Add regression test for #6623

Closes phpstan/phpstan#6623

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-6623.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace Bug6623;
+
+use function PHPStan\Testing\assertType;
+
+class World {
+	protected function bar(): void {}
+}
+
+class HelloWorld extends World
+{
+	private ?int $foo = null;
+
+	public function useFoo(): void
+	{
+		if (is_null($this->foo)) {
+			throw new \LogicException();
+		}
+		assertType('int', $this->foo);
+		$this->bar();
+		assertType('int', $this->foo);
+	}
+}