BUG #5477: CREATE DOMAIN NOT NULL constraints not always enforced for INSERT with subquery

The following bug has been logged online:

Bug reference: 5477
Logged by: Matt Nourse
Email address: matthew(at)nplus1(dot)com(dot)au
PostgreSQL version: 8.4
Operating system: Linux (Debian and Red Hat)
Description: CREATE DOMAIN NOT NULL constraints not always enforced
for INSERT with subquery
Details:

Set up:

CREATE DOMAIN test_id_domain INT NOT NULL;

CREATE TABLE test_state(id test_id_domain PRIMARY KEY, display_value
varchar(20) NOT NULL);

CREATE TABLE test_city(state_id test_id_domain REFERENCES test_state(id));

This produces an error as expected:

INSERT INTO test_city(state_id) VALUES (NULL);

This successfully inserts a NULL value into the state_id field:

INSERT INTO test_city(state_id) VALUES ((SELECT id FROM test_state WHERE
display_value = 'Nonexistent state'));

When I use simpler subqueries (eg SELECT 1 WHERE 1 = 0), the NOT NULL
constraint is enforced.

If I remove the test_id_domain domain and replace its use with INT NOT NULL,
the constraint is enforced.

Thanks and regards,
Matt