Add regression test for #12401

Closes phpstan/phpstan#12401

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-12401.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12401;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param array{a: string, b: string}|array{c: string} $data
+ */
+function test(array $data): void {
+	if (isset($data['a'])) {
+		assertType('array{a: string, b: string}', $data);
+	} else {
+		assertType('array{c: string}', $data);
+	}
+
+	if (isset($data['a'])) {
+		assertType('array{a: string, b: string}', $data);
+	} elseif (isset($data['c'])) {
+		assertType('array{c: string}', $data);
+	} else {
+		assertType('*NEVER*', $data);
+	}
+}