Superuser can't revoke role granted by non-superuser

Hi,

Here is a self-contained example with 17.2, however I assume that 16 and master will exhibit similar behaviour.

postgres=# create user a with createrole;
CREATE ROLE
postgres=# create user b with createrole;
CREATE ROLE
postgres=# set role a;
SET
postgres=> create user aa;
CREATE ROLE
postgres=> set role b;
SET
postgres=> create user bb;
CREATE ROLE
postgres=> grant bb to aa;
GRANT ROLE
postgres=> \drg
               List of role grants
 Role name | Member of |   Options    | Grantor  
-----------+-----------+--------------+----------
 a         | aa        | ADMIN        | postgres
 aa        | bb        | INHERIT, SET | b
 b         | bb        | ADMIN        | postgres
(3 rows)

postgres=> reset role;
RESET
postgres=# revoke bb from aa;
WARNING:  role "aa" has not been granted membership in role "bb" by role "postgres"
REVOKE ROLE
postgres=# \drg
               List of role grants
 Role name | Member of |   Options    | Grantor  
-----------+-----------+--------------+----------
 a         | aa        | ADMIN        | postgres
 aa        | bb        | INHERIT, SET | b
 b         | bb        | ADMIN        | postgres
(3 rows)

IMO, superusers should be able to revoke privileges it didn't grant.