Mutual inclusion of xkeycheck.h yvals_core.h is intentional to ensure that xkeycheck.h has the proper value for _STL_COMPILER_PREPROCESSOR even if it's included before/without including yvals_core.h. If we don't want it to be possible to include xkeycheck.h without including yvals_core.h, we should fuse the headers.

Honestly I'd prefer to dump xkeycheck.h and request a #pragma to instruct the preprocessor to gripe at the user about keyword definitions. xkeycheck.h is a poor solution to this problem:

or

and I consequently don't think it provides good return on investment. Other STLs get by just fine without a solution for this problem - they haven't exactly been banging on our door demanding since we open-sourced ;)