Add regression test for #1311

Closes phpstan/phpstan#1311

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

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-1311.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace Bug1311;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @param array<int, array{a: string}> $sets
+	 *
+	 * @return array<int, array{a: string, b: bool}>
+	 */
+	public function sayHello(array $sets): array
+	{
+		foreach ($sets as &$set) {
+			$set['b'] = false;
+		}
+
+		assertType('array<int, array{a: string, b: false}>', $sets);
+
+		return $sets;
+	}
+}