Standing Document 6 recommends a series of macros for checking the status of various standard features in the implementation. While clang seems to have implemented this for pure language features, libc++ does not yet have support for this. For example, although libc++ does support compile-time integer sequences as specified in n3658, the <utility> header does not define __cpp_lib_integer_sequence as recommended by SD6.
@Marshall Ping. I'll make the changes, but I want your opinion on this because I have these feature test macros in my LFTS work.
We do all of these now, thanks to Eric's work.