Other options are preserved by ALTER (and CLUSTER ON is and most obviously
should be preserved by CLUSTER's rewrite), so I think (SET) CLUSTER should be
preserved by ALTER, too.
As far as I can see, this should be the responsibility of something in the
vicinity of ATPostAlterTypeParse/RememberIndexForRebuilding.
Attach patch sketches a fix.
ts=# SET client_min_messages=debug; DROP TABLE t; CREATE TABLE t(i int); CREATE INDEX ON t(i)WITH(fillfactor=11,
vacuum_cleanup_index_scale_factor=12);CLUSTER t USING t_i_key; ALTER TABLE t ALTER i TYPE bigint; \d t
SET
DEBUG: drop auto-cascades to type t
DEBUG: drop auto-cascades to type t[]
DEBUG: drop auto-cascades to index t_i_idx
DROP TABLE
CREATE TABLE
DEBUG: building index "t_i_idx" on table "t" serially
CREATE INDEX
ERROR: index "t_i_key" for table "t" does not exist
DEBUG: rewriting table "t"
DEBUG: building index "t_i_idx" on table "t" serially
DEBUG: drop auto-cascades to type pg_temp_3091172777
DEBUG: drop auto-cascades to type pg_temp_3091172777[]
ALTER TABLE
Table "public.t"
Column | Type | Collation | Nullable | Default
--------+--------+-----------+----------+---------
i | bigint | | |
Indexes:
"t_i_idx" btree (i) WITH (fillfactor='11', vacuum_cleanup_index_scale_factor='12')