// dependent_false_v
template <typename...>
constexpr bool dependent_false_v = false;

// type_list
template <typename... Ts>
struct type_list {};

// type_index_pair
template <typename T, size_t idx>
struct type_index_pair {
   using type = T;
   static constexpr size_t value = idx;
};

// indexate
template <typename S, typename... Ts>
struct indexate_impl {
   static_assert(dependent_false_v<S>,
       "indexate_impl requires index_sequence as first template argument");
};

template <size_t... idxs, typename... Ts>
struct indexate_impl<std::index_sequence<idxs...>, Ts...> {
   static_assert(sizeof...(idxs) == sizeof...(Ts),
       "pack sizes should be same");

   using type = type_list<type_index_pair<Ts, idxs>...>;
};

template <typename... Ts>
struct indexate {
   using type = typename indexate_impl<
       std::make_index_sequence<sizeof...(Ts)>, Ts...>::type;
};

template <typename... Ts>
using indexate_t = typename indexate<Ts...>::type;

// get_types
template <typename T>
struct get_types {…