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

[Mark de Wever 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>
----------------
mordante wrote:

Based on http://eel.is/c++draft/description#structure.requirements-9 this seems 
to be allowed. Then we can write "proper" SFINAE tests. I agree with 
@frederick-vs-ja the current way gives horrible diagnostics based on 
`noexcept`. 

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