Several feature-test macros are available in C++ versions of C headers (e.g. __cpp_lib_byte is available in <cstddef>). However, the current standard wording doesn't seem to make the distinction between C++ library feature-test macros and macros from C, so it's not very clear that whether <stddef.h> and its friends are required to provide __cpp_lib_* macros in C++.

Presumably, name.h should provide macros shown in the synopsis of its corresponding cname (as required in C), but should not be required to provide C++ library feature-test macros because such requirement would generally require implementations of the C standard library to change. I think we should make clarification in [support.c.headers.general].