Add regression test for #11441 (assert on UnionType)

Author: yt-catpaw

tests/PHPStan/Analyser/nsrt/bug-11441.php

@@ -0,0 +1,39 @@
+<?php declare(strict_types=1);
+
+namespace Bug11441;
+
+use function PHPStan\Testing\assertType;
+
+class Foo {
+    public function getParam(): ?string {
+        return 'foo';
+    }
+
+    /** @phpstan-assert !null $this->getParam() */
+    public function checkNotNull(): void {
+        if ($this->getParam() === null) {
+            throw new \Exception();
+        }
+    }
+}
+
+class Bar {
+    public function getParam(): ?int {
+        return 1;
+    }
+
+    /** @phpstan-assert !null $this->getParam() */
+    public function checkNotNull(): void {
+        if ($this->getParam() === null) {
+            throw new \Exception();
+        }
+    }
+}
+
+function test(Foo|Bar $obj): void {
+    assertType('int|string|null', $obj->getParam());
+
+    $obj->checkNotNull();
+
+    assertType('int|string', $obj->getParam());
+}