c++/8007: Internal compiler error in walk_tree

[Giovanni Bajo]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8007

The ICE is now gone on both 3.3 and 3.4, and the code is rejected. The only
reason why I keep this PR open is that I'm not sure if the code is legal or
not. With GCC 2.95, A<> gets instantiated as A<int (B::)()>, which I am not
sure it's a legal type. Can some GCC extension guru comment on this?

Giovanni Bajo