-Wold-style-cast for g++, please ?

[Christian Millour]

old-style casts are notoriously hard to find. The best detector is
probably the c++ parser itself. I wonder if a warning could be added
to this end. It would be invaluable for those who have to upgrade
legacy code.

I know there are legitimate uses for old-style casts (e.g. downcasting
from a private base) so it should be standalone (i.e. not make it to
-W, -Wall or -pedantic).

What do you think ?