[PATCH] stdlib: Suppress gcc diagnostic that char8_t is a keyword in C++20 in uchar.h.

[Tom Honermann]

On 7/24/22 1:11 AM, Tom Honermann via Libc-alpha wrote:
> gcc 13 issues the following diagnostic for the uchar.h header when the
> -Wc++20-compat option is enabled in C++ modes that do not enable char8_t
> as a builtin type (C++17 and earlier by default; subject to _GNU_SOURCE
> and the gcc -f[no-]char8_t option).
>    warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]
> This change modifies the uchar.h header to suppress the diagnostic through
> the use of '#pragma GCC diagnostic' directives for gcc 10 and later (the
> -Wc++20-compat option was added in gcc version 10).  Unfortunately, a bug
> in gcc currently prevents those directives from having the intended effect
> as reported athttps://gcc.gnu.org/PR106423.  A patch for that issue has
> been submitted and is available in the email thread archive linked below.
>    https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598736.html

The above linked gcc patch has landed 
<https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=60468d6cd46a3bd3afe8ff856f82afcd4c65a217> 
for gcc 13; the diagnostic suppression in the patch below (committed for 
2.37 and as a post 2.36 update to release/2.36/master 
<https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/release/2.36/master>) 
will become effective with that release.

Tom.

> ---
>   wcsmbs/uchar.h | 8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/wcsmbs/uchar.h b/wcsmbs/uchar.h
> index c37e8619a0..5f7139f279 100644
> --- a/wcsmbs/uchar.h
> +++ b/wcsmbs/uchar.h
> @@ -34,8 +34,16 @@
>   /* Declare the C2x char8_t typedef in C2x modes, but only if the C++
>     __cpp_char8_t feature test macro is not defined.  */
>   #if __GLIBC_USE (ISOC2X) && !defined __cpp_char8_t
> +#if __GNUC_PREREQ (10, 0) && defined __cplusplus
> +/* Suppress the diagnostic regarding char8_t being a keyword in C++20.  */
> +# pragma GCC diagnostic push
> +# pragma GCC diagnostic ignored "-Wc++20-compat"
> +#endif
>   /* Define the 8-bit character type.  */
>   typedef unsigned char char8_t;
> +#if __GNUC_PREREQ (10, 0) && defined __cplusplus
> +# pragma GCC diagnostic pop
> +#endif
>   #endif
>   
>   #ifndef __USE_ISOCXX11