In place method to find the index of a type in a tuple

flat_index_finder.cpp

template <class T, std::size_t I, class Tuple>  // 1
constexpr bool match_v = std::is_same_v<T, std::tuple_element_t<I, Tuple>>;

template <class T, class Tuple,  // 2
          class Idxs = std::make_index_sequence<std::tuple_size_v<Tuple>>>
struct type_index;

template <class T, template <class...> class Tuple, class... Args,  // 3
          std::size_t... Is>
struct type_index<T, Tuple<Args...>, std::index_sequence<Is...>>
    : std::integral_constant<std::size_t,
                             ((Is * match_v<T, Is, Tuple<Args...>>) + ... + 0)> {
  static_assert(1 == (match_v<T, Is, Tuple<Args...>> + ... + 0),
                "T doesn't appear once in Tuple");
};