PhantomTypeをSwiftでもなんとかして使いたい

PhantomType.swift

protocol LoginState { }
final class Login: LoginState { }
final class Logout: LoginState { }

class LoginStateAction<State>: LoginStateActionType { }

protocol LoginStateActionType {
    func loginState() -> LoginStateAction<Login>?
    func logoutState() -> LoginStateAction<Logout>?
}

extension LoginStateActionType {
    func loginState() -> LoginStateAction<Login>? {
        return self as? LoginStateAction<Login>
    }
    func logoutState() -> LoginStateAction<Logout>? {
        return self as? LoginStateAction<Logout>
    }
}

extension LoginStateAction where State: Login {
    func logout() -> LoginStateAction<Logout> {
        return LoginStateAction<Logout>()
    }
}

extension LoginStateAction where State: Logout {
    func login() -> LoginStateAction<Login> {
        return LoginStateAction<Login>()
    }
}

let action: LoginStateActionType = LoginStateAction<Login>() // !!! ここを実現するためにassociatedTypeを使わない
action.loginState()?.logout()
action.logoutState()?.login()

状態がふえるときつそう