gistfile1.swift

class A {
    func test() {
        print("A")
    }
}

class B: A {
    override func test() {
        print("B")
    }
}

let block = { (b: B?) -> Void in
    if let b = b {
        b.test()
    }
}

let a: A? = nil

//...

block(a as? B?) // Value of optional type 'B??' not unwrapped; did you mean to use '!' or '?'?