BUG #19858: pg_dump --clean --if-exists fails on NOT VALID domain constraint

The following bug has been logged on the website:

Bug reference: 19858
Logged by: Atish Prakash
Email address: 125atishprak(at)gmail(dot)com
PostgreSQL version: 18.6
Operating system: Linux x86_64
Description:

The pg_dump documentation says --if-exists suppresses "does not exist"
errors
when restoring with --clean. A separately dumped domain constraint is not
covered,
because the command it generates has no guard on the domain itself.

CREATE DOMAIN d AS int;
ALTER DOMAIN d ADD CONSTRAINT k CHECK (VALUE > 0) NOT VALID;

pg_dump --clean --if-exists mydb > dump.sql

The dump starts with:

ALTER DOMAIN public.d DROP CONSTRAINT IF EXISTS k;
DROP DOMAIN IF EXISTS public.d;

Restoring into an empty database gives:

ERROR: type "public.d" does not exist

With psql -v ON_ERROR_STOP=1 the restore stops at that point, before
creating
anything. Other objects in the same test, using ALTER TABLE IF EXISTS and
DROP ...
IF EXISTS, gave no such error.

Expected: no "does not exist" error with --if-exists.

ALTER DOMAIN has no IF EXISTS form for the domain, so the fix could be to
add one,
or to skip the constraint drop since DROP DOMAIN follows it. I have not
written a
patch.

Found with dump and restore differential tests. I reproduced and checked the
result myself.