[RoboSeldon]

Works with Clang and GCC.

Obviously, Visual Studio doesn't work (they have almost complete support for C99 though, so maybe in a few years they will include C11 support).

[detaro]

Unlikely, unless they need it for C++. Even C99 only gets better when they need parts of it for C++ anyways, and is not developed further.

[cremno]

Only library-wise: the reason for _Generic <tgmath.h> is missing, <complex.h> exists but can't be used. Some lesser used and sometimes useful C99 language features/changes are still unimplemented. Even excluding features C11 made optional (for good reasons) like VLA or complex.