https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109606
--- Comment #3 from Alexander <akaraevz at mail dot ru> --- (In reply to Jonathan Wakely from comment #1) > Yes. Using libstdc++ headers with older **or newer** versions of GCC is > completely unsupported. For other compilers, they need to be sufficiently > close to GCC, which means they should support built-ins added to GCC 10 by > now. Thanks you for explanation. Usage of `_GLIBCXX_HAVE_BUILTIN_IS_SAME` confused me, I expected that all `__same_as` usages should be guarded by this flag. > That was added some years ago, and I've been thinking of simplifying it. > > Do you actually see a problem with any real compiler? There's no problem with any real compiler (at least for me), however the problem exists for ReSharper C++ (tracking issue: https://youtrack.jetbrains.com/issue/RSCPP-34115). Amongst other, ReSharper has its own preprocessor and also constexpr evaluation engine where `__same_as` built-in was marked as supported only in clang. We missed that gcc-10 started supporting it too. It wasn't an issue before, because we simply returned `false` from `__has_builtin(__same_as)` for gcc and everything worked fine with fallback to naive implementation (i.e. for `std::is_same`). Of course, we'll fix this issue on our side, providing `__same_as` for gcc-10 (and later) too.
