sum metafunction C++17

sum.cpp

#include <variant>

template <typename ...Args>
struct sum_;

template <typename Last>
struct sum_<Last> {
    using type = Last;
};

template <typename A, typename B, typename ...Rest>
struct sum_<A, B, Rest...> {
    using type = typename sum_<std::variant<A, B>, Rest...>::type;
};

template <typename ...A, typename B, typename ...Rest>
struct sum_<std::variant<A...>, B, Rest...> {
    using type = typename sum_<std::variant<A..., B>, Rest...>::type;
};

template <typename ...A, typename ...B, typename ...Rest>
struct sum_<std::variant<A...>, std::variant<B...>, Rest...> {
    using type = typename sum_<std::variant<A..., B...>, Rest...>::type;
};

template <typename A, typename ...B, typename ...Rest>
struct sum_<A, std::variant<B...>, Rest...> {
    using type = typename sum_<std::variant<A, B...>, Rest...>::type;
};

template <typename ...T>
using sum = typename sum_<T...>::type;

int main() {
    using type_t = sum<int, char, double>;
    using type_2 = sum<float, short>;
    static_assert(
        std::is_same_v<sum<int, char, double>, std::variant<int, char, double>>
    );
    static_assert(
        std::is_same_v<
            sum<type_t, float, short>,
            std::variant<int, char, double, float, short>>);
    static_assert(
        std::is_same_v<
            sum<type_t, type_2>,
            std::variant<int, char, double, float, short>>);
    static_assert(
        std::is_same_v<
            sum<int, char, type_2>,
            std::variant<int, char, float, short>>);
    return 0;
}