C2x features status

[Marek Polacek]

On Fri, Oct 21, 2022 at 08:31:09PM +0200, Florian Weimer via Gcc wrote:
> * Joseph Myers:
> 
> > I'm working on adding various C2x features to the C front end (and 
> > elsewhere in GCC as applicable).
> >
> > I suspect I won't get all the C2x features done for GCC 13.  If anyone 
> > else is interested in adding C2x features, I'd encourage looking at some 
> > of the following, which I may well not get to for GCC 13 (and posting here 
> > to avoid duplication of effort if working on such a feature):
> >
> > * Bit-precise integer types (_BitInt) (see bug 102989) (integrated version 
> > based on N2763, plus literal suffixes from N2775 and bit-fields from 
> > N2969).  Would require working with back-end maintainers and upstream ABI 
> > groups, where available, to get ABIs defined for as many architectures as 
> > possible, as well as some default ABI choice in GCC for architectures that 
> > haven't defined the ABI for these types.
> >
> > * [[unsequenced]] and [[reproducible]] attributes for function types.  See 
> > N2956.  These are supposed to be similar to const and pure attributes, at 
> > least in the absence of pointer and array function parameters (but note 
> > they never affect type compatibility).
> >
> > * Tag compatibility (N3037, alternative wording).  Martin Uecker might 
> > have patches for a draft version of this?
> >
> > * Preprocessor #embed (N3017) (see bug 105863).
> 
> Do you have a list of C2X features that are likely to impact autoconf
> tests?  Or planned changes in the GCC 13 and 14 default language modes
> that reject constructs previous accepted as an extension?

At least this one:

commit 0a91bdaf177409a2a5e7895bce4f0e7091b4b3ca
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Sep 7 13:56:25 2022 +0000

    c: New C2x keywords

which says:

    As with the removal of unprototyped functions, this change has a high
    risk of breaking some old code and people doing GNU/Linux distribution
    builds may wish to see how much is broken in a build with a -std=gnu2x
    default.
 
> I'm asking because I'm working on the implicit function declaration
> problem once more, and other things could be piggybacked on the tool
> support over time.  See the parallel “C89isms in the test suite” thread.
> 
> I wonder if anything went into the default C2X language mode already
> that could be similarly disruptive as the removal of implicit ints?  In
> that case, I should probably backport that change into my GCC test
> version.  (To avoid chasing ghosts, it's based off GCC 12, I've decided
> to decouple it from our planned switch to GCC 13.)
> 
> Thanks,
> Florian
> 

Marek