protocol P { static var value: Int { get } } extension P { static var value: Int { return 3 } } protocol Q { static var value: Bool { get } } extension Q { static var value: Bool { return true } } class A : P { } class B : A, Q { } class C : B { static var value: String = "str" } let a: Int = C.value let b: Bool = C.value let c: String = C.value print(a, b, c)