Re: [RFC] Add stdckdint.h header for C23

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Jun 13, 2023 at 07:54:25PM -0700, Paul Eggert wrote:
> I don't see how you could implement __has_include_next(<stdckdint.h>) for
> arbitrary non-GCC compilers, which is what we'd need for glibc users. For
> glibc internals we can use "#include_next" more readily, since we assume a
> new-enough GCC. I.e. we could do something like this:

Indeed, limits.h uses
#if defined __GNUC__ && !defined _GCC_LIMITS_H_
/* `_GCC_LIMITS_H_' is what GCC's file defines.  */
# include_next <limits.h>
#endif

I'd just do:
#if defined (__has_include_next) && !defined (ckd_add)
# if __has_include_next (<stdckdint.h>)
#  include_next <stdckdint.h>
# endif
#endif

and then deal with the fallback (if the C library really needs one,
does it need to support a C23 feature in arbitrary compiler which
doesn't support C23?).
A fallback should start with using __builtin_{add,sub,mul}_overflow
without extra checking if compiler supports it, with or without
use of __has_builtin.  GCC 6 and later has __builtin_{add,sub,mul}_overflow,
only GCC 10 has __has_builtin, I think clang has added those shortly
after they were added in GCC 6 (though, __builtin_{add,sub,mul}_overflow_p
which has been added in GCC 7 wasn't added to clang).

	Jakub