Add regression test for #6991

Closes phpstan/phpstan#6991

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

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-6991.php

@@ -0,0 +1,29 @@
+<?php declare(strict_types = 1); // lint >= 8.0
+
+namespace Bug6991;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	public int $more = 1;
+
+	public ?string $key = null;
+}
+
+class Other {
+	public ?HelloWorld $optional = null;
+}
+
+function test(Other $object): int
+{
+	$key = $object->optional?->key;
+
+	if (!$key) {
+		return 0;
+	}
+
+	assertType('Bug6991\HelloWorld', $object->optional);
+
+	return $object->optional->more * 100;
+}