BUG #18970: Atempt to alter type of table column used in row type with check leads to assertion failure

The following bug has been logged on the website:

Bug reference: 18970
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 18beta1
Operating system: Ubuntu 24.04
Description:

The following script:
CREATE TABLE t1(a int);
CREATE TABLE t2(b t1 CHECK ((b).a IS NOT NULL));
ALTER TABLE t1 ALTER COLUMN a TYPE numeric;
triggers
2025-06-28 06:52:21.201 UTC [2233016] LOG: statement: ALTER TABLE t1 ALTER
COLUMN a TYPE numeric;
TRAP: failed Assert("lockmode != NoLock || IsBootstrapProcessingMode() ||
CheckRelationLockedByMe(r, AccessShareLock, true)"), File: "relation.c",
Line: 67, PID: 2233016
ExceptionalCondition at assert.c:52:13
relation_open at relation.c:72:6
transformAlterTableStmt at parse_utilcmd.c:3543:8
ATPostAlterTypeParse at tablecmds.c:15600:20
ATPostAlterTypeCleanup at tablecmds.c:15478:3
ATRewriteCatalogs at tablecmds.c:5336:11
ATController at tablecmds.c:4882:2
AlterTable at tablecmds.c:4535:1
...
with an assert-enabled build, and fails with just
ERROR: cannot alter table "t1" because column "t2.b" uses its row type
with no asserts.
Reproduced starting from commit b04aeb0a0, which added the Assert.