struct ObservableA<T> { func subscribe(onNext:(T) -> () = { _ in }) { } } ObservableA<(Int, Int)>().subscribe(onNext: { (a, b) in // ok }) struct ObservableB<T> { func subscribe(onNext:((T) -> ())? = nil) { } } ObservableB<(Int, Int)>().subscribe(onNext: { (a, b) in // 死ーん })