[clang] [libcxx] [libc++] Implement LWG3528 (`make_from_tuple` can perform (the equivalent of) a C-style cast) (PR #85263)

[A. Jiang via cfe-commits]

================
@@ -1386,9 +1386,19 @@ inline _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) apply(_Fn&& __f, _Tuple&&
         std::forward<_Tuple>(__t),
         typename __make_tuple_indices<tuple_size_v<remove_reference_t<_Tuple>>>::type{}))
 
+#if _LIBCPP_STD_VER >= 20
 template <class _Tp, class _Tuple, size_t... _Idx>
 inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp __make_from_tuple_impl(_Tuple&& __t, __tuple_indices<_Idx...>)
+  noexcept(noexcept(_Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...)))
+  requires is_constructible_v<_Tp, decltype(std::get<_Idx>(std::forward<_Tuple>(__t)))...> {
+  return _Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...);
+}
+#else
+template <class _Tp, class _Tuple, size_t... _Idx>
+inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp __make_from_tuple_impl(_Tuple&& __t, __tuple_indices<_Idx...>, 
+    enable_if_t<is_constructible_v<_Tp, decltype(std::get<_Idx>(std::forward<_Tuple>(__t)))...>> * = nullptr)
     _LIBCPP_NOEXCEPT_RETURN(_Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...))
+#endif // _LIBCPP_STD_VER >= 20
 
 template <class _Tp, class _Tuple>
----------------
frederick-vs-ja wrote:

Just making `__make_from_tuple_impl` SFINAE-friendly is insufficient and leads to worse diagnostic messages.

I guess we can write the following to make `make_from_tuple` SFINAE-friendly. `(requires` can be used since C++20 to make the implemenation clearer.)

```C++
template <class _Tp, class _Tuple,
    class _Seq = typename __make_tuple_indices<tuple_size_v<remove_reference_t<_Tuple>>>::type, class = void>
inline constexpr bool __can_make_from_tuple = false;
template <class _Tp, class _Tuple, size_t... _Idx>
inline constexpr bool __can_make_from_tuple<
    _Tp, _Tuple, index_sequence<_Idx...>,
    enable_if_t<is_constructible_v<_Tp, decltype(std::get<_Idx>(std::declval<_Tuple>()))...>>> = true;

template <class _Tp, class _Tuple, enable_if_t<__can_make_from_tuple<_Tp, _Tuple>, int> = 0>
inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp make_from_tuple(_Tuple&& __t)
    _LIBCPP_NOEXCEPT_RETURN(std::__make_from_tuple_impl<_Tp>(
        std::forward<_Tuple>(__t), typename __make_tuple_indices<tuple_size_v<remove_reference_t<_Tuple>>>::type{}))
```

And perhaps we shouldn't touch `__make_from_tuple_impl` if it's OK to constrain `make_from_tuple`.

https://github.com/llvm/llvm-project/pull/85263