Add regression test for #6120

Closes phpstan/phpstan#6120

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

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-6120.php

@@ -0,0 +1,27 @@
+<?php declare(strict_types = 1); // lint >= 8.0
+
+namespace Bug6120;
+
+use Generator;
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @var Generator<int, string> $gen
+	 */
+	private ?Generator $gen = null;
+
+	public function setGenerator(Generator $gen): void {
+		$this->gen = $gen;
+	}
+
+	public function sayHello(): void
+	{
+		while ($v = $this->gen?->current()) {
+			assertType('Generator<int, string, mixed, mixed>', $this->gen);
+			echo $v;
+			$this->gen->next();
+		}
+	}
+}