template chaining

chaining.cpp

template <typename... Ts>
struct box {
    using t = box<Ts...>;
};

template <typename... Ts>
struct flatten;

template <typename T>
struct flatten<T> {
    using t = box<T>;
};

template <typename... Ts>
struct flatten<box<Ts...>> {
    using t = box<Ts...>;
};

template <typename... Ls, typename... Rs>
struct flatten<box<Ls...>, box<Rs...>> {
    using t = box<Ls..., Rs...>;
};

template <typename... Ls, typename R>
struct flatten<box<Ls...>, R> {
    using t = box<Ls..., R>;
};

template <typename L, typename... Rs>
struct flatten<L, box<Rs...>> {
    using t = box<L, Rs...>;
};

template <typename L, typename R>
struct flatten<L, R> {
    using t = box<L, R>;
};

template <typename T>
struct unwrap {
    using t = T;
};

template <typename... Ts>
struct unwrap<box<Ts...>> {
    using t = box<Ts...>;
};

template <typename L, typename R>
struct chain {
    using t = flatten<
        typename unwrap<L>::t,
        typename unwrap<R>::t
    >::t;
};

template <template<typename...> class Clas, typename... Ts>
struct apply {
    using t = Clas<Ts...>;
};

template <template<typename...> class Clas, typename... Ts>
struct apply<Clas, box<Ts...>> {
    using t = Clas<Ts...>;
};