I've drafted a proposal to replace undefined behavior of special cases
of UCNs with conditional support of more general concepts. This is
intended to supersede the recommendation in N3801 that specified
undefined behavior be changed to required diagnosis. The motivations are
essentially that there are many ill-specified behaviors that would be
unaffected by changing the specified cases, and that required diagnosis
requires excessive implementation, where no further implementation
should be required (following the principle of "standardize existing
practice").

It also incorporates other fixes, and should resolve all UCN-related
active DRs except for CWG DR 411 which does not seem substantive. This
link updates the one recently posted to the std-proposals list:

https://www.dropbox.com/s/472qnd9q3da2qyi/Fixing_UCNs.pdf

Another paper covering other instances of undefined behavior and holes
in the preprocessor specification will hopefully follow soon.