do {

    class A {
    }

    class B : A {
    }

    class C : B {
    }

    func test(f: (C) -> A) -> String {
        return "Passed"
    }

    let closure1: (A) -> A = { _ in A() }
    let closure2: (B) -> B = { _ in B() }

    test(f: closure1)   // "Passed"
    test(f: closure2)   // "Passed"
}