires()
      *
      * @return void
      */
-    public function setSuggests(array $suggests)
+    public function setSuggests(array $suggests): void
     {
         $this->suggests = $suggests;
     }
@@ -579,7 +579,7 @@ public function setSuggests(array $suggests)
     /**
      * @inheritDoc
      */
-    public function getSuggests()
+    public function getSuggests(): array
     {
         return $this->suggests;
     }
@@ -593,7 +593,7 @@ public function getSuggests()
      *
      * @phpstan-param AutoloadRules $autoload
      */
-    public function setAutoload(array $autoload)
+    public function setAutoload(array $autoload): void
     {
         $this->autoload = $autoload;
     }
@@ -601,7 +601,7 @@ public function setAutoload(array $autoload)
     /**
      * @inheritDoc
      */
-    public function getAutoload()
+    public function getAutoload(): array
     {
         return $this->autoload;
     }
@@ -615,7 +615,7 @@ public function getAutoload()
      *
      * @phpstan-param DevAutoloadRules $devAutoload
      */
-    public function setDevAutoload(array $devAutoload)
+    public function setDevAutoload(array $devAutoload): void
     {
         $this->devAutoload = $devAutoload;
     }
@@ -623,7 +623,7 @@ public function setDevAutoload(array $devAutoload)
     /**
      * @inheritDoc
      */
-    public function getDevAutoload()
+    public function getDevAutoload(): array
     {
         return $this->devAutoload;
     }
@@ -635,7 +635,7 @@ public function getDevAutoload()
      *
      * @return void
      */
-    public function setIncludePaths(array $includePaths)
+    public function setIncludePaths(array $includePaths): void
     {
         $this->includePaths = $includePaths;
     }
@@ -643,7 +643,7 @@ public function setIncludePaths(array $includePaths)
     /**
      * @inheritDoc
      */
-    public function getIncludePaths()
+    public function getIncludePaths(): array
     {
         return $this->includePaths;
     }
@@ -655,7 +655,7 @@ public function getIncludePaths()
      *
      * @return void
      */
-    public function setNotificationUrl(string $notificationUrl)
+    public function setNotificationUrl(string $notificationUrl): void
     {
         $this->notificationUrl = $notificationUrl;
     }
@@ -663,7 +663,7 @@ public function setNotificationUrl(string $notificationUrl)
     /**
      * @inheritDoc
      */
-    public function getNotificationUrl()
+    public function getNotificationUrl(): ?string
     {
         return $this->notificationUrl;
     }
@@ -673,7 +673,7 @@ public function getNotificationUrl()
      *
      * @return void
      */
-    public function setIsDefaultBranch(bool $defaultBranch)
+    public function setIsDefaultBranch(bool $defaultBranch): void
     {
         $this->isDefaultBranch = $defaultBranch;
     }
@@ -681,7 +681,7 @@ public function setIsDefaultBranch(bool $defaultBranch)
     /**
      * @inheritDoc
      */
-    public function isDefaultBranch()
+    public function isDefaultBranch(): bool
     {
         return $this->isDefaultBranch;
     }
@@ -689,7 +689,7 @@ public function isDefaultBranch()
     /**
      * @inheritDoc
      */
-    public function setSourceDistReferences(string $reference)
+    public function setSourceDistReferences(string $reference): void
     {
         $this->setSourceReference($reference);
 
@@ -715,7 +715,7 @@ public function setSourceDistReferences(string $reference)
      *
      * @return void
      */
-    public function replaceVersion(string $version, string $prettyVersion)
+    public function replaceVersion(string $version, string $prettyVersion): void
     {
         $this->version = $version;
         $this->prettyVersion = $prettyVersion;
@@ -735,7 +735,7 @@ public function replaceVersion(string $version, string $prettyVersion)
      *
      * @phpstan-param list<array{url: string, preferred: bool}>|null $mirrors
      */
-    protected function getUrls(?string $url, ?array $mirrors, ?string $ref, ?string $type, string $urlType)
+    protected function getUrls(?string $url, ?array $mirrors, ?string $ref, ?string $type, string $urlType): array
     {
         if (!$url) {
             return array();
diff --git a/src/Composer/Package/PackageInterface.php b/src/Composer/Package/PackageInterface.php
index 50512bfe513e..31a0e78fb8f9 100644
--- a/src/Composer/Package/PackageInterface.php
+++ b/src/Composer/Package/PackageInterface.php
@@ -17,6 +17,8 @@
 /**
  * Defines the essential information a package has that is used during solving/installation
  *
+ * PackageInterface & derivatives are considered internal, you may use them in type hints but extending/implementing them is not recommended and not supported. Things may change without notice.
+ *
  * @author Jordi Boggiano <j.boggiano@seld.be>
  *
  * @phpstan-type AutoloadRules    array{psr-0?: array<string, string|string[]>, psr-4?: array<string, string|string[]>, classmap?: list<string>, files?: list<string>, exclude-from-classmap?: list<string>}
@@ -33,14 +35,14 @@ interface PackageInterface
      *
      * @return string package name
      */
-    public function getName();
+    public function getName(): string;
 
     /**
      * Returns the package's pretty (i.e. with proper case) name
      *
      * @return string package name
      */
-    public function getPrettyName();
+    public function getPrettyName(): string;
 
     /**
      * Returns a set of names that could refer to this package
@@ -52,7 +54,7 @@ public function getPrettyName();
      *
      * @return string[] An array of strings referring to this package
      */
-    public function getNames(bool $provides = true);
+    public function getNames(bool $provides = true): array;
 
     /**
      * Allows the solver to set an id for this package to refer to it.
@@ -61,42 +63,42 @@ public function getNames(bool $provides = true);
      *
      * @return void
      */
-    public function setId(int $id);
+    public function setId(int $id): void;
 
     /**
      * Retrieves the package's id set through setId
      *
      * @return int The previously set package id
      */
-    public function getId();
+    public function getId(): int;
 
     /**
      * Returns whether the package is a development virtual package or a concrete one
      *
      * @return bool
      */
-    public function isDev();
+    public function isDev(): bool;
 
     /**
      * Returns the package type, e.g. library
      *
      * @return string The package type
      */
-    public function getType();
+    public function getType(): string;
 
     /**
      * Returns the package targetDir property
      *
      * @return ?string The package targetDir
      */
-    public function getTargetDir();
+    public function getTargetDir(): ?string;
 
     /**
      * Returns the package extra data
      *
      * @return mixed[] The package extra data
      */
-    public function getExtra();
+    public function getExtra(): array;
 
     /**
      * Sets source from which this package was installed (source/dist).
@@ -106,7 +108,7 @@ public function getExtra();
      *
      * @return void
      */
-    public function setInstallationSource(?string $type);
+    public function setInstallationSource(?string $type): void;
 
     /**
      * Returns source from which this package was installed (source/dist).
@@ -114,110 +116,110 @@ public function setInstallationSource(?string $type);
      * @return ?string source/dist
      * @phpstan-return 'source'|'dist'|null
      */
-    public function getInstallationSource();
+    public function getInstallationSource(): ?string;
 
     /**
      * Returns the repository type of this package, e.g. git, svn
      *
      * @return ?string The repository type
      */
-    public function getSourceType();
+    public function getSourceType(): ?string;
 
     /**
      * Returns the repository url of this package, e.g. git://github.com/naderman/composer.git
      *
      * @return ?string The repository url
      */
-    public function getSourceUrl();
+    public function getSourceUrl(): ?string;
 
     /**
      * Returns the repository urls of this package including mirrors, e.g. git://github.com/naderman/composer.git
      *
      * @return string[]
      */
-    public function getSourceUrls();
+    public function getSourceUrls(): array;
 
     /**
      * Returns the repository reference of this package, e.g. master, 1.0.0 or a commit hash for git
      *
      * @return ?string The repository reference
      */
-    public function getSourceReference();
+    public function getSourceReference(): ?string;
 
     /**
      * Returns the source mirrors of this package
      *
      * @return ?array<int, array{url: string, preferred: bool}>
      */
-    public function getSourceMirrors();
+    public function getSourceMirrors(): ?array;
 
     /**
      * @param  null|array<int, array{url: string, preferred: bool}> $mirrors
      * @return void
      */
-    public function setSourceMirrors(?array $mirrors);
+    public function setSourceMirrors(?array $mirrors): void;
 
     /**
      * Returns the type of the distribution archive of this version, e.g. zip, tarball
      *
      * @return ?string The repository type
      */
-    public function getDistType();
+    public function getDistType(): ?string;
 
     /**
      * Returns the url of the distribution archive of this version
      *
      * @return ?string
      */
-    public function getDistUrl();
+    public function getDistUrl(): ?string;
 
     /**
      * Returns the urls of the distribution archive of this version, including mirrors
      *
      * @return string[]
      */
-    public function getDistUrls();
+    public function getDistUrls(): array;
 
     /**
      * Returns the reference of the distribution archive of this version, e.g. master, 1.0.0 or a commit hash for git
      *
      * @return ?string
      */
-    public function getDistReference();
+    public function getDistReference(): ?string;
 
     /**
      * Returns the sha1 checksum for the distribution archive of this version
      *
      * @return ?string
      */
-    public function getDistSha1Checksum();
+    public function getDistSha1Checksum(): ?string;
 
     /**
      * Returns the dist mirrors of this package
      *
      * @return ?array<int, array{url: string, preferred: bool}>
      */
-    public function getDistMirrors();
+    public function getDistMirrors(): ?array;
 
     /**
      * @param  null|array<int, array{url: string, preferred: bool}> $mirrors
      * @return void
      */
-    public function setDistMirrors(?array $mirrors);
+    public function setDistMirrors(?array $mirrors): void;
 
     /**
      * Returns the version of this package
      *
      * @return string version
      */
-    public function getVersion();
+    public function getVersion(): string;
 
     /**
      * Returns the pretty (i.e. non-normalized) version string of this package
      *
      * @return string version
      */
-    public function getPrettyVersion();
+    public function getPrettyVersion(): string;
 
     /**
      * Returns the pretty version string plus a git or hg commit hash of this package
@@ -230,14 +232,14 @@ public function getPrettyVersion();
      *
      * @phpstan-param self::DISPLAY_SOURCE_REF_IF_DEV|self::DISPLAY_SOURCE_REF|self::DISPLAY_DIST_REF $displayMode
      */
-    public function getFullPrettyVersion(bool $truncate = true, int $displayMode = self::DISPLAY_SOURCE_REF_IF_DEV);
+    public function getFullPrettyVersion(bool $truncate = true, int $displayMode = self::DISPLAY_SOURCE_REF_IF_DEV): string;
 
     /**
      * Returns the release date of the package
      *
      * @return ?\DateTimeInterface
      */
-    public function getReleaseDate();
+    public function getReleaseDate(): ?\DateTimeInterface;
 
     /**
      * Returns the stability of this package: one of (dev, alpha, beta, RC, stable)
@@ -246,7 +248,7 @@ public function getReleaseDate();
      *
      * @phpstan-return 'stable'|'RC'|'beta'|'alpha'|'dev'
      */
-    public function getStability();
+    public function getStability(): string;
 
     /**
      * Returns a set of links to packages which need to be installed before
@@ -254,7 +256,7 @@ public function getStability();
      *
      * @return array<string, Link> A map of package links defining required packages, indexed by the require package's name
      */
-    public function getRequires();
+    public function getRequires(): array;
 
     /**
      * Returns a set of links to packages which must not be installed at the
@@ -262,7 +264,7 @@ public function getRequires();
      *
      * @return Link[] An array of package links defining conflicting packages
      */
-    public function getConflicts();
+    public function getConflicts(): array;
 
     /**
      * Returns a set of links to virtual packages that are provided through
@@ -270,7 +272,7 @@ public function getConflicts();
      *
      * @return Link[] An array of package links defining provided packages
      */
-    public function getProvides();
+    public function getProvides(): array;
 
     /**
      * Returns a set of links to packages which can alternatively be
@@ -278,7 +280,7 @@ public function getProvides();
      *
      * @return Link[] An array of package links defining replaced packages
      */
-    public function getReplaces();
+    public function getReplaces(): array;
 
     /**
      * Returns a set of links to packages which are required to develop
@@ -286,7 +288,7 @@ public function getReplaces();
      *
      * @return array<string, Link> A map of package links defining packages required for development, indexed by the require package's name
      */
-    public function getDevRequires();
+    public function getDevRequires(): array;
 
     /**
      * Returns a set of package names and reasons why they are useful in
@@ -295,7 +297,7 @@ public function getDevRequires();
      * @return array An array of package suggestions with descriptions
      * @phpstan-return array<string, string>
      */
-    public function getSuggests();
+    public function getSuggests(): array;
 
     /**
      * Returns an associative array of autoloading rules
@@ -308,7 +310,7 @@ public function getSuggests();
      * @return array Mapping of autoloading rules
      * @phpstan-return AutoloadRules
      */
-    public function getAutoload();
+    public function getAutoload(): array;
 
     /**
      * Returns an associative array of dev autoloading rules
@@ -321,7 +323,7 @@ public function getAutoload();
      * @return array Mapping of dev autoloading rules
      * @phpstan-return DevAutoloadRules
      */
-    public function getDevAutoload();
+    public function getDevAutoload(): array;
 
     /**
      * Returns a list of directories which should get added to PHP's
@@ -329,7 +331,7 @@ public function getDevAutoload();
      *
      * @return string[]
      */
-    public function getIncludePaths();
+    public function getIncludePaths(): array;
 
     /**
      * Stores a reference to the repository that owns the package
@@ -338,61 +340,61 @@ public function getIncludePaths();
      *
      * @return void
      */
-    public function setRepository(RepositoryInterface $repository);
+    public function setRepository(RepositoryInterface $repository): void;
 
     /**
      * Returns a reference to the repository that owns the package
      *
      * @return ?RepositoryInterface
      */
-    public function getRepository();
+    public function getRepository(): ?RepositoryInterface;
 
     /**
      * Returns the package binaries
      *
      * @return string[]
      */
-    public function getBinaries();
+    public function getBinaries(): array;
 
     /**
      * Returns package unique name, constructed from name and version.
      *
      * @return string
      */
-    public function getUniqueName();
+    public function getUniqueName(): string;
 
     /**
      * Returns the package notification url
      *
      * @return ?string
      */
-    public function getNotificationUrl();
+    public function getNotificationUrl(): ?string;
 
     /**
      * Converts the package into a readable and unique string
      *
      * @return string
      */
-    public function __toString();
+    public function __toString(): string;
 
     /**
      * Converts the package into a pretty readable string
      *
      * @return string
      */
-    public function getPrettyString();
+    public function getPrettyString(): string;
 
     /**
      * @return bool
      */
-    public function isDefaultBranch();
+    public function isDefaultBranch(): bool;
 
     /**
      * Returns a list of options to download package dist files
      *
      * @return mixed[]
      */
-    public function getTransportOptions();
+    public function getTransportOptions(): array;
 
     /**
      * Configures the list of options to download package dist files
@@ -401,32 +403,32 @@ public function getTransportOptions();
      *
      * @return void
      */
-    public function setTransportOptions(array $options);
+    public function setTransportOptions(array $options): void;
 
     /**
      * @return void
      */
-    public function setSourceReference(?string $reference);
+    public function setSourceReference(?string $reference): void;
 
     /**
      * @return void
      */
-    public function setDistUrl(?string $url);
+    public function setDistUrl(?string $url): void;
 
     /**
      * @return void
      */
-    public function setDistType(?string $type);
+    public function setDistType(?string $type): void;
 
     /**
      * @return void
      */
-    public function setDistReference(?string $reference);
+    public function setDistReference(?string $reference): void;
 
     /**
      * Set dist and source references and update dist URL for ones that contain a reference
      *
      * @return void
      */
-    public function setSourceDistReferences(string $reference);
+    public function setSourceDistReferences(string $reference): void;
 }
diff --git a/src/Composer/Package/RootPackageInterface.php b/src/Composer/Package/RootPackageInterface.php
index a1783f87579e..9e74e4ddc8f3 100644
--- a/src/Composer/Package/RootPackageInterface.php
+++ b/src/Composer/Package/RootPackageInterface.php
@@ -15,6 +15,8 @@
 /**
  * Defines additional fields that are only needed for the root package
  *
+ * PackageInterface & derivatives are considered internal, you may use them in type hints but extending/implementing them is not recommended and not supported. Things may change without notice.
+ *
  * @author Jordi Boggiano <j.boggiano@seld.be>
  *
  * @phpstan-import-type AutoloadRules from PackageInterface
@@ -27,14 +29,14 @@ interface RootPackageInterface extends CompletePackageInterface
      *
      * @return array<array{package: string, version: string, alias: string, alias_normalized: string}>
      */
-    public function getAliases();
+    public function getAliases(): array;
 
     /**
      * Returns the minimum stability of the package
      *
      * @return string
      */
-    public function getMinimumStability();
+    public function getMinimumStability(): string;
 
     /**
      * Returns the stability flags to apply to dependencies
@@ -43,7 +45,7 @@ public function getMinimumStability();
      *
      * @return array<string, BasePackage::STABILITY_*>
      */
-    public function getStabilityFlags();
+    public function getStabilityFlags(): array;
 
     /**
      * Returns a set of package names and source references that must be enforced on them
@@ -52,21 +54,21 @@ public function getStabilityFlags();
      *
      * @return array<string, string>
      */
-    public function getReferences();
+    public function getReferences(): array;
 
     /**
      * Returns true if the root package prefers picking stable packages over unstable ones
      *
      * @return bool
      */
-    public function getPreferStable();
+    public function getPreferStable(): bool;
 
     /**
      * Returns the root package's configuration
      *
      * @return mixed[]
      */
-    public function getConfig();
+    public function getConfig(): array;
 
     /**
      * Set the required packages
@@ -75,7 +77,7 @@ public function getConfig();
      *
      * @return void
      */
-    public function setRequires(array $requires);
+    public function setRequires(array $requires): void;
 
     /**
      * Set the recommended packages
@@ -84,7 +86,7 @@ public function setRequires(array $requires);
      *
      * @return void
      */
-    public function setDevRequires(array $devRequires);
+    public function setDevRequires(array $devRequires): void;
 
     /**
      * Set the conflicting packages
@@ -93,7 +95,7 @@ public function setDevRequires(array $devRequires);
      *
      * @return void
      */
-    public function setConflicts(array $conflicts);
+    public function setConflicts(array $conflicts): void;
 
     /**
      * Set the provided virtual packages
@@ -102,7 +104,7 @@ public function setConflicts(array $conflicts);
      *
      * @return void
      */
-    public function setProvides(array $provides);
+    public function setProvides(array $provides): void;
 
     /**
      * Set the packages this one replaces
@@ -111,7 +113,7 @@ public function setProvides(array $provides);
      *
      * @return void
      */
-    public function setReplaces(array $replaces);
+    public function setReplaces(array $replaces): void;
 
     /**
      * Set the repositories
@@ -120,7 +122,7 @@ public function setReplaces(array $replaces);
      *
      * @return void
      */
-    public function setRepositories(array $repositories);
+    public function setRepositories(array $repositories): void;
 
     /**
      * Set the autoload mapping
@@ -130,7 +132,7 @@ public function setRepositories(array $repositories);
      *
      * @return void
      */
-    public function setAutoload(array $autoload);
+    public function setAutoload(array $autoload): void;
 
     /**
      * Set the dev autoload mapping
@@ -140,7 +142,7 @@ public function setAutoload(array $autoload);
      *
      * @return void
      */
-    public function setDevAutoload(array $devAutoload);
+    public function setDevAutoload(array $devAutoload): void;
 
     /**
      * Set the stabilityFlags
@@ -149,7 +151,7 @@ public function setDevAutoload(array $devAutoload);
      *
      * @return void
      */
-    public function setStabilityFlags(array $stabilityFlags);
+    public function setStabilityFlags(array $stabilityFlags): void;
 
     /**
      * Set the minimumStability
@@ -158,7 +160,7 @@ public function setStabilityFlags(array $stabilityFlags);
      *
      * @return void
      */
-    public function setMinimumStability(string $minimumStability);
+    public function setMinimumStability(string $minimumStability): void;
 
     /**
      * Set the preferStable
@@ -167,7 +169,7 @@ public function setMinimumStability(string $minimumStability);
      *
      * @return void
      */
-    public function setPreferStable(bool $preferStable);
+    public function setPreferStable(bool $preferStable): void;
 
     /**
      * Set the config
@@ -176,7 +178,7 @@ public function setPreferStable(bool $preferStable);
      *
      * @return void
      */
-    public function setConfig(array $config);
+    public function setConfig(array $config): void;
 
     /**
      * Set the references
@@ -185,7 +187,7 @@ public function setConfig(array $config);
      *
      * @return void
      */
-    public function setReferences(array $references);
+    public function setReferences(array $references): void;
 
     /**
      * Set the aliases
@@ -194,7 +196,7 @@ public function setReferences(array $references);
      *
      * @return void
      */
-    public function setAliases(array $aliases);
+    public function setAliases(array $aliases): void;
 
     /**
      * Set the suggested packages
@@ -203,12 +205,12 @@ public function setAliases(array $aliases);
      *
      * @return void
      */
-    public function setSuggests(array $suggests);
+    public function setSuggests(array $suggests): void;
 
     /**
      * @param mixed[] $extra
      *
      * @return void
      */
-    public function setExtra(array $extra);
+    public function setExtra(array $extra): void;
 }
diff --git a/src/Composer/Package/Version/VersionParser.php b/src/Composer/Package/Version/VersionParser.php
index 44b7dce62e6b..faa5002c785c 100644
--- a/src/Composer/Package/Version/VersionParser.php
+++ b/src/Composer/Package/Version/VersionParser.php
@@ -28,7 +28,7 @@ class VersionParser extends SemverVersionParser
     /**
      * @inheritDoc
      */
-    public function parseConstraints($constraints)
+    public function parseConstraints($constraints): ConstraintInterface
     {
         if (!isset(self::$constraints[$constraints])) {
             self::$constraints[$constraints] = parent::parseConstraints($constraints);
@@ -47,7 +47,7 @@ public function parseConstraints($constraints)
      *
      * @return list<array{name: string, version?: string}>
      */
-    public function parseNameVersionPairs(array $pairs)
+    public function parseNameVersionPairs(array $pairs): array
     {
         $pairs = array_values($pairs);
         $result = array();
@@ -76,7 +76,7 @@ public function parseNameVersionPairs(array $pairs)
      *
      * @return bool
      */
-    public static function isUpgrade(string $normalizedFrom, string $normalizedTo)
+    public static function isUpgrade(string $normalizedFrom, string $normalizedTo): bool
     {
         if ($normalizedFrom === $normalizedTo) {
             return true;
diff --git a/src/Composer/Repository/RepositoryManager.php b/src/Composer/Repository/RepositoryManager.php
index 913768028576..0f119931ee7b 100644
--- a/src/Composer/Repository/RepositoryManager.php
+++ b/src/Composer/Repository/RepositoryManager.php
@@ -62,7 +62,7 @@ public function __construct(IOInterface $io, Config $config, HttpDownloader $htt
      *
      * @return PackageInterface|null
      */
-    public function findPackage(string $name, $constraint)
+    public function findPackage(string $name, $constraint): ?PackageInterface
     {
         foreach ($this->repositories as $repository) {
             /** @var RepositoryInterface $repository */
@@ -82,7 +82,7 @@ public function findPackage(string $name, $constraint)
      *
      * @return PackageInterface[]
      */
-    public function findPackages(string $name, $constraint)
+    public function findPackages(string $name, $constraint): array
     {
         $packages = array();
 
@@ -100,7 +100,7 @@ public function findPackages(string $name, $constraint)
      *
      * @return void
      */
-    public function addRepository(RepositoryInterface $repository)
+    public function addRepository(RepositoryInterface $repository): void
     {
         $this->repositories[] = $repository;
     }
@@ -114,7 +114,7 @@ public function addRepository(RepositoryInterface $repository)
      *
      * @return void
      */
-    public function prependRepository(RepositoryInterface $repository)
+    public function prependRepository(RepositoryInterface $repository): void
     {
         array_unshift($this->repositories, $repository);
     }
@@ -128,7 +128,7 @@ public function prependRepository(RepositoryInterface $repository)
      * @throws \InvalidArgumentException if repository for provided type is not registered
      * @return RepositoryInterface
      */
-    public function createRepository(string $type, array $config, string $name = null)
+    public function createRepository(string $type, array $config, string $name = null): RepositoryInterface
     {
         if (!isset($this->repositoryClasses[$type])) {
             throw new \InvalidArgumentException('Repository type is not registered: '.$type);
@@ -162,7 +162,7 @@ public function createRepository(string $type, array $config, string $name = nul
      *
      * @return void
      */
-    public function setRepositoryClass(string $type, $class)
+    public function setRepositoryClass(string $type, $class): void
     {
         $this->repositoryClasses[$type] = $class;
     }
@@ -172,7 +172,7 @@ public function setRepositoryClass(string $type, $class)
      *
      * @return RepositoryInterface[]
      */
-    public function getRepositories()
+    public function getRepositories(): array
     {
         return $this->repositories;
     }
@@ -184,7 +184,7 @@ public function getRepositories()
      *
      * @return void
      */
-    public function setLocalRepository(InstalledRepositoryInterface $repository)
+    public function setLocalRepository(InstalledRepositoryInterface $repository): void
     {
         $this->localRepository = $repository;
     }
@@ -194,7 +194,7 @@ public function setLocalRepository(InstalledRepositoryInterface $repository)
      *
      * @return InstalledRepositoryInterface
      */
-    public function getLocalRepository()
+    public function getLocalRepository(): InstalledRepositoryInterface
     {
         return $this->localRepository;
     }
diff --git a/src/Composer/Script/Event.php b/src/Composer/Script/Event.php
index 3c1bccdb2c98..ed6b7f0ba50d 100644
--- a/src/Composer/Script/Event.php
+++ b/src/Composer/Script/Event.php
@@ -67,7 +67,7 @@ public function __construct(string $name, Composer $composer, IOInterface $io, b
      *
      * @return Composer
      */
-    public function getComposer()
+    public function getComposer(): Composer
     {
         return $this->composer;
     }
@@ -77,7 +77,7 @@ public function getComposer()
      *
      * @return IOInterface
      */
-    public function getIO()
+    public function getIO(): IOInterface
     {
         return $this->io;
     }
@@ -87,7 +87,7 @@ public function getIO()
      *
      * @return bool
      */
-    public function isDevMode()
+    public function isDevMode(): bool
     {
         return $this->devMode;
     }
diff --git a/src/Composer/Util/Git.php b/src/Composer/Util/Git.php
index 7c9482a5c947..be86b3dd01d6 100644
--- a/src/Composer/Util/Git.php
+++ b/src/Composer/Util/Git.php
@@ -49,7 +49,7 @@ public function __construct(IOInterface $io, Config $config, ProcessExecutor $pr
      *
      * @return void
      */
-    public function runCommand(callable $commandCallable, string $url, ?string $cwd, bool $initialClone = false)
+    public function runCommand(callable $commandCallable, string $url, ?string $cwd, bool $initialClone = false): void
     {
         // Ensure we are allowed to use this URL by config
         $this->config->prohibitUrlByConfig($url, $this->io);
@@ -272,7 +272,7 @@ public function runCommand(callable $commandCallable, string $url, ?string $cwd,
      *
      * @return bool
      */
-    public function syncMirror(string $url, string $dir)
+    public function syncMirror(string $url, string $dir): bool
     {
         if (Platform::getEnv('COMPOSER_DISABLE_NETWORK') && Platform::getEnv('COMPOSER_DISABLE_NETWORK') !== 'prime') {
             $this->io->writeError('<warning>Aborting git mirror sync of '.$url.' as network is disabled</warning>');
@@ -317,7 +317,7 @@ public function syncMirror(string $url, string $dir)
      *
      * @return bool
      */
-    public function fetchRefOrSyncMirror(string $url, string $dir, string $ref)
+    public function fetchRefOrSyncMirror(string $url, string $dir, string $ref): bool
     {
         if ($this->checkRefIsInMirror($dir, $ref)) {
             return true;
@@ -333,7 +333,7 @@ public function fetchRefOrSyncMirror(string $url, string $dir, string $ref)
     /**
      * @return string
      */
-    public static function getNoShowSignatureFlag(ProcessExecutor $process)
+    public static function getNoShowSignatureFlag(ProcessExecutor $process): string
     {
         $gitVersion = self::getVersion($process);
         if ($gitVersion && version_compare($gitVersion, '2.10.0-rc0', '>=')) {
@@ -395,7 +395,7 @@ private function isAuthenticationFailure(string $url, array &$match): bool
     /**
      * @return void
      */
-    public static function cleanEnv()
+    public static function cleanEnv(): void
     {
         // added in git 1.7.1, prevents prompting the user for username/password
         if (Platform::getEnv('GIT_ASKPASS') !== 'echo') {
@@ -458,7 +458,7 @@ private function throwException($message, string $url): void
      *
      * @return string|null The git version number, if present.
      */
-    public static function getVersion(ProcessExecutor $process)
+    public static function getVersion(ProcessExecutor $process): ?string
     {
         if (false === self::$version) {
             self::$version = null;
diff --git a/src/Composer/Util/Perforce.php b/src/Composer/Util/Perforce.php
index 058180783858..93ac38a68fdb 100644
--- a/src/Composer/Util/Perforce.php
+++ b/src/Composer/Util/Perforce.php
@@ -85,7 +85,7 @@ public function __construct($repoConfig, string $port, string $path, ProcessExec
      *
      * @return self
      */
-    public static function create($repoConfig, string $port, string $path, ProcessExecutor $process, IOInterface $io)
+    public static function create($repoConfig, string $port, string $path, ProcessExecutor $process, IOInterface $io): self
     {
         return new Perforce($repoConfig, $port, $path, $process, Platform::isWindows(), $io);
     }
@@ -96,7 +96,7 @@ public static function create($repoConfig, string $port, string $path, ProcessEx
      *
      * @return bool
      */
-    public static function checkServerExists(string $url, ProcessExecutor $processExecutor)
+    public static function checkServerExists(string $url, ProcessExecutor $processExecutor): bool
     {
         return 0 === $processExecutor->execute('p4 -p ' . ProcessExecutor::escape($url) . ' info -s', $ignoredOutput);
     }
@@ -106,7 +106,7 @@ public static function checkServerExists(string $url, ProcessExecutor $processEx
      *
      * @return void
      */
-    public function initialize($repoConfig)
+    public function initialize($repoConfig): void
     {
         $this->uniquePerforceClientName = $this->generateUniquePerforceClientName();
         if (!$repoConfig) {
@@ -138,7 +138,7 @@ public function initialize($repoConfig)
      *
      * @return void
      */
-    public function initializeDepotAndBranch(?string $depot, ?string $branch)
+    public function initializeDepotAndBranch(?string $depot, ?string $branch): void
     {
         if (isset($depot)) {
             $this->p4Depot = $depot;
@@ -159,7 +159,7 @@ public function generateUniquePerforceClientName()
     /**
      * @return void
      */
-    public function cleanupClientSpec()
+    public function cleanupClientSpec(): void
     {
         $client = $this->getClient();
         $task = 'client -d ' . ProcessExecutor::escape($client);
@@ -176,7 +176,7 @@ public function cleanupClientSpec()
      *
      * @return int
      */
-    protected function executeCommand($command)
+    protected function executeCommand($command): int
     {
         $this->commandResult = '';
 
@@ -186,7 +186,7 @@ protected function executeCommand($command)
     /**
      * @return string
      */
-    public function getClient()
+    public function getClient(): string
     {
         if (!isset($this->p4Client)) {
             $cleanStreamName = str_replace(array('//', '/', '@'), array('', '_', ''), $this->getStream());
@@ -199,7 +199,7 @@ public function getClient()
     /**
      * @return string
      */
-    protected function getPath()
+    protected function getPath(): string
     {
         return $this->path;
     }
@@ -209,7 +209,7 @@ protected function getPath()
      *
      * @return void
      */
-    public function initializePath(string $path)
+    public function initializePath(string $path): void
     {
         $this->path = $path;
         $fs = $this->getFilesystem();
@@ -219,7 +219,7 @@ public function initializePath(string $path)
     /**
      * @return string
      */
-    protected function getPort()
+    protected function getPort(): string
     {
         return $this->p4Port;
     }
@@ -229,7 +229,7 @@ protected function getPort()
      *
      * @return void
      */
-    public function setStream(string $stream)
+    public function setStream(string $stream): void
     {
         $this->p4Stream = $stream;
         $index = strrpos($stream, '/');
@@ -242,7 +242,7 @@ public function setStream(string $stream)
     /**
      * @return bool
      */
-    public function isStream()
+    public function isStream(): bool
     {
         return is_string($this->p4DepotType) && (strcmp($this->p4DepotType, 'stream') === 0);
     }
@@ -250,7 +250,7 @@ public function isStream()
     /**
      * @return string
      */
-    public function getStream()
+    public function getStream(): string
     {
         if (!isset($this->p4Stream)) {
             if ($this->isStream()) {
@@ -268,7 +268,7 @@ public function getStream()
      *
      * @return string
      */
-    public function getStreamWithoutLabel(string $stream)
+    public function getStreamWithoutLabel(string $stream): string
     {
         $index = strpos($stream, '@');
         if ($index === false) {
@@ -289,7 +289,7 @@ public function getP4ClientSpec()
     /**
      * @return string|null
      */
-    public function getUser()
+    public function getUser(): ?string
     {
         return $this->p4User;
     }
@@ -299,7 +299,7 @@ public function getUser()
      *
      * @return void
      */
-    public function setUser(?string $user)
+    public function setUser(?string $user): void
     {
         $this->p4User = $user;
     }
@@ -307,7 +307,7 @@ public function setUser(?string $user)
     /**
      * @return void
      */
-    public function queryP4User()
+    public function queryP4User(): void
     {
         $this->getUser();
         if (strlen((string) $this->p4User) > 0) {
@@ -365,7 +365,7 @@ protected function getP4variable(string $name)
     /**
      * @return string|null
      */
-    public function queryP4Password()
+    public function queryP4Password(): ?string
     {
         if (isset($this->p4Password)) {
             return $this->p4Password;
@@ -400,7 +400,7 @@ public function generateP4Command(string $command, bool $useClient = true)
     /**
      * @return bool
      */
-    public function isLoggedIn()
+    public function isLoggedIn(): bool
     {
         $command = $this->generateP4Command('login -s', false);
         $exitCode = $this->executeCommand($command);
@@ -423,7 +423,7 @@ public function isLoggedIn()
     /**
      * @return void
      */
-    public function connectClient()
+    public function connectClient(): void
     {
         $p4CreateClientCommand = $this->generateP4Command(
             'client -i < ' . str_replace(" ", "\\ ", $this->getP4ClientSpec())
@@ -436,7 +436,7 @@ public function connectClient()
      *
      * @return void
      */
-    public function syncCodeBase(?string $sourceReference)
+    public function syncCodeBase(?string $sourceReference): void
     {
         $prevDir = Platform::getCwd();
         chdir($this->path);
@@ -453,7 +453,7 @@ public function syncCodeBase(?string $sourceReference)
      *
      * @return void
      */
-    public function writeClientSpecToFile($spec)
+    public function writeClientSpecToFile($spec): void
     {
         fwrite($spec, 'Client: ' . $this->getClient() . PHP_EOL . PHP_EOL);
         fwrite($spec, 'Update: ' . date('Y/m/d H:i:s') . PHP_EOL . PHP_EOL);
@@ -479,7 +479,7 @@ public function writeClientSpecToFile($spec)
     /**
      * @return void
      */
-    public function writeP4ClientSpec()
+    public function writeP4ClientSpec(): void
     {
         $clientSpec = $this->getP4ClientSpec();
         $spec = fopen($clientSpec, 'w');
@@ -498,7 +498,7 @@ public function writeP4ClientSpec()
      *
      * @return void
      */
-    protected function read($pipe, $name)
+    protected function read($pipe, $name): void
     {
         if (feof($pipe)) {
             return;
@@ -514,7 +514,7 @@ protected function read($pipe, $name)
      *
      * @return int
      */
-    public function windowsLogin(?string $password)
+    public function windowsLogin(?string $password): int
     {
         $command = $this->generateP4Command(' login -a');
 
@@ -526,7 +526,7 @@ public function windowsLogin(?string $password)
     /**
      * @return void
      */
-    public function p4Login()
+    public function p4Login(): void
     {
         $this->queryP4User();
         if (!$this->isLoggedIn()) {
@@ -565,7 +565,7 @@ public function getComposerInformation(string $identifier)
      *
      * @return string|null
      */
-    public function getFileContent(string $file, string $identifier)
+    public function getFileContent(string $file, string $identifier): ?string
     {
         $path = $this->getFilePath($file, $identifier);
 
@@ -586,7 +586,7 @@ public function getFileContent(string $file, string $identifier)
      *
      * @return string|null
      */
-    public function getFilePath(string $file, string $identifier)
+    public function getFilePath(string $file, string $identifier): ?string
     {
         $index = strpos($identifier, '@');
         if ($index === false) {
@@ -646,7 +646,7 @@ public function getBranches()
     /**
      * @return array<string, string>
      */
-    public function getTags()
+    public function getTags(): array
     {
         $command = $this->generateP4Command('labels');
         $this->executeCommand($command);
@@ -666,7 +666,7 @@ public function getTags()
     /**
      * @return bool
      */
-    public function checkStream()
+    public function checkStream(): bool
     {
         $command = $this->generateP4Command('depots', false);
         $this->executeCommand($command);
@@ -690,7 +690,7 @@ public function checkStream()
      * @param  string     $reference
      * @return mixed|null
      */
-    protected function getChangeList(string $reference)
+    protected function getChangeList(string $reference): mixed
     {
         $index = strpos($reference, '@');
         if ($index === false) {
@@ -713,7 +713,7 @@ protected function getChangeList(string $reference)
      * @param  string     $toReference
      * @return mixed|null
      */
-    public function getCommitLogs(string $fromReference, string $toReference)
+    public function getCommitLogs(string $fromReference, string $toReference): mixed
     {
         $fromChangeList = $this->getChangeList($fromReference);
         if ($fromChangeList === null) {
@@ -734,7 +734,7 @@ public function getCommitLogs(string $fromReference, string $toReference)
     /**
      * @return Filesystem
      */
-    public function getFilesystem()
+    public function getFilesystem(): Filesystem
     {
         if (null === $this->filesystem) {
             $this->filesystem = new Filesystem($this->process);
@@ -746,7 +746,7 @@ public function getFilesystem()
     /**
      * @return void
      */
-    public function setFilesystem(Filesystem $fs)
+    public function setFilesystem(Filesystem $fs): void
     {
         $this->filesystem = $fs;
     }
diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index 05060bbfbeee..429337708c0a 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -68,7 +68,7 @@ public function __construct(IOInterface $io = null)
      * @param  null|string $cwd     the working directory
      * @return int     statuscode
      */
-    public function execute($command, &$output = null, ?string $cwd = null)
+    public function execute($command, &$output = null, ?string $cwd = null): int
     {
         if (func_num_args() > 1) {
             return $this->doExecute($command, $cwd, false, $output);
@@ -84,7 +84,7 @@ public function execute($command, &$output = null, ?string $cwd = null)
      * @param  null|string $cwd     the working directory
      * @return int     statuscode
      */
-    public function executeTty($command, ?string $cwd = null)
+    public function executeTty($command, ?string $cwd = null): int
     {
         if (Platform::isTty()) {
             return $this->doExecute($command, $cwd, true);
@@ -121,7 +121,10 @@ private function doExecute($command, ?string $cwd, bool $tty, &$output = null):
             }
         }
 
-        $callback = is_callable($output) ? $output : array($this, 'outputHandler');
+        $callback = is_callable($output) ? $output : function (string $type, string $buffer): void {
+            $this->outputHandler($type, $buffer);
+        };
+
         $process->run($callback);
 
         if ($this->captureOutput && !is_callable($output)) {
@@ -140,7 +143,7 @@ private function doExecute($command, ?string $cwd, bool $tty, &$output = null):
      * @param  string              $cwd     the working directory
      * @return PromiseInterface
      */
-    public function executeAsync($command, ?string $cwd = null)
+    public function executeAsync($command, ?string $cwd = null): PromiseInterface
     {
         if (!$this->allowAsync) {
             throw new \LogicException('You must use the ProcessExecutor instance which is part of a Composer\Loop instance to be able to run async processes');
@@ -206,6 +209,25 @@ public function executeAsync($command, ?string $cwd = null)
         return $promise;
     }
 
+    protected function outputHandler(string $type, string $buffer): void
+    {
+        if ($this->captureOutput) {
+            return;
+        }
+
+        if (null === $this->io) {
+            echo $buffer;
+
+            return;
+        }
+
+        if (Process::ERR === $type) {
+            $this->io->writeErrorRaw($buffer, false);
+        } else {
+            $this->io->writeRaw($buffer, false);
+        }
+    }
+
     /**
      * @param  int  $id
      * @return void
@@ -253,7 +275,7 @@ private function startJob(int $id): void
      * @param  ?int $index job id
      * @return void
      */
-    public function wait($index = null)
+    public function wait($index = null): void
     {
         while (true) {
             if (!$this->countActiveJobs($index)) {
@@ -324,7 +346,7 @@ private function markJobDone(): void
      * @param  null|string  $output
      * @return string[]
      */
-    public function splitLines(?string $output)
+    public function splitLines(?string $output): array
     {
         $output = trim((string) $output);
 
@@ -336,42 +358,15 @@ public function splitLines(?string $output)
      *
      * @return string
      */
-    public function getErrorOutput()
+    public function getErrorOutput(): string
     {
         return $this->errorOutput;
     }
 
-    /**
-     * @private
-     *
-     * @param Process::ERR|Process::OUT $type
-     * @param string                    $buffer
-     *
-     * @return void
-     */
-    public function outputHandler($type, string $buffer)
-    {
-        if ($this->captureOutput) {
-            return;
-        }
-
-        if (null === $this->io) {
-            echo $buffer;
-
-            return;
-        }
-
-        if (Process::ERR === $type) {
-            $this->io->writeErrorRaw($buffer, false);
-        } else {
-            $this->io->writeRaw($buffer, false);
-        }
-    }
-
     /**
      * @return int the timeout in seconds
      */
-    public static function getTimeout()
+    public static function getTimeout(): int
     {
         return static::$timeout;
     }
@@ -380,7 +375,7 @@ public static function getTimeout()
      * @param  int  $timeout the timeout in seconds
      * @return void
      */
-    public static function setTimeout(int $timeout)
+    public static function setTimeout(int $timeout): void
     {
         static::$timeout = $timeout;
     }
@@ -392,7 +387,7 @@ public static function setTimeout(int $timeout)
      *
      * @return string The escaped argument
      */
-    public static function escape($argument)
+    public static function escape($argument): string
     {
         return self::escapeArgument($argument);
     }
diff --git a/tests/Composer/Test/Mock/ProcessExecutorMock.php b/tests/Composer/Test/Mock/ProcessExecutorMock.php
index 97f4c3f56551..897d889ce4fb 100644
--- a/tests/Composer/Test/Mock/ProcessExecutorMock.php
+++ b/tests/Composer/Test/Mock/ProcessExecutorMock.php
@@ -135,7 +135,9 @@ private function doExecute($command, string $cwd, bool $tty, &$output = null)
         $this->captureOutput = func_num_args() > 3;
         $this->errorOutput = '';
 
-        $callback = is_callable($output) ? $output : array($this, 'outputHandler');
+        $callback = is_callable($output) ? $output : function (string $type, string $buffer): void {
+            $this->outputHandler($type, $buffer);
+        };
 
         $commandString = is_array($command) ? implode(' ', $command) : $command;
         $this->log[] = $commandString;