This script:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET cp1251 COLLATE cp1251_ukrainian_ci);

INSERT INTO t1 VALUES (' 1'),('`1');

SELECT * FROM t1 WHERE a IN (1,2,3);

returns one row:

+------+

| a    |

+------+

|  1   |

+------+

Now if I make the condition even stroger:

SELECT * FROM t1 WHERE a IN (1,2,3) AND a=' 1';

It erroneously returns two rows:

+------+

| a    |

+------+

|  1   |

| `1   |

+------+

It should return one row.

The problem happens in equal fields propagation in this code:

  if (!item || !has_compatible_context(item))

    item= this; 

Item_func_in does not set cmp_type of args[0], so has_compatible_context() passes and the field gets erroneously replaced to the string constant which makes Item_func_in::val_int() always evaluate to TRUE.