create user ''@localhost;
create user c;
grant select on mysql.* to c;
create role r1;
grant r1 to c;
connect  c,localhost,c,,,,,;
select user(), current_user();
user()	current_user()
c@localhost	@localhost
select user from mysql.user group by user;
ERROR 42000: SELECT command denied to user ''@'localhost' for table 'user'
set role r1;
ERROR OP000: Invalid role specification `r1`
disconnect c;
connection default;
drop role r1;
drop user c;
drop user ''@localhost;