Add non regression test

Author: VincentLanglet

tests/PHPStan/Rules/Functions/ExistingClassesInClosureTypehintsRuleTest.php

@@ -353,4 +353,9 @@ public function testDeprecatedImplicitlyNullableParameterType(): void
 		]);
 	}
 
+	public function testBug5206(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-5206.php'], []);
+	}
+
 }

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!';
+	}
+}