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

[Mark de Wever via libcxx-commits]

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@@ -1423,8 +1423,16 @@ inline _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) apply(_Fn&& __f, _Tuple&&
         typename __make_tuple_indices<tuple_size_v<remove_reference_t<_Tuple>>>::type{}))
 
 template <class _Tp, class _Tuple, size_t... _Idx>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp __make_from_tuple_impl(_Tuple&& __t, __tuple_indices<_Idx...>)
-    _LIBCPP_NOEXCEPT_RETURN(_Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...))
+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))...)))
+#if _LIBCPP_STD_VER >= 20
+  requires is_constructible_v<_Tp, decltype(std::get<_Idx>(std::forward<_Tuple>(__t)))...>
+#endif // _LIBCPP_STD_VER >= 20
+{
+  static_assert(is_constructible_v<_Tp, decltype(std::get<_Idx>(std::forward<_Tuple>(__t)))...>, 
+                "Cannot constructible target type from the fields of the argument tuple.");
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mordante wrote:

Since the LWG issue uses SFINAE to remove the function from the overload set we should do the same in earlier C++ versions. That also allows to use a SFINAE based test.

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