template <typename ... Functions>
struct function_array : public Functions ... {

    template <std::size_t Index>
    using at = std::tuple_element_t<Index, std::tuple<Functions ...>>;

    template <std::size_t Index, typename ... Args>
    constexpr decltype(auto) call(Args&& ... args) const
    noexcept(noexcept(std::declval<function_array>().at<Index>::operator()(std::declval<Args>() ...))) {
        return at<Index>::operator()(std::forward<Args>(args) ...);
    }

};
template <typename ... Functions> function_array(Functions ...) -> function_array<Functions ...>;