RFC: New C++ Attribute: final

[Robert Dewar]

> How about: experimental extensions must only be enabled if a certain
> flag is given in the command line?  Like that of signatures, just to
> mention an example of an extension that didn't go forward.  Then we
> still serve as a testbed, but we don't have backward-compatibility
> issues to worry about.

The switch -gnatX acts this way for Ada, and is currently being used
to enable tentatively approved extensions for Ada0Y (the next version
of Ada).