BUG #2971: 8.1.7/8.2.2 break constraint checking for 'update'

The following bug has been logged online:

Bug reference:      2971
Logged by:          Martin Pitt
Email address:      martin@piware.de
PostgreSQL version: 8.2.2
Operating system:   Linux
Description:        8.1.7/8.2.2 break constraint checking for 'update'
Details:

Originally reported on https://launchpad.net/bugs/83505:

db> create table foo (bar VARCHAR(20) NOT NULL check (bar in
('FOO','BAR')));
CREATE TABLE

db> insert into foo (bar) values ('FOO');
INSERT 0 1

db> update foo set bar = 'BAR';
ERROR: attribute 1 has wrong type
DETAIL: Table has type character varying, but query expects character
varying.

This worked in the previous versions (8.2.1/8.1.6). I checked that it does
not affect 7.4.16.

Thank you!