diff --git a/.gitattributes b/.gitattributes
index 21be40c..5d5606d 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,6 +1,7 @@
 /.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
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 71802dd..43c9cb3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -30,3 +30,25 @@ jobs:
         if: ${{ matrix.php >= 7.3 }}
       - run: vendor/bin/phpunit --coverage-text -c phpunit.xml.legacy
         if: ${{ matrix.php < 7.3 }}
+
+  PHPStan:
+    name: PHPStan (PHP ${{ matrix.php }})
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        php:
+          - 8.2
+          - 8.1
+          - 8.0
+          - 7.4
+          - 7.3
+          - 7.2
+          - 7.1
+    steps:
+      - uses: actions/checkout@v3
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+      - run: composer install
+      - run: vendor/bin/phpstan
diff --git a/README.md b/README.md
index 5c13bc5..1453a09 100644
--- a/README.md
+++ b/README.md
@@ -426,6 +426,12 @@ To run the test suite, go to the project root and run:
 vendor/bin/phpunit
 ```
 
+On top of this, we use PHPStan on max level to ensure type safety across the project:
+
+```bash
+vendor/bin/phpstan
+```
+
 ## License
 
 MIT, see [LICENSE file](LICENSE).
diff --git a/composer.json b/composer.json
index 2729de2..e208dd3 100644
--- a/composer.json
+++ b/composer.json
@@ -31,6 +31,7 @@
         "react/promise": "^3.0 || ^2.8 || ^1.2.1"
     },
     "require-dev": {
+        "phpstan/phpstan": "1.10.18 || 1.4.10",
         "phpunit/phpunit": "^9.5 || ^7.5"
     },
     "autoload": {
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 0000000..b7f8ddb
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,11 @@
+parameters:
+    level: max
+
+    paths:
+        - src/
+        - tests/
+
+    reportUnmatchedIgnoredErrors: false
+    ignoreErrors:
+        # ignore generic usage like `PromiseInterface<T>` until fixed upstream
+        - '/^PHPDoc .* contains generic type React\\Promise\\PromiseInterface<.+> but interface React\\Promise\\PromiseInterface is not generic\.$/'
diff --git a/src/functions.php b/src/functions.php
index 28c16d0..0096496 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -56,6 +56,8 @@ function await(PromiseInterface $promise)
     $resolved = null;
     $exception = null;
     $rejected = false;
+
+    /** @var bool $loopStarted */
     $loopStarted = false;
 
     $promise->then(
@@ -294,7 +296,7 @@ function delay(float $seconds): void
  * });
  * ```
  *
- * @param callable(...$args):\Generator<mixed,PromiseInterface,mixed,mixed> $function
+ * @param callable(mixed ...$args):(\Generator<mixed,PromiseInterface,mixed,mixed>|mixed) $function
  * @param mixed ...$args Optional list of additional arguments that will be passed to the given `$function` as is
  * @return PromiseInterface<mixed>
  * @since 3.0.0
@@ -313,6 +315,7 @@ function coroutine(callable $function, ...$args): PromiseInterface
 
     $promise = null;
     $deferred = new Deferred(function () use (&$promise) {
+        /** @var ?PromiseInterface $promise */
         if ($promise instanceof PromiseInterface && \method_exists($promise, 'cancel')) {
             $promise->cancel();
         }
@@ -333,6 +336,7 @@ function coroutine(callable $function, ...$args): PromiseInterface
             return;
         }
 
+        /** @var mixed $promise */
         $promise = $generator->current();
         if (!$promise instanceof PromiseInterface) {
             $next = null;
@@ -342,6 +346,7 @@ function coroutine(callable $function, ...$args): PromiseInterface
             return;
         }
 
+        assert($next instanceof \Closure);
         $promise->then(function ($value) use ($generator, $next) {
             $generator->send($value);
             $next();
@@ -364,6 +369,7 @@ function coroutine(callable $function, ...$args): PromiseInterface
  */
 function parallel(iterable $tasks): PromiseInterface
 {
+    /** @var array<int,PromiseInterface> $pending */
     $pending = [];
     $deferred = new Deferred(function () use (&$pending) {
         foreach ($pending as $promise) {
@@ -425,6 +431,7 @@ function series(iterable $tasks): PromiseInterface
 {
     $pending = null;
     $deferred = new Deferred(function () use (&$pending) {
+        /** @var ?PromiseInterface $pending */
         if ($pending instanceof PromiseInterface && \method_exists($pending, 'cancel')) {
             $pending->cancel();
         }
@@ -437,9 +444,9 @@ function series(iterable $tasks): PromiseInterface
         assert($tasks instanceof \Iterator);
     }
 
-    /** @var callable():void $next */
     $taskCallback = function ($result) use (&$results, &$next) {
         $results[] = $result;
+        /** @var \Closure $next */
         $next();
     };
 
@@ -453,9 +460,11 @@ function series(iterable $tasks): PromiseInterface
             $task = $tasks->current();
             $tasks->next();
         } else {
+            assert(\is_array($tasks));
             $task = \array_shift($tasks);
         }
 
+        assert(\is_callable($task));
         $promise = \call_user_func($task);
         assert($promise instanceof PromiseInterface);
         $pending = $promise;
@@ -469,13 +478,14 @@ function series(iterable $tasks): PromiseInterface
 }
 
 /**
- * @param iterable<callable(mixed=):PromiseInterface<mixed>> $tasks
+ * @param iterable<(callable():PromiseInterface<mixed>)|(callable(mixed):PromiseInterface<mixed>)> $tasks
  * @return PromiseInterface<mixed>
  */
 function waterfall(iterable $tasks): PromiseInterface
 {
     $pending = null;
     $deferred = new Deferred(function () use (&$pending) {
+        /** @var ?PromiseInterface $pending */
         if ($pending instanceof PromiseInterface && \method_exists($pending, 'cancel')) {
             $pending->cancel();
         }
@@ -498,9 +508,11 @@ function waterfall(iterable $tasks): PromiseInterface
             $task = $tasks->current();
             $tasks->next();
         } else {
+            assert(\is_array($tasks));
             $task = \array_shift($tasks);
         }
 
+        assert(\is_callable($task));
         $promise = \call_user_func_array($task, func_get_args());
         assert($promise instanceof PromiseInterface);
         $pending = $promise;
diff --git a/tests/AwaitTest.php b/tests/AwaitTest.php
index da9079a..397a0fa 100644
--- a/tests/AwaitTest.php
+++ b/tests/AwaitTest.php
@@ -8,7 +8,7 @@
 
 class AwaitTest extends TestCase
 {
-    public function testAwaitThrowsExceptionWhenPromiseIsRejectedWithException()
+    public function testAwaitThrowsExceptionWhenPromiseIsRejectedWithException(): void
     {
         $promise = new Promise(function () {
             throw new \Exception('test');
@@ -19,7 +19,7 @@ public function testAwaitThrowsExceptionWhenPromiseIsRejectedWithException()
         React\Async\await($promise);
     }
 
-    public function testAwaitThrowsUnexpectedValueExceptionWhenPromiseIsRejectedWithFalse()
+    public function testAwaitThrowsUnexpectedValueExceptionWhenPromiseIsRejectedWithFalse(): void
     {
         if (!interface_exists('React\Promise\CancellablePromiseInterface')) {
             $this->markTestSkipped('Promises must be rejected with a \Throwable instance since Promise v3');
@@ -34,7 +34,7 @@ public function testAwaitThrowsUnexpectedValueExceptionWhenPromiseIsRejectedWith
         React\Async\await($promise);
     }
 
-    public function testAwaitThrowsUnexpectedValueExceptionWhenPromiseIsRejectedWithNull()
+    public function testAwaitThrowsUnexpectedValueExceptionWhenPromiseIsRejectedWithNull(): void
     {
         if (!interface_exists('React\Promise\CancellablePromiseInterface')) {
             $this->markTestSkipped('Promises must be rejected with a \Throwable instance since Promise v3');
@@ -49,7 +49,7 @@ public function testAwaitThrowsUnexpectedValueExceptionWhenPromiseIsRejectedWith
         React\Async\await($promise);
     }
 
-    public function testAwaitThrowsErrorWhenPromiseIsRejectedWithError()
+    public function testAwaitThrowsErrorWhenPromiseIsRejectedWithError(): void
     {
         $promise = new Promise(function ($_, $reject) {
             throw new \Error('Test', 42);
@@ -61,7 +61,7 @@ public function testAwaitThrowsErrorWhenPromiseIsRejectedWithError()
         React\Async\await($promise);
     }
 
-    public function testAwaitReturnsValueWhenPromiseIsFullfilled()
+    public function testAwaitReturnsValueWhenPromiseIsFullfilled(): void
     {
         $promise = new Promise(function ($resolve) {
             $resolve(42);
@@ -70,7 +70,7 @@ public function testAwaitReturnsValueWhenPromiseIsFullfilled()
         $this->assertEquals(42, React\Async\await($promise));
     }
 
-    public function testAwaitReturnsValueWhenPromiseIsFulfilledEvenWhenOtherTimerStopsLoop()
+    public function testAwaitReturnsValueWhenPromiseIsFulfilledEvenWhenOtherTimerStopsLoop(): void
     {
         $promise = new Promise(function ($resolve) {
             Loop::addTimer(0.02, function () use ($resolve) {
@@ -84,7 +84,7 @@ public function testAwaitReturnsValueWhenPromiseIsFulfilledEvenWhenOtherTimerSto
         $this->assertEquals(2, React\Async\await($promise));
     }
 
-    public function testAwaitWithAlreadyFulfilledPromiseWillReturnWithoutRunningLoop()
+    public function testAwaitWithAlreadyFulfilledPromiseWillReturnWithoutRunningLoop(): void
     {
         $now = true;
 
@@ -100,7 +100,7 @@ public function testAwaitWithAlreadyFulfilledPromiseWillReturnWithoutRunningLoop
         $this->assertTrue($now);
     }
 
-    public function testAwaitWithAlreadyFulfilledPromiseWillReturnWithoutStoppingLoop()
+    public function testAwaitWithAlreadyFulfilledPromiseWillReturnWithoutStoppingLoop(): void
     {
         $ticks = 0;
 
@@ -128,7 +128,7 @@ public function testAwaitWithAlreadyFulfilledPromiseWillReturnWithoutStoppingLoo
         $this->assertEquals(2, $ticks);
     }
 
-    public function testAwaitWithPendingPromiseThatWillResolveWillStopLoopBeforeLastTimerFinishes()
+    public function testAwaitWithPendingPromiseThatWillResolveWillStopLoopBeforeLastTimerFinishes(): void
     {
         $promise = new Promise(function ($resolve) {
             Loop::addTimer(0.02, function () use ($resolve) {
@@ -159,7 +159,7 @@ public function testAwaitWithPendingPromiseThatWillResolveWillStopLoopBeforeLast
         $this->assertEquals(1, $ticks);
     }
 
-    public function testAwaitWithAlreadyRejectedPromiseWillReturnWithoutStoppingLoop()
+    public function testAwaitWithAlreadyRejectedPromiseWillReturnWithoutStoppingLoop(): void
     {
         $ticks = 0;
 
@@ -191,7 +191,7 @@ public function testAwaitWithAlreadyRejectedPromiseWillReturnWithoutStoppingLoop
         $this->assertEquals(2, $ticks);
     }
 
-    public function testAwaitWithPendingPromiseThatWillRejectWillStopLoopBeforeLastTimerFinishes()
+    public function testAwaitWithPendingPromiseThatWillRejectWillStopLoopBeforeLastTimerFinishes(): void
     {
         $promise = new Promise(function ($_, $reject) {
             Loop::addTimer(0.02, function () use (&$reject) {
@@ -227,7 +227,7 @@ public function testAwaitWithPendingPromiseThatWillRejectWillStopLoopBeforeLastT
         $this->assertEquals(1, $ticks);
     }
 
-    public function testAwaitShouldNotCreateAnyGarbageReferencesForResolvedPromise()
+    public function testAwaitShouldNotCreateAnyGarbageReferencesForResolvedPromise(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
@@ -244,7 +244,7 @@ public function testAwaitShouldNotCreateAnyGarbageReferencesForResolvedPromise()
         $this->assertEquals(0, gc_collect_cycles());
     }
 
-    public function testAwaitShouldNotCreateAnyGarbageReferencesForRejectedPromise()
+    public function testAwaitShouldNotCreateAnyGarbageReferencesForRejectedPromise(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
@@ -265,7 +265,7 @@ public function testAwaitShouldNotCreateAnyGarbageReferencesForRejectedPromise()
         $this->assertEquals(0, gc_collect_cycles());
     }
 
-    public function testAwaitShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithNullValue()
+    public function testAwaitShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithNullValue(): void
     {
         if (!interface_exists('React\Promise\CancellablePromiseInterface')) {
             $this->markTestSkipped('Promises must be rejected with a \Throwable instance since Promise v3');
diff --git a/tests/CoroutineTest.php b/tests/CoroutineTest.php
index adc82bc..c9b7439 100644
--- a/tests/CoroutineTest.php
+++ b/tests/CoroutineTest.php
@@ -9,7 +9,7 @@
 
 class CoroutineTest extends TestCase
 {
-    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsWithoutGenerator()
+    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsWithoutGenerator(): void
     {
         $promise = coroutine(function () {
             return 42;
@@ -18,10 +18,10 @@ public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsWithoutGene
         $promise->then($this->expectCallableOnceWith(42));
     }
 
-    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsImmediately()
+    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsImmediately(): void
     {
         $promise = coroutine(function () {
-            if (false) {
+            if (false) { // @phpstan-ignore-line
                 yield;
             }
             return 42;
@@ -30,7 +30,7 @@ public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsImmediately
         $promise->then($this->expectCallableOnceWith(42));
     }
 
-    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsAfterYieldingPromise()
+    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsAfterYieldingPromise(): void
     {
         $promise = coroutine(function () {
             $value = yield resolve(42);
@@ -40,7 +40,7 @@ public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsAfterYieldi
         $promise->then($this->expectCallableOnceWith(42));
     }
 
-    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsWithoutGenerator()
+    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsWithoutGenerator(): void
     {
         $promise = coroutine(function () {
             throw new \RuntimeException('Foo');
@@ -49,10 +49,10 @@ public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsWithoutGenera
         $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException('Foo')));
     }
 
-    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsImmediately()
+    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsImmediately(): void
     {
         $promise = coroutine(function () {
-            if (false) {
+            if (false) { // @phpstan-ignore-line
                 yield;
             }
             throw new \RuntimeException('Foo');
@@ -61,7 +61,7 @@ public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsImmediately()
         $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException('Foo')));
     }
 
-    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsAfterYieldingPromise()
+    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsAfterYieldingPromise(): void
     {
         $promise = coroutine(function () {
             $reason = yield resolve('Foo');
@@ -71,7 +71,7 @@ public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsAfterYielding
         $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException('Foo')));
     }
 
-    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsAfterYieldingRejectedPromise()
+    public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsAfterYieldingRejectedPromise(): void
     {
         $promise = coroutine(function () {
             try {
@@ -84,7 +84,7 @@ public function testCoroutineReturnsRejectedPromiseIfFunctionThrowsAfterYielding
         $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException('Foo')));
     }
 
-    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsAfterYieldingRejectedPromise()
+    public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsAfterYieldingRejectedPromise(): void
     {
         $promise = coroutine(function () {
             try {
@@ -97,7 +97,7 @@ public function testCoroutineReturnsFulfilledPromiseIfFunctionReturnsAfterYieldi
         $promise->then($this->expectCallableOnceWith(42));
     }
 
-    public function testCoroutineReturnsRejectedPromiseIfFunctionYieldsInvalidValue()
+    public function testCoroutineReturnsRejectedPromiseIfFunctionYieldsInvalidValue(): void
     {
         $promise = coroutine(function () {
             yield 42;
@@ -106,7 +106,7 @@ public function testCoroutineReturnsRejectedPromiseIfFunctionYieldsInvalidValue(
         $promise->then(null, $this->expectCallableOnceWith(new \UnexpectedValueException('Expected coroutine to yield React\Promise\PromiseInterface, but got integer')));
     }
 
-    public function testCancelCoroutineWillReturnRejectedPromiseWhenCancellingPendingPromiseRejects()
+    public function testCancelCoroutineWillReturnRejectedPromiseWhenCancellingPendingPromiseRejects(): void
     {
         $promise = coroutine(function () {
             yield new Promise(function () { }, function () {
@@ -114,12 +114,13 @@ public function testCancelCoroutineWillReturnRejectedPromiseWhenCancellingPendin
             });
         });
 
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
 
         $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException('Operation cancelled')));
     }
 
-    public function testCancelCoroutineWillReturnFulfilledPromiseWhenCancellingPendingPromiseRejectsInsideCatchThatReturnsValue()
+    public function testCancelCoroutineWillReturnFulfilledPromiseWhenCancellingPendingPromiseRejectsInsideCatchThatReturnsValue(): void
     {
         $promise = coroutine(function () {
             try {
@@ -131,12 +132,13 @@ public function testCancelCoroutineWillReturnFulfilledPromiseWhenCancellingPendi
             }
         });
 
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
 
         $promise->then($this->expectCallableOnceWith(42));
     }
 
-    public function testCancelCoroutineWillReturnPendigPromiseWhenCancellingFirstPromiseRejectsInsideCatchThatYieldsSecondPromise()
+    public function testCancelCoroutineWillReturnPendigPromiseWhenCancellingFirstPromiseRejectsInsideCatchThatYieldsSecondPromise(): void
     {
         $promise = coroutine(function () {
             try {
@@ -150,12 +152,13 @@ public function testCancelCoroutineWillReturnPendigPromiseWhenCancellingFirstPro
             }
         });
 
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
 
         $promise->then($this->expectCallableNever(), $this->expectCallableNever());
     }
 
-    public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorReturns()
+    public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorReturns(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
@@ -165,7 +168,7 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorRet
         gc_collect_cycles();
 
         $promise = coroutine(function () {
-            if (false) {
+            if (false) { // @phpstan-ignore-line
                 yield;
             }
             return 42;
@@ -176,7 +179,7 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorRet
         $this->assertEquals(0, gc_collect_cycles());
     }
 
-    public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithExceptionImmediately()
+    public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithExceptionImmediately(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
@@ -195,7 +198,7 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseReject
         $this->assertEquals(0, gc_collect_cycles());
     }
 
-    public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithExceptionOnCancellation()
+    public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithExceptionOnCancellation(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
@@ -209,13 +212,14 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseReject
             });
         });
 
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
         unset($promise);
 
         $this->assertEquals(0, gc_collect_cycles());
     }
 
-    public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorThrowsBeforeFirstYield()
+    public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorThrowsBeforeFirstYield(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
@@ -225,7 +229,7 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorThr
 
         $promise = coroutine(function () {
             throw new \RuntimeException('Failed', 42);
-            yield;
+            yield; // @phpstan-ignore-line
         });
 
         unset($promise);
@@ -233,7 +237,7 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorThr
         $this->assertEquals(0, gc_collect_cycles());
     }
 
-    public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorYieldsInvalidValue()
+    public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorYieldsInvalidValue(): void
     {
         if (class_exists('React\Promise\When')) {
             $this->markTestSkipped('Not supported on legacy Promise v1 API');
diff --git a/tests/DelayTest.php b/tests/DelayTest.php
index d34c16e..0091943 100644
--- a/tests/DelayTest.php
+++ b/tests/DelayTest.php
@@ -8,7 +8,7 @@
 
 class DelayTest extends TestCase
 {
-    public function testDelayBlocksForGivenPeriod()
+    public function testDelayBlocksForGivenPeriod(): void
     {
         $time = microtime(true);
         delay(0.02);
@@ -17,7 +17,7 @@ public function testDelayBlocksForGivenPeriod()
         $this->assertEqualsWithDelta(0.02, $time, 0.01);
     }
 
-    public function testDelaySmallPeriodBlocksForCloseToZeroSeconds()
+    public function testDelaySmallPeriodBlocksForCloseToZeroSeconds(): void
     {
         $time = microtime(true);
         delay(0.000001);
@@ -26,7 +26,7 @@ public function testDelaySmallPeriodBlocksForCloseToZeroSeconds()
         $this->assertLessThan(0.01, $time);
     }
 
-    public function testDelayNegativePeriodBlocksForCloseToZeroSeconds()
+    public function testDelayNegativePeriodBlocksForCloseToZeroSeconds(): void
     {
         $time = microtime(true);
         delay(-1);
@@ -35,7 +35,7 @@ public function testDelayNegativePeriodBlocksForCloseToZeroSeconds()
         $this->assertLessThan(0.01, $time);
     }
 
-    public function testDelayRunsOtherEventsWhileWaiting()
+    public function testDelayRunsOtherEventsWhileWaiting(): void
     {
         $buffer = 'a';
         Loop::addTimer(0.001, function () use (&$buffer) {
diff --git a/tests/ParallelTest.php b/tests/ParallelTest.php
index 1a5759b..c1ed553 100644
--- a/tests/ParallelTest.php
+++ b/tests/ParallelTest.php
@@ -6,10 +6,11 @@
 use React\EventLoop\Loop;
 use React\Promise\Promise;
 use function React\Promise\reject;
+use function React\Promise\resolve;
 
 class ParallelTest extends TestCase
 {
-    public function testParallelWithoutTasks()
+    public function testParallelWithoutTasks(): void
     {
         $tasks = array();
 
@@ -18,11 +19,11 @@ public function testParallelWithoutTasks()
         $promise->then($this->expectCallableOnceWith(array()));
     }
 
-    public function testParallelWithoutTasksFromEmptyGeneratorResolvesWithEmptyArray()
+    public function testParallelWithoutTasksFromEmptyGeneratorResolvesWithEmptyArray(): void
     {
         $tasks = (function () {
-            if (false) {
-                yield;
+            if (false) { // @phpstan-ignore-line
+                yield function () { return resolve(null); };
             }
         })();
 
@@ -31,7 +32,7 @@ public function testParallelWithoutTasksFromEmptyGeneratorResolvesWithEmptyArray
         $promise->then($this->expectCallableOnceWith([]));
     }
 
-    public function testParallelWithTasks()
+    public function testParallelWithTasks(): void
     {
         $tasks = array(
             function () {
@@ -63,7 +64,7 @@ function () {
         $timer->assertInRange(0.1, 0.2);
     }
 
-    public function testParallelWithTasksFromGeneratorResolvesWithArrayOfFulfillmentValues()
+    public function testParallelWithTasksFromGeneratorResolvesWithArrayOfFulfillmentValues(): void
     {
         $tasks = (function () {
             yield function () {
@@ -95,7 +96,7 @@ public function testParallelWithTasksFromGeneratorResolvesWithArrayOfFulfillment
         $timer->assertInRange(0.1, 0.2);
     }
 
-    public function testParallelWithErrorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
+    public function testParallelWithErrorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks(): void
     {
         $called = 0;
 
@@ -127,12 +128,12 @@ function () use (&$called) {
         $this->assertSame(2, $called);
     }
 
-    public function testParallelWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
+    public function testParallelWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks(): void
     {
         $called = 0;
 
         $tasks = (function () use (&$called) {
-            while (true) {
+            while (true) { // @phpstan-ignore-line
                 yield function () use (&$called) {
                     return reject(new \RuntimeException('Rejected ' . ++$called));
                 };
@@ -146,7 +147,7 @@ public function testParallelWithErrorFromInfiniteGeneratorReturnsPromiseRejected
         $this->assertSame(1, $called);
     }
 
-    public function testParallelWithErrorWillCancelPendingPromises()
+    public function testParallelWithErrorWillCancelPendingPromises(): void
     {
         $cancelled = 0;
 
@@ -175,7 +176,7 @@ function () use (&$cancelled) {
         $this->assertSame(1, $cancelled);
     }
 
-    public function testParallelWillCancelPendingPromisesWhenCallingCancelOnResultingPromise()
+    public function testParallelWillCancelPendingPromisesWhenCallingCancelOnResultingPromise(): void
     {
         $cancelled = 0;
 
@@ -193,12 +194,13 @@ function () use (&$cancelled) {
         );
 
         $promise = React\Async\parallel($tasks);
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
 
         $this->assertSame(2, $cancelled);
     }
 
-    public function testParallelWithDelayedErrorReturnsPromiseRejectedWithExceptionFromTask()
+    public function testParallelWithDelayedErrorReturnsPromiseRejectedWithExceptionFromTask(): void
     {
         $called = 0;
 
diff --git a/tests/SeriesTest.php b/tests/SeriesTest.php
index 404c907..25aa104 100644
--- a/tests/SeriesTest.php
+++ b/tests/SeriesTest.php
@@ -6,10 +6,11 @@
 use React\EventLoop\Loop;
 use React\Promise\Promise;
 use function React\Promise\reject;
+use function React\Promise\resolve;
 
 class SeriesTest extends TestCase
 {
-    public function testSeriesWithoutTasks()
+    public function testSeriesWithoutTasks(): void
     {
         $tasks = array();
 
@@ -18,11 +19,11 @@ public function testSeriesWithoutTasks()
         $promise->then($this->expectCallableOnceWith(array()));
     }
 
-    public function testSeriesWithoutTasksFromEmptyGeneratorResolvesWithEmptyArray()
+    public function testSeriesWithoutTasksFromEmptyGeneratorResolvesWithEmptyArray(): void
     {
         $tasks = (function () {
-            if (false) {
-                yield;
+            if (false) { // @phpstan-ignore-line
+                yield function () { return resolve(null); };
             }
         })();
 
@@ -31,7 +32,7 @@ public function testSeriesWithoutTasksFromEmptyGeneratorResolvesWithEmptyArray()
         $promise->then($this->expectCallableOnceWith([]));
     }
 
-    public function testSeriesWithTasks()
+    public function testSeriesWithTasks(): void
     {
         $tasks = array(
             function () {
@@ -63,7 +64,7 @@ function () {
         $timer->assertInRange(0.10, 0.20);
     }
 
-    public function testSeriesWithTasksFromGeneratorResolvesWithArrayOfFulfillmentValues()
+    public function testSeriesWithTasksFromGeneratorResolvesWithArrayOfFulfillmentValues(): void
     {
         $tasks = (function () {
             yield function () {
@@ -95,7 +96,7 @@ public function testSeriesWithTasksFromGeneratorResolvesWithArrayOfFulfillmentVa
         $timer->assertInRange(0.10, 0.20);
     }
 
-    public function testSeriesWithError()
+    public function testSeriesWithError(): void
     {
         $called = 0;
 
@@ -126,12 +127,12 @@ function () use (&$called) {
         $this->assertSame(1, $called);
     }
 
-    public function testSeriesWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
+    public function testSeriesWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks(): void
     {
         $called = 0;
 
         $tasks = (function () use (&$called) {
-            while (true) {
+            while (true) { // @phpstan-ignore-line
                 yield function () use (&$called) {
                     return reject(new \RuntimeException('Rejected ' . ++$called));
                 };
@@ -145,14 +146,15 @@ public function testSeriesWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWi
         $this->assertSame(1, $called);
     }
 
-    public function testSeriesWithErrorFromInfiniteIteratorAggregateReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
+    public function testSeriesWithErrorFromInfiniteIteratorAggregateReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks(): void
     {
         $tasks = new class() implements \IteratorAggregate {
+            /** @var int */
             public $called = 0;
 
             public function getIterator(): \Iterator
             {
-                while (true) {
+                while (true) { // @phpstan-ignore-line
                     yield function () {
                         return reject(new \RuntimeException('Rejected ' . ++$this->called));
                     };
@@ -167,7 +169,7 @@ public function getIterator(): \Iterator
         $this->assertSame(1, $tasks->called);
     }
 
-    public function testSeriesWillCancelFirstPendingPromiseWhenCallingCancelOnResultingPromise()
+    public function testSeriesWillCancelFirstPendingPromiseWhenCallingCancelOnResultingPromise(): void
     {
         $cancelled = 0;
 
@@ -185,6 +187,7 @@ function () use (&$cancelled) {
         );
 
         $promise = React\Async\series($tasks);
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
 
         $this->assertSame(1, $cancelled);
diff --git a/tests/TestCase.php b/tests/TestCase.php
index ee0f476..c7bf729 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -3,45 +3,43 @@
 namespace React\Tests\Async;
 
 use PHPUnit\Framework\MockObject\MockBuilder;
+use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase as BaseTestCase;
 
 class TestCase extends BaseTestCase
 {
-    protected function expectCallableOnce()
+    protected function expectCallableOnce(): callable
     {
         $mock = $this->createCallableMock();
-        $mock
-            ->expects($this->once())
-            ->method('__invoke');
+        $mock->expects($this->once())->method('__invoke');
+        assert(is_callable($mock));
 
         return $mock;
     }
 
-    protected function expectCallableOnceWith($value)
+    /** @param mixed $value */
+    protected function expectCallableOnceWith($value): callable
     {
         $mock = $this->createCallableMock();
-        $mock
-            ->expects($this->once())
-            ->method('__invoke')
-            ->with($value);
+        $mock->expects($this->once())->method('__invoke')->with($value);
+        assert(is_callable($mock));
 
         return $mock;
     }
 
-    protected function expectCallableNever()
+    protected function expectCallableNever(): callable
     {
         $mock = $this->createCallableMock();
-        $mock
-            ->expects($this->never())
-            ->method('__invoke');
+        $mock->expects($this->never())->method('__invoke');
+        assert(is_callable($mock));
 
         return $mock;
     }
 
-    protected function createCallableMock()
+    protected function createCallableMock(): MockObject
     {
         if (method_exists(MockBuilder::class, 'addMethods')) {
-            // PHPUnit 9+
+            // @phpstan-ignore-next-line PHPUnit 9+
             return $this->getMockBuilder(\stdClass::class)->addMethods(['__invoke'])->getMock();
         } else {
             // PHPUnit < 9
diff --git a/tests/Timer.php b/tests/Timer.php
index 0a37a73..3e381cc 100644
--- a/tests/Timer.php
+++ b/tests/Timer.php
@@ -4,8 +4,13 @@
 
 class Timer
 {
+    /** @var TestCase */
     private $testCase;
+
+    /** @var float */
     private $start;
+
+    /** @var float */
     private $stop;
 
     public function __construct(TestCase $testCase)
@@ -13,32 +18,32 @@ public function __construct(TestCase $testCase)
         $this->testCase = $testCase;
     }
 
-    public function start()
+    public function start(): void
     {
         $this->start = microtime(true);
     }
 
-    public function stop()
+    public function stop(): void
     {
         $this->stop = microtime(true);
     }
 
-    public function getInterval()
+    public function getInterval(): float
     {
         return $this->stop - $this->start;
     }
 
-    public function assertLessThan($milliseconds)
+    public function assertLessThan(float $milliseconds): void
     {
         $this->testCase->assertLessThan($milliseconds, $this->getInterval());
     }
 
-    public function assertGreaterThan($milliseconds)
+    public function assertGreaterThan(float $milliseconds): void
     {
         $this->testCase->assertGreaterThan($milliseconds, $this->getInterval());
     }
 
-    public function assertInRange($minMs, $maxMs)
+    public function assertInRange(float $minMs, float $maxMs): void
     {
         $this->assertGreaterThan($minMs);
         $this->assertLessThan($maxMs);
diff --git a/tests/WaterfallTest.php b/tests/WaterfallTest.php
index 2fbbc23..8aa6c6f 100644
--- a/tests/WaterfallTest.php
+++ b/tests/WaterfallTest.php
@@ -6,10 +6,11 @@
 use React\EventLoop\Loop;
 use React\Promise\Promise;
 use function React\Promise\reject;
+use function React\Promise\resolve;
 
 class WaterfallTest extends TestCase
 {
-    public function testWaterfallWithoutTasks()
+    public function testWaterfallWithoutTasks(): void
     {
         $tasks = array();
 
@@ -18,11 +19,11 @@ public function testWaterfallWithoutTasks()
         $promise->then($this->expectCallableOnceWith(null));
     }
 
-    public function testWaterfallWithoutTasksFromEmptyGeneratorResolvesWithNull()
+    public function testWaterfallWithoutTasksFromEmptyGeneratorResolvesWithNull(): void
     {
         $tasks = (function () {
-            if (false) {
-                yield;
+            if (false) { // @phpstan-ignore-line
+                yield function () { return resolve(null); };
             }
         })();
 
@@ -31,7 +32,7 @@ public function testWaterfallWithoutTasksFromEmptyGeneratorResolvesWithNull()
         $promise->then($this->expectCallableOnceWith(null));
     }
 
-    public function testWaterfallWithTasks()
+    public function testWaterfallWithTasks(): void
     {
         $tasks = array(
             function ($foo = 'foo') {
@@ -70,7 +71,7 @@ function ($bar) {
         $timer->assertInRange(0.15, 0.30);
     }
 
-    public function testWaterfallWithTasksFromGeneratorResolvesWithFinalFulfillmentValue()
+    public function testWaterfallWithTasksFromGeneratorResolvesWithFinalFulfillmentValue(): void
     {
         $tasks = (function () {
             yield function ($foo = 'foo') {
@@ -109,7 +110,7 @@ public function testWaterfallWithTasksFromGeneratorResolvesWithFinalFulfillmentV
         $timer->assertInRange(0.15, 0.30);
     }
 
-    public function testWaterfallWithError()
+    public function testWaterfallWithError(): void
     {
         $called = 0;
 
@@ -140,12 +141,12 @@ function () use (&$called) {
         $this->assertSame(1, $called);
     }
 
-    public function testWaterfallWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
+    public function testWaterfallWithErrorFromInfiniteGeneratorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks(): void
     {
         $called = 0;
 
         $tasks = (function () use (&$called) {
-            while (true) {
+            while (true) { // @phpstan-ignore-line
                 yield function () use (&$called) {
                     return reject(new \RuntimeException('Rejected ' . ++$called));
                 };
@@ -159,14 +160,15 @@ public function testWaterfallWithErrorFromInfiniteGeneratorReturnsPromiseRejecte
         $this->assertSame(1, $called);
     }
 
-    public function testWaterfallWithErrorFromInfiniteIteratorAggregateReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
+    public function testWaterfallWithErrorFromInfiniteIteratorAggregateReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks(): void
     {
         $tasks = new class() implements \IteratorAggregate {
+            /** @var int */
             public $called = 0;
 
             public function getIterator(): \Iterator
             {
-                while (true) {
+                while (true) { // @phpstan-ignore-line
                     yield function () {
                         return reject(new \RuntimeException('Rejected ' . ++$this->called));
                     };
@@ -181,7 +183,7 @@ public function getIterator(): \Iterator
         $this->assertSame(1, $tasks->called);
     }
 
-    public function testWaterfallWillCancelFirstPendingPromiseWhenCallingCancelOnResultingPromise()
+    public function testWaterfallWillCancelFirstPendingPromiseWhenCallingCancelOnResultingPromise(): void
     {
         $cancelled = 0;
 
@@ -199,6 +201,7 @@ function () use (&$cancelled) {
         );
 
         $promise = React\Async\waterfall($tasks);
+        assert(method_exists($promise, 'cancel'));
         $promise->cancel();
 
         $this->assertSame(1, $cancelled);

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