Added regression test

Author: staabm

tests/PHPStan/Rules/Classes/ImpossibleInstanceOfRuleTest.php

@@ -556,4 +556,15 @@ public function testNewIsAlwaysFinalClass(): void
 		]);
 	}
 
+	public function testBug13469(): void
+	{
+		$this->treatPhpDocTypesAsCertain = false;
+		$this->analyse([__DIR__ . '/data/bug-13469.php'], [
+			[
+				'Instanceof between Bug13469\Foo and Stringable will always evaluate to true.',
+				23,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Classes/data/bug-13469.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Bug13469;
+
+use Stringable;
+
+trait FooTrait
+{
+	public function __toString(): string
+	{
+		return 'foo';
+	}
+}
+
+class Foo
+{
+	use FooTrait;
+}
+
+function doFoo() {
+	$foo = new Foo();
+
+	var_dump($foo instanceof Stringable);
+}
+