Add non regression test for #5206

Author: VincentLanglet

tests/PHPStan/Rules/Functions/ExistingClassesInArrowFunctionTypehintsRuleTest.php

@@ -311,4 +311,17 @@ public function testNever(): void
 		$this->analyse([__DIR__ . '/data/arrow-function-never.php'], $errors);
 	}
 
+	public function testBug5206(): void
+	{
+		$errors = [];
+		if (PHP_VERSION_ID < 80000) {
+			$errors[] = [
+				'Parameter $mixed of anonymous function has invalid type Bug5206\mixed.',
+				9,
+			];
+		}
+
+		$this->analyse([__DIR__ . '/data/bug-5206.php'], $errors);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-5206.php

@@ -0,0 +1,11 @@
+<?php declare(strict_types = 1);
+
+namespace Bug5206;
+
+class HelloWorld
+{
+	public function sayHello(): \Closure
+	{
+		return fn (mixed $mixed) => '`mixed` type!';
+	}
+}