Re: GCC support for extensions from later standards

[Jason Merrill <jason@redhat.com>]

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On Wed, Aug 2, 2023 at 12:02 PM Nikolas Klauser <nikolasklauser@berlin.de>
wrote:

> Hi everyone!
>
> I'm working on libc++ and we are currently discussing using language
> extensions from later standards (
> https://discourse.llvm.org/t/rfc-use-language-extensions-from-future-standards-in-libc/71898/4).
> By that I mean things like using `if constexpr` with `-std=c++11`. GCC has
> quite a lot of these kinds of conforming extensions, but doesn't document
> them AFAICT. While discussing using these extensions, the question came up
> what GCCs support policy for these is. Aaron was kind enough to answer
> these questions for us on the Clang side. Since I couldn't find anything in
> the documentation, I thought I'd ask here.
>
> So, here are my questions:
>
> Do you expect that these extensions will ever be removed for some reason?
> If yes, what could those reasons be?
>

Potentially, if they don't actually work properly in earlier standard
modes.  I recently noticed that while we allow DMI and =default in C++03
mode with a pedwarn, combining them doesn't work.

Some of the extensions are needed by libstdc++ and are therefore well
tested; these are extremely unlikely to ever be removed.  libstdc++ folks,
is there a list of these?

Would you be interested in documenting them?
>

That would be useful, yes.

There is a patch in review to add __has_feature/__has_extension to G++,
which would seem like a suitable context for this documentation.

Aaron noted that we should ask the Clang folks before using them, so they
> can evaluated whether the extension makes sense, since they might not be
> aware of them, and some might be broken. So I'd be interested whether you
> would also like us to ask whether you want to actually support these
> extensions.
>

Sounds good.

Jason