#include <future>
#include <iostream>
#include <tuple>
template <class X, class Tuple>
class Idx;
template <class X, class... T>
class Idx<X, std::tuple<T...>> {
template <std::size_t... idx>
static constexpr ssize_t find_idx(std::index_sequence<idx...>) {
return -1 + ((std::is_same<X, T>::value ? idx + 1 : 0) + ...);
}
public:
static constexpr ssize_t value = find_idx(std::index_sequence_for<T...>{});
};
int main() {
using T = std::tuple<int, char, std::future<void>>;
std::cout << Idx<std::promise<void>, T>::value << '\n'
<< Idx<int, T>::value << '\n'
<< Idx<char, T>::value << '\n'
<< Idx<std::future<void>, T>::value << std::endl;
//constexpr auto idx = Idx<unsigned, T>::value;
//static_assert(idx == -1, "Type not found");
}
