On Mon, 7 Aug 2023 at 14:04, Jonathan Wakely <jwakely....@gmail.com> wrote: > > On Sun, 6 Aug 2023 at 20:43, Jonathan Wakely <jwakely....@gmail.com> wrote: > > > > 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. > > Oh, but we do use __decltype in a few places.
I thought of another one: we use __constinit in some C++11 code. But only inside the library, not in headers. I've recorded these at https://gcc.gnu.org/wiki/LibstdcxxHacking for now.
