I believe Matt's suggestion is the right long term approach, assuming
we have an implementation of #nospam. For the mid-term, I think that
using bits/cpp_type_traits.h is the most viable option -- with proper
uglification.
I wonder if this does have to be "long term"? If we think that we
have a good specification of preprocessor scoping, I bet it won't
be that hard to implement.
I don't attend EWG sessions, so I don't have a good sense of how it
was received. Did it look to you as if the feature is ready to
implement as Bjarne proposed it?