[PATCH] c++: Add __reference_con{struc,ver}ts_from_temporary [PR104477]

[Stephan Bergmann]

On 7/15/22 22:25, Marek Polacek via Gcc-patches wrote:
> Yeah, I guess so.  But I've already pushed the patch.

This commit obviously breaks using libstdc++ with Clang (in -std=c++2b 
mode), which doesn't implement those new builtins.  Something like the 
below would fix that,

> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index b1a1deecf66..a6e028b42ec 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -3505,6 +3505,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>    template<typename _Tp>
>      inline constexpr bool is_scoped_enum_v = is_scoped_enum<_Tp>::value;
>  
> +#if __has_builtin(__reference_constructs_from_temporary) && __has_builtin(__reference_converts_from_temporary)
>  #define __cpp_lib_reference_from_temporary 202202L
>  
>    /// True if _Tp is a reference type, a _Up value can be bound to _Tp in
> @@ -3544,6 +3545,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>    template<typename _Tp, typename _Up>
>      inline constexpr bool reference_converts_from_temporary_v
>        = reference_converts_from_temporary<_Tp, _Up>::value;
> +#endif
>  #endif // C++23
>  
>  #if _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
> diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
> index 5edca2f3007..7c4b7f7cc6d 100644
> --- a/libstdc++-v3/include/std/version
> +++ b/libstdc++-v3/include/std/version
> @@ -304,7 +304,9 @@
>  #define __cpp_lib_byteswap 202110L
>  #define __cpp_lib_constexpr_typeinfo 202106L
>  #define __cpp_lib_is_scoped_enum 202011L
> +#if __has_builtin(__reference_constructs_from_temporary) && __has_builtin(__reference_converts_from_temporary)
>  #define __cpp_lib_reference_from_temporary 202202L
> +#endif
>  
>  #if _GLIBCXX_HOSTED
>  #define __cpp_lib_adaptor_iterator_pair_constructor 202106L