The proximate cause of this complaint:
http://archives.postgresql.org/pgsql-general/2008-12/msg00283.php
seems to be that the polymorphic-type code doesn't consider a domain
over an enum type to match an ANYENUM function argument.
ISTM this is probably wrong: we need such a domain to act like its base
type for matching purposes. There is an analogous problem with a domain
over an array type failing to match ANYARRAY; conversely, such a domain
is considered to match ANYNONARRAY which it likely should not.
Comments? If this is agreed to be a bug, should we consider
back-patching it? (I'd vote not, I think, because the behavioral
change could conceivably break some apps that work now.)
regards, tom lane