https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51146
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> --- Libc++ uses complex<_Tp>(log(abs(__x)), arg(__x)); Even for C++11, which is based on C99, ::clog is not a reserved name in C++ because C++ specifies that <complex.h> just includes the C++ <complex> header, so defines std::complex and none of the C99 complex routines exist in C++. This is not a universally popular feature of C++. But it does mean that a strictly conforming C++ program can use ::clog for whatever purpose it wants. However, in practice, our implementation doesn't exist in a vacuum and depends on an underlying system library (which is usually based on POSIX) and so those symbols are essentially reserved too, even if the C++ standard doesn't say so. E.g. if you define a symbol called ::fdopen or ::open you're asking for trouble, even though those are not in ISO C++. If you want a pure ISO C++ environment that doesn't rely on additional features of POSIX and C99, maybe you need to look elsewhere.
