Robert Haas <robertmhaas@gmail.com> writes:
> I am not too sure that the distinction between implicit casts and
> assignment casts is all that useful;
We've been there and done that; it doesn't work. The current scheme
was invented specifically because a two-way design didn't work.
http://archives.postgresql.org/pgsql-hackers/2002-09/msg00900.php
regards, tom lane