Add regression test for collateral fix #12500

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGhnJQpUWRRpg6H5LuWJkA

Author: ondrejmirtes

tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php

@@ -1415,4 +1415,9 @@ public function testBug12984(): void
 		$this->analyse([__DIR__ . '/data/bug-12984.php'], []);
 	}
 
+	public function testBug12500(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-12500.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-12500.php

@@ -0,0 +1,17 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12500;
+
+class HelloWorld
+{
+	/**
+	 * @param array<string,string|int> $input
+	 * @return array<string,string>
+	 */
+	public function clean(array $input): array {
+		foreach ($input as $k => $v) {
+			if (\is_int($v)) { $input[$k] = 'was-int'; }
+		}
+		return $input;
+	}
+}