12dev and 11.1:
postgres=# CREATE TABLE t(i int)PARTITION BY RANGE(i);
postgres=# CREATE INDEX ON t(i) WITH(fillfactor=11);
postgres=# ALTER INDEX t_i_idx SET (fillfactor=12);
ERROR: 42809: "t_i_idx" is not a table, view, materialized view, or index
LOCATION: ATWrongRelkindError, tablecmds.c:5031
I can't see that's deliberate, but I found an earlier problem report; however,
discussion regarding the ALTER behavior seems to have been eclipsed due to 2nd,
separate issue with pageinspect.
https://www.postgresql.org/message-id/flat/CAKcux6mb6AZjMVyohnta6M%2BfdkUB720Gq8Wb6KPZ24FPDs7qzg%40mail.gmail.com
Justin