BUG #17872: Dropping an attribute of a composite type breaks indexes over the type silently

The following bug has been logged on the website:

Bug reference: 17872
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 15.2
Operating system: Ubuntu 22.04
Description:

The following script:
CREATE TYPE ctype AS (i int, j int);
CREATE TABLE ctbl(a int, cf ctype);
CREATE UNIQUE INDEX ctbl_idx ON ctbl(cf);
INSERT INTO ctbl VALUES (1, '(1, 2)'::ctype), (2, '(1, 1)'::ctype);
ALTER TYPE ctype DROP ATTRIBUTE j;

Results in the UNIQUE constraint broken:
SELECT ctid, * FROM ctbl;
ctid | a | cf
-------+---+-----
(0,1) | 1 | (1)
(0,2) | 2 | (1)

\d+ ctbl
Table "public.ctbl"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+----------+-------------+--------------+-------------
a | integer | | | | plain |
| |
cf | ctype | | | | extended |
| |
Indexes:
"ctbl_idx" UNIQUE, btree (cf)
Access method: heap

And the index corruption detected by amcheck:
CREATE EXTENSION amcheck;
SELECT bt_index_parent_check(oid, true, true) FROM pg_class
WHERE relname = 'ctbl_idx';
ERROR: could not find tuple using search from root page in index
"ctbl_idx"
DETAIL: Index tid=(1,1) points to heap tid=(0,2) page lsn=0/187BF80.

(Interestingly enough, amcheck doesn't detect a corruption if
VALUES (1, '(1, 1)'::ctype), (2, '(1, 2)'::ctype) are inserted.)

The same is observed with a composite type representing a table row,
for example:
CREATE TABLE ijtbl(i int, j int);
CREATE TABLE ttbl(a int, cf ijtbl);
CREATE UNIQUE INDEX ttbl_idx ON ttbl(cf);
INSERT INTO ttbl VALUES (1, '(1, 2)'::ijtbl), (2, '(1, 1)'::ijtbl);
ALTER TABLE ijtbl DROP COLUMN j;

But if an index created over a set of ordinary columns, it is removed
automatically when a column in the set is dropped.
CREATE TABLE tbl(a int, i int, j int);
CREATE UNIQUE INDEX tbl_idx ON tbl(i, j);
INSERT INTO tbl VALUES (1, 1, 2), (2, 1, 1);
\d+ tbl
Table "public.tbl"
Column | Type | Collation | Nullable | Default | Storage | Compression |
Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
a | integer | | | | plain | |
|
i | integer | | | | plain | |
|
j | integer | | | | plain | |
|
Indexes:
"tbl_idx" UNIQUE, btree (i, j)
Access method: heap

ALTER TABLE tbl DROP COLUMN j;
\d+ tbl
Table "public.tbl"
Column | Type | Collation | Nullable | Default | Storage | Compression |
Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
a | integer | | | | plain | |
|
i | integer | | | | plain | |
|
Access method: heap

As I can see, in this case pg_depend contains dependencies of the index
on specific table columns, but for composite types there exists only a
dependency on a composite type as a whole.