alias do not exist and a query do not report an error

MariaDB [test]> select user from mysql.user m where user in (select user from mysql.user m2 group by s.id);

+---------------+

| user          |

+---------------+

| data_engineer |

| ro1           |

as you see there is no alias for 's' table.

MariaDB [test]> explain extended select user from mysql.user m where user in (se

lect user from mysql.user m2 group by s.id);

+------+--------------+-------------+-------+---------------+---------+---------

+------+------+----------+------------------------------------------------------

--------+

| id   | select_type  | table       | type  | possible_keys | key     | key_len

| ref  | rows | filtered | Extra

        |

+------+--------------+-------------+-------+---------------+---------+---------

+------+------+----------+------------------------------------------------------

--------+

|    1 | PRIMARY      | <subquery2> | ALL   | distinct_key  | NULL    | NULL

| NULL |   20 |   100.00 |

        |

|    1 | PRIMARY      | m           | index | NULL          | PRIMARY | 420

| NULL |   20 |   100.00 | Using where; Using index; Using join buffer (flat, BN

L join) |

|    2 | MATERIALIZED | m2          | index | NULL          | PRIMARY | 420

| NULL |   20 |   100.00 | Using index

        |

+------+--------------+-------------+-------+---------------+---------+---------

+------+------+----------+------------------------------------------------------

--------+

3 rows in set, 1 warning (0.023 sec)

MariaDB [test]> show warnings\G

*************************** 1. row ***************************

  Level: Note

   Code: 1003

Message: select `mysql`.`m`.`User` AS `user` from `mysql`.`user` `m` semi join (

`mysql`.`user` `m2`) where `mysql`.`m`.`User` = `mysql`.`m2`.`User`

1 row in set (0.001 sec)