Re: [PATCH] libstdc++: Define _GLIBCXX_HAS_BUILTIN_TRAIT

[Patrick Palka <ppalka@redhat.com>]

On Tue, 18 Jul 2023, Ken Matsui via Libstdc++ wrote:

> This patch defines _GLIBCXX_HAS_BUILTIN_TRAIT, which will be used as a
> flag to toggle built-in traits in the type_traits header. Through this
> macro function and _GLIBCXX_NO_BUILTIN_TRAITS macro, we can switch the
> use of built-in traits without needing to modify the source code.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/c++config (_GLIBCXX_HAS_BUILTIN_TRAIT): Define.

The ChangeLog entry should also mention the change to _GLIBCXX_HAS_BUILTIN,
e.g.

	(_GLIBCXX_HAS_BUILTIN): Keep defined.

> 
> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
> ---
>  libstdc++-v3/include/bits/c++config | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
> index dd47f274d5f..de13f61db71 100644
> --- a/libstdc++-v3/include/bits/c++config
> +++ b/libstdc++-v3/include/bits/c++config
> @@ -854,7 +854,11 @@ namespace __gnu_cxx
>  # define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
>  #endif
>  
> -#undef _GLIBCXX_HAS_BUILTIN
> +// Returns true if _GLIBCXX_NO_BUILTIN_TRAITS is not defined and the compiler
> +// has a corresponding built-in type trait. _GLIBCXX_NO_BUILTIN_TRAITS is
> +// defined to disable the use of built-in traits.
> +#define _GLIBCXX_HAS_BUILTIN_TRAIT(BT)  \
> +  (!defined(_GLIBCXX_NO_BUILTIN_TRAITS) && _GLIBCXX_HAS_BUILTIN(BT))

Since we don't expect _GLIBCXX_NO_BUILTIN_TRAITS to get
defined/undefined in the middle of preprocessing, perhaps we should
factor out the _GLIBCXX_NO_BUILTIN_TRAITS test from the macro function
and instead conditionally define the macro function to 0 according
_GLIBCXX_NO_BUILTIN_TRAITS?

>  
>  // Mark code that should be ignored by the compiler, but seen by Doxygen.
>  #define _GLIBCXX_DOXYGEN_ONLY(X)
> -- 
> 2.41.0
> 
>