@swift-4.2.4
import Foundation class Cat { @objc func nya() -> Int { return 3 } } class Dog { @objc func wan() -> Int { return 4 } } func f(_ a: AnyObject) { let x = a.nya() let y = a.wan?() print(type(of: x), x) // Int 3 print(type(of: y), y) // Optional<Int> nil } f(Cat())