simple tuple container using C++17

tuple17.cpp

template <std::size_t N, class T>
struct tuple_elem
{
    constexpr tuple_elem(T&& val)
    : val_{ std::forward<T>(val) }
    {}

    template <auto I, std::enable_if_t<I == N>* = nullptr>
    auto constexpr& operator[](constant<I>)
    { return val_; }

    template <auto I, std::enable_if_t<I == N>* = nullptr>
    auto constexpr& operator[](constant<I>) const
    { return val_; }

private:
    std::decay_t<T> val_;
};

template <class, class...>
struct tuple_impl;

template <std::size_t... Is, class... Ts>
struct tuple_impl<std::index_sequence<Is...>, Ts...>
: tuple_elem<Is, Ts>...
{
    using tuple_elem<Is, Ts>::operator[]...;

    constexpr tuple_impl(Ts&&... ts)
    : tuple_elem<Is, Ts>{ std::forward<Ts>(ts) }...
    {}
};

template <class... Ts>
struct tuple
: tuple_impl<std::make_index_sequence<sizeof...(Ts)>, Ts...>
{
    static auto constexpr size = sizeof...(Ts);

    constexpr tuple(Ts&&... ts)
    : tuple_impl<std::make_index_sequence<size>, Ts...>
        { std::forward<Ts>(ts)... }
    {}
};

template <class... Ts>
tuple(Ts&&...) -> tuple<Ts...>;