Domain checks not always working when used in compound type

Hello All,

Today I was experimenting with domain and compound types.

-- Creating a domain type with NOT NULL constraint and at least 2
characters.
CREATE DOMAIN domain_a AS VARCHAR(10) NOT NULL
  CHECK (LENGTH(TRIM(value)) > 2);

-- Using the domain within a compound type. The NOT NULL constraint
should be
-- inherited from the domain, therefore not repeated here for column a.
CREATE TYPE compound_ab AS (
  a    domain_a,
  b    varchar(10)
);

-- Creating a table using the compound type.
CREATE TABLE test1 (
  name  VARCHAR(25) NOT NULL UNIQUE,
  ab    compound_ab,
  id    INTEGER GENERATED ALWAYS AS IDENTITY
);

-- Insert into table without providing a value for ab, thus (ab).a is
NULL, which shouldn't be allowed.
INSERT INTO test1 VALUES ('person1');
INSERT 0 1

-- Select proves that (ab).a is NULL - violating the NOT NULL constraint.
SELECT *, (ab).a FROM test1;
  name   | a
---------+---
 person1 | ¤

¤ is set as a marker for NULL values in order to make it distinguishable
from an empty string.

-- Directly inserting a NULL gives an error, so the constraint is in place.
INSERT INTO test1 (name, ab.a, ab.b)
  VALUES ('person2', NULL, 'something');
ERROR 23502: domain domain_a doesn't allow NULL values

INSERT INTO test1
  VALUES ('person3', '(,something)');
ERROR 23502: domain domain_a doesn't allow NULL values

-- But it can be circumvented by only feeding ab.b, but not ab.a.
INSERT INTO test1 (name, ab.b)
  VALUES ('person3', 'something');

SELECT *, (ab).a FROM test1;
  name   |      ab      | id | a
---------+--------------+----+---
 person1 | ¤            |  1 | ¤
 person3 | (,something) |  2 | ¤

The tests have been carried out with PostgreSQL 16.1.

Is the behaviour as expected? I think it isn't.

Regards,

Holger

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