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

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 19 Jul 2022 at 11:08, Jonathan Wakely wrote:
>
> On Sun, 17 Jul 2022 at 22:13, Stephan Bergmann via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > 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,
>
> Thanks, Stephan, I'll fix this.

This patch doesn't work, because __has_builtin doesn't detect the new
built-ins. I have a patch that solves that, so we can make the change
to the library headers.

>
> >
> > > 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
> >