std::tuple_element

template< std::size_t I, class T >
struct tuple_element; // not defined

template< std::size_t I, class T >
struct tuple_element< I, const T > {
using type = typename
std::add_const<typename std::tuple_element<I, T>::type>::type;
};

template< std::size_t I, class T >
struct tuple_element< I, volatile T > {
using type = typename
std::add_volatile<typename std::tuple_element<I, T>::type>::type;
};

template< std::size_t I, class T >
struct tuple_element< I, const volatile T > {
using type = typename
std::add_cv<typename std::tuple_element<I, T>::type>::type;
};

#include <array>
#include <cstddef>
#include <iostream>
#include <ranges>
#include <tuple>
#include <type_traits>
#include <utility>
 
template<typename T1, typename T2, typename T3>
struct Triple
{
    T1 t1;
    T2 t2;
    T3 t3;
};
 
// A specialization of std::tuple_element for program-defined type Triple:
template<std::size_t I, typename T1, typename T2, typename T3>
    struct std::tuple_element<I, Triple<T1, T2, T3>>
    { static_assert(false, "Invalid index"); }; 
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<0, Triple<T1, T2, T3>> { using type = T1; };
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<1, Triple<T1, T2, T3>> { using type = T2; };
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<2, Triple<T1, T2, T3>> { using type = T3; };
 
 
template<typename... Args> struct TripleTypes
{
    static_assert(3 == sizeof...(Args), "Expected exactly 3 type names");
    template<std::size_t N>
    using type = typename std::tuple_element_t<N, Triple<Args...>>;
};
 
int main()
{
    TripleTypes<char, int, float>::type<1> i{42};
    std::cout << i << '\n';
 
    using Tri = Triple<int, char, short>; //< Program-defined type
    static_assert(std::is_same_v<std::tuple_element_t<0, Tri>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Tri>, char> &&
                  std::is_same_v<std::tuple_element_t<2, Tri>, short>);
 
    using Tuple = std::tuple<int, char, short>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Tuple>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Tuple>, char> &&
                  std::is_same_v<std::tuple_element_t<2, Tuple>, short>);
 
    using Array3 = std::array<int, 3>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Array3>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Array3>, int> &&
                  std::is_same_v<std::tuple_element_t<2, Array3>, int>);
 
    using Pair = std::pair<Tuple, Tri>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Pair>, Tuple> &&
                  std::is_same_v<std::tuple_element_t<1, Pair>, Tri>);
 
    using Sub = std::ranges::subrange<int*, int*>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Sub>, int*> &&
                  std::is_same_v<std::tuple_element_t<1, Sub>, int*>);
}