Hybrid functions create a column of an impossible type DOUBLE(256,4)

CREATE OR REPLACE TABLE t1 (a DOUBLE(255,4),b DOUBLE(255,3));

CREATE OR REPLACE TABLE t2 AS SELECT COALESCE(a,b) FROM t1;

SHOW CREATE TABLE t2;

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

| Table | Create Table                                                                                            |

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

| t2    | CREATE TABLE `t2` (

  `COALESCE(a,b)` double(256,4) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

CREATE OR REPLACE TABLE t1 (a DOUBLE(256,4));

ERROR 1439 (42000): Display width out of range for 'a' (max = 255)