`index_subquery` optimization leads to inconsistent results

CREATE TABLE t18 (

    c1 BIT,

    c2 BIT NOT NULL,

    UNIQUE (c2, c1)

);

INSERT INTO t18 (c1, c2) VALUES

    (b'0', b'1'),

    (b'1', b'1'),

    (b'0', b'0');

mysql> SELECT INET_NTOA('2016-12-24 09:00:00') IN (SELECT c2 FROM t18) AS result;

+--------+

| result |

+--------+

|      0 |

+--------+

-- Test 2: Without index (CORRECT - returns TRUE)

mysql> SELECT INET_NTOA('2016-12-24 09:00:00') IN (SELECT c2 FROM t18 USE INDEX ()) AS result;

+--------+

| result |

+--------+

|      1 |

+--------+

-- With index (incorrect result)

mysql> EXPLAIN EXTENDED SELECT INET_NTOA('2016-12-24 09:00:00') IN (SELECT c2 FROM t18);

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

| id | select_type | table | type          | key  | key_len | ref  | rows | Extra                    |

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

|  1 | PRIMARY     | NULL  | NULL          | NULL | NULL    | NULL | NULL | No tables used           |

|  2 | SUBQUERY    | t18   | index_subquery| c2   | 1       | const |    2 | Using index; Using where |

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

-- Without index (correct result)

mysql> EXPLAIN EXTENDED SELECT INET_NTOA('2016-12-24 09:00:00') IN (SELECT c2 FROM t18 USE INDEX ());

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

| id | select_type | table | type | key | key_len | ref  | rows | Extra       |

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

|  1 | PRIMARY     | NULL  | NULL | NULL | NULL    | NULL | NULL | No tables   |

|  2 | SUBQUERY    | t18   | ALL  | NULL | NULL    | NULL |    3 | Using where |

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