CHANGE COLUMN IF EXISTS does not work and throws wrong warning

MariaDB [test]> create table t1 (a int);

Query OK, 0 rows affected (0.20 sec)

MariaDB [test]> alter table t1 change column a b bigint;

Query OK, 0 rows affected (0.45 sec)               

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> drop table t1;

Query OK, 0 rows affected (0.10 sec)

MariaDB [test]> create table t1 (a int);

Query OK, 0 rows affected (0.24 sec)

MariaDB [test]> alter table t1 change column if exists a b bigint;

Query OK, 0 rows affected, 1 warning (0.00 sec)

Records: 0  Duplicates: 0  Warnings: 1

MariaDB [test]> show warnings;

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

| Level | Code | Message                    |

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

| Note  | 1054 | Unknown column 'a' in 't1' |

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

1 row in set (0.01 sec)

MariaDB [test]> show create table t1;

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

| Table | Create Table                                                                          |

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

| t1    | CREATE TABLE `t1` (

  `a` int(11) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

MariaDB [test]> alter table t1 change column a b bigint;

Query OK, 0 rows affected (0.47 sec)               

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> show create table t1;

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

| Table | Create Table                                                                             |

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

| t1    | CREATE TABLE `t1` (

  `b` bigint(20) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.01 sec)

create table t1 (a int);

alter table t1 change column if exists a b bigint;

show warnings;

show create table t1;

revision-id: bar@mariadb.org-20130424142022-u4xhikvoqggze9b0

revno: 3745

branch-nick: 10.0