ALTER TYPE ... USING(NULL) / NOT NULL violation

If a column type is being changed with USING (NULL) clause,
NOT NULL constraint seems not to be checked:

fduch(at)~=# CREATE table test (x integer not null);
CREATE TABLE
fduch(at)~=# INSERT INTO test VALUES (1);
INSERT 0 1
fduch(at)~=# ALTER TABLE test ALTER x TYPE boolean USING (NULL);
ALTER TABLE
fduch(at)~=# \d test
Table "public.test"
Column | Type | Modifiers
--------+---------+-----------
x | boolean | not null

fduch(at)~=# SELECT * from test;
x
---

(1 row)

fduch(at)~=# SELECT version();
version
------------------------------------------------------------------------------------------------
PostgreSQL 8.1.4 on i386-portbld-freebsd6.1, compiled by GCC cc (GCC) 3.4.4 [FreeBSD] 20050518
(1 row)

8.0 branch has the same behavour.

--
Fduch M. Pravking