[Bug c/59850] Support sparse-style pointer address spaces (type attributes)

["tromey at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59850

--- Comment #9 from Tom Tromey <tromey at gcc dot gnu.org> ---
(In reply to Josh Triplett from comment #7)

> I can't think of a legitimate reason to have a null pointer constant in a
> non-zero address space; there's already a null pointer constant, NULL,
> effectively in all address spaces, so why would you want to redefine it?

> That isn't a null pointer constant, since it isn't (void *); it can't be
> converted to any other pointer type without warning, and I don't think it's
> unreasonable to say it can't be converted to any other address space without
> warning either.

Thanks.  While the one case did seem borderline to me, overall my
concern is really about trying to understand all the cases, so I can
document the feature nicely.