https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79433
--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Marc Glisse from comment #19) > (In reply to Jonathan Wakely from comment #16) > > Even if we moved our headers to separate directories, it wouldn't make > > __has_include sufficient.. > > Could you explain why? It would be a pain for other compilers using > libstdc++ to add a suitable list of directories, but I don't immediately see > why that would fail. As I wrote: Or you could include it, and it doesn't define anything, because all its contents are hidden behind a #if __cplusplus > 201402L check (which is what happens with libc++). So you include the header, try to use the library types it is supposed to define, and get a compiler error. i.e. it wouldn't make __has_include sufficient for portable checks. A solution that only work for GCC isn't a very good solution, because the whole point of the feature-test macros is to have portable checks that work across implementations. I've spoken to the libc++ maintainers and they are strongly opposed to lots of different sub-directories.
