Add regression test for #5128

Closes phpstan/phpstan#5128

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-5128.php

@@ -0,0 +1,18 @@
+<?php declare(strict_types = 1);
+
+namespace Bug5128;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param array{a: string}|array{b: string} $array
+ */
+function a(array $array): string {
+	if (isset($array['a'])) {
+		assertType('array{a: string}', $array);
+		return $array['a'];
+	}
+
+	assertType('array{b: string}', $array);
+	return $array['b'];
+}