On Sun, 6 Aug 2023 at 20:20, Jason Merrill via Libstdc++ <libstd...@gcc.gnu.org> wrote: > > On Wed, Aug 2, 2023 at 12:02 PM Nikolas Klauser <nikolasklau...@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?
We use variadic templates and long long in C++98. We use a DMI in __gnu_cxx::__mutex even in C++98. I don't think we unconditionally use anything else, because we can't rely on it being available when using non-GCC compilers, or when compiling with -Wsystem-headers -pedantic. We don't use if-constexpr before C++17 for example. > > 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
