BUG #5688: ALTER TABLE ALTER col TYPE newtype fails if col is named in an UPDATE OF col trigger

The following bug has been logged online:

Bug reference:      5688
Logged by:          Daniel Grace
Email address:      dgrace@wingsnw.com
PostgreSQL version: 9.0.0
Operating system:   Windows XP 32-bit
Description:        ALTER TABLE ALTER col TYPE newtype fails if col is named
in an UPDATE OF col trigger
Details:

Given the following state:

CREATE TABLE foo (
    bar TEXT,
        baz TEXT
);

CREATE OR REPLACE FUNCTION foo_trigger_proc() RETURNS TRIGGER LANGUAGE
PLPGSQL AS $$BEGIN RETURN NEW; END$$;
CREATE TRIGGER foo_trigger BEFORE INSERT OR UPDATE OF bar ON foo FOR EACH
ROW EXECUTE PROCEDURE foo_trigger_proc();

The following happens:

> ALTER TABLE foo ALTER bar TYPE VARCHAR;
Fails:
ERROR: unexpected object depending on column: trigger foo_trigger on table
foo
SQL state: XX000

> ALTER TABLE foo ALTER baz TYPE VARCHAR;
Succeeds (because baz is not named in the trigger)

> ALTER TABLE foo DROP bar;
Correctly produces an error message:
ERROR:  cannot drop table foo column bar because other objects depend on it
DETAIL:  trigger foo_trigger on table foo depends on table foo column bar

> ALTER TABLE foo DROP bar CASCADE;
Correctly removes bar and foo_trigger

ALTER TABLE foo RENAME bar and other variations of variations of ALTER
column seem to function correctly.