Re: [PATCH] Define alignof_slot using _Alignof when using C11 or newer

[Khem Raj <raj.khem@gmail.com>]

On Sat, Jan 14, 2023 at 4:17 PM Bruno Haible <bruno@clisp.org> wrote:
>
> Khem Raj wrote:
> > [2] https://reviews.llvm.org/D133574
>
> Thanks for the heads-up.
>
> > WG14 N2350 made very clear that it is an UB having type definitions
> > within "offsetof" [1].
> > [1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
>
> This document is not normative; it is merely a discussion document.
>
> What's (quasi) normative is
>
>   * n3047.pdf = C23, which says in 7.21.(3)
>     "If the specified type defines a new type or if the specified member is
>      a bit-field, the behavior is undefined."
>
>   * n2176.pdf = C18, which in 7.19.(3) merely says
>     "If the specified member is a bit-field, the behavior is undefined."
>
> So, only C23 and higher require to avoid offsetof with a new type.
>
> Since gnulib/lib/stdalign.h mentions an _Alignof bug in older versions of
> GCC and clang and some of these older versions (e.g. GCC 4.8.5) support C11,
> it is better to enable the workaround only for C23 and higher, not for
> C11 and higher.
>

I think if its enabled only for c2x and newer, clang will still errors out  for
__STDC_VERSION__= 201710L
I have sent out a patch which replicates the logic from
gnulib/lib/stdalign.h.in which should
address this as well.

> > +#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && !defined(__cplusplus)
>
> The '!defined(__cplusplus)' part is redundant here.
>

right. Dropped in v2.

> Bruno
>
>
>