BUG #19446: Domain DEFAULT not reflected in system catalogs and information_schema (PG 18.3)

The following bug has been logged on the website:

Bug reference:      19446
Logged by:          Xianghang Zheng
Email address:      zheng_xianghang@163.com
PostgreSQL version: 18.3
Operating system:   Linux x86_64
Description:

1. PostgreSQL Version
PostgreSQL 18.3 (x86_64)
2. Operating System
Linux x86_64
3. Problem Description
When a column uses a domain with DEFAULT value, all system catalogs fail to
reflect the default:
- pg_attribute.atthasdef = false
- No entries in pg_attrdef
- information_schema.columns.column_default is empty
But the default value works correctly at runtime.
This is a metadata bug.
4. Steps to Reproduce
CREATE DOMAIN info_domain AS int DEFAULT 100;
CREATE TABLE t (col info_domain);
SELECT attname, atthasdef
FROM pg_attribute
WHERE attrelid = 't'::regclass AND attnum > 0;
SELECT * FROM pg_attrdef WHERE adrelid = 't'::regclass;
SELECT column_name, column_default
FROM information_schema.columns
WHERE table_name = 't' AND column_name = 'col';
INSERT INTO t DEFAULT VALUES;
SELECT * FROM t;
5. Actual Result
- atthasdef = f
- pg_attrdef returns empty
- column_default is empty
- But the default value 100 works
6. Expected Result
System catalogs should show the domain's DEFAULT correctly.
7. Test Output
postgres=# CREATE DOMAIN info_domain AS int DEFAULT 100;
CREATE DOMAIN
postgres=# CREATE TABLE t (col info_domain);
CREATE TABLE
postgres=# SELECT attname, atthasdef FROM pg_attribute WHERE attrelid =
't'::regclass AND attnum > 0;
 attname | atthasdef
---------+-----------
 col     | f
(1 row)
postgres=# SELECT * FROM pg_attrdef WHERE adrelid = 't'::regclass;
 oid | adrelid | adnum | adbin
-----+---------+-------+-------
(0 rows)
postgres=# SELECT column_name, column_default FROM
information_schema.columns WHERE table_name = 't' AND column_name = 'col';
 column_name | column_default
-------------+----------------
 col         |
(1 row)
postgres=# INSERT INTO t DEFAULT VALUES;
INSERT 0 1
postgres=# SELECT * FROM t;
 col
-----
 100
(1 row)