Add regression test for #13000

Closes phpstan/phpstan#13000

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-13000.php

@@ -0,0 +1,13 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13000;
+
+use function PHPStan\Testing\assertType;
+
+function (): void {
+	$r = [];
+	foreach (['a' => '1', 'b' => '2'] as $key => $val) {
+		$r[$key] = $val;
+	}
+	assertType("array{a?: '1'|'2', b?: '1'|'2'}", $r);
+};