Functionally test enforcing Throwable as rejection

tests/FunctionalRejectTest.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace React\Promise;
+
+use stdClass;
+
+class FunctionalRejectTest extends TestCase
+{
+    public function nonThrowables()
+    {
+        yield '1' => [1];
+        yield 'true' => [true];
+        yield 'stdClass' => [new stdClass()];
+    }
+
+    /**
+     * @test
+     * @dataProvider nonThrowables
+     */
+    public function shouldThrowWhenCalledWithANonException($input)
+    {
+        $errorCollector = new ErrorCollector();
+        $errorCollector->start();
+
+        (new Promise(function ($_, $reject) use ($input) {
+            $reject($input);
+        }))->done($this->expectCallableNever());
+
+        $errors = $errorCollector->stop();
+
+        $this->assertEquals(E_USER_ERROR, $errors[0]['errno']);
+        $this->assertContains(
+            'TypeError: Argument 1 passed to React\Promise\reject() must implement interface Throwable',
+            $errors[0]['errstr']
+        );
+    }
+}