protocol MyViewProtocol { var button: Button { get } } protocol HasMyViewProtocol: MyViewProtocol { associatedtype MyView: MyViewProtocol var myView: MyView { get } } extension HasMyViewProtocol { var button: Button { return myView.button } }protocol MyViewProtocol { var button: Button { get } } protocol HasMyViewProtocol: MyViewProtocol { associatedtype MyView: MyViewProtocol var myView: MyView { get } } extension HasMyViewProtocol { var button: Button { return myView.button } }extension MyViewProtocol に置けば全部使えるようになる