Add regression test for #12562

Closes phpstan/phpstan#12562

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

tests/PHPStan/Analyser/nsrt/bug-12562.php

@@ -0,0 +1,38 @@
+<?php declare(strict_types = 1); // lint >= 8.0
+
+namespace Bug12562;
+
+use function PHPStan\Testing\assertType;
+
+class UserV1
+{
+	public function toV2(): UserV2
+	{
+		return new UserV2();
+	}
+}
+
+class UserV2
+{
+	public function toV2(): self
+	{
+		return $this;
+	}
+}
+
+class UserV2a extends UserV2
+{
+	/**
+	 * @return $this
+	 */
+	public function toV2(): self
+	{
+		return $this;
+	}
+}
+
+function doSomething(UserV1|UserV2 $user, UserV1|UserV2a $user2): void
+{
+	assertType('Bug12562\UserV2', $user->toV2());
+	assertType('Bug12562\UserV2', $user2->toV2());
+}