grant '' to foo@localhost;
ERROR OP000: Invalid role specification ``.
create user ''@localhost;
create role r1;
grant r1 to ''@localhost;
select current_user;
current_user
@localhost
show grants;
Grants for @localhost
GRANT r1 TO ''@'localhost'
GRANT USAGE ON *.* TO ''@'localhost'
drop role r1;
drop user ''@localhost;