[1.x] Add basic template annotations

.gitattributes

@@ -1,6 +1,8 @@
 /.gitattributes export-ignore
 /.github/ export-ignore
 /.gitignore export-ignore
+/phpstan.neon.dist export-ignore
 /phpunit.xml.dist export-ignore
 /phpunit.xml.legacy export-ignore
 /tests/ export-ignore
+/types/ export-ignore

.github/workflows/ci.yml

@@ -44,3 +44,18 @@ jobs:
       - run: composer self-update --2.2 # downgrade Composer for HHVM
       - run: hhvm $(which composer) install
       - run: hhvm vendor/bin/phpunit
+
+  PHPStan:
+    name: PHPStan
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        php:
+          - 8.1
+    steps:
+      - uses: actions/checkout@v3
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+      - run: composer require phpstan/phpstan
+      - run: vendor/bin/phpstan

phpstan.neon.dist

@@ -0,0 +1,4 @@
+parameters:
+  paths:
+    - types
+  level: max

src/React/Promise/PromiseInterface.php

@@ -2,7 +2,13 @@
 
 namespace React\Promise;
 
+/** @template T */
 interface PromiseInterface
 {
+    /**
+     * @template TReturn of mixed
+     * @param callable(T): TReturn $fulfilledHandler
+     * @return (TReturn is PromiseInterface ? TReturn : PromiseInterface<TReturn>)
+     */
     public function then($fulfilledHandler = null, $errorHandler = null, $progressHandler = null);
 }

types/PromiseInterface.php

@@ -0,0 +1,9 @@
+<?php
+
+use function PHPStan\Testing\assertType;
+use function React\Promise\resolve;
+
+$passThroughBoolFn = static fn (bool $bool): bool => $bool;
+
+assertType('React\Promise\PromiseInterface<bool>', resolve(true));
+assertType('React\Promise\PromiseInterface<bool>', resolve(true)->then($passThroughBoolFn));