Add asserts

Author: VincentLanglet

tests/PHPStan/Analyser/nsrt/bug-12447.php

@@ -0,0 +1,15 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12447;
+
+use function PHPStan\Testing\assertType;
+
+assertType('mixed', $data);
+$data[] = 1;
+assertType('mixed', $data);
+
+function (): void {
+	assertType('*ERROR*', $data);
+	$data[] = 1;
+	assertType('array{1}', $data);
+};