Background information to this posting was in a very recent summary.

Anyone who proposes an extension to C++ bears the onus of convincing
the standardization committee that the extension can be implemented
realistically.  I have never written a compiler.  Those of you out
there who have, what are your reactions to how easily ~const could be
implemented?

My hunch is that it would be quite easy.  At present, when the
compiler needs to decide whether an entity x is const, it must descend
a tree from the largest enclosing object to the member containing x,
until it hits a "const" specifier.  If it never his a const specifier,
x is not const; otherwise it is.

With ~const, the compiler would not be able to stop at the first const
specifier.  Instead, it would have to continue descending the tree,
looking for ~const specifiers, and so on.

Is this wild guess incorrect?
--
  Tom Ngo
  ngo@harvard.harvard.edu
  617/495-1768 lab number, leave message