foreign key silently becomes broken

MariaDB [test]> create table t1(f1 int ,primary key(f1))engine=innodb;

Query OK, 0 rows affected (0.18 sec)

MariaDB [test]> create table t2(f2 int,f3 int, key t(f2),key t1(f2,f3),foreign key(f2) references t1(f1))engine=innodb;

Query OK, 0 rows affected (0.17 sec)

MariaDB [test]> show create table t2;

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

| Table | Create Table                                                                                                                                                                                                                      |

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

| t2    | CREATE TABLE `t2` (

  `f2` int(11) DEFAULT NULL,

  `f3` int(11) DEFAULT NULL,

  KEY `t` (`f2`),

  KEY `t1` (`f2`,`f3`),

  CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`f2`) REFERENCES `t1` (`f1`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

MariaDB [test]> alter table t2 drop key t1;

Query OK, 0 rows affected (0.08 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t2 drop key t;

ERROR 1553 (HY000): Cannot drop index 't': needed in a foreign key constraint

MariaDB [test]> SET foreign_key_checks = 0;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> alter table t2 drop key t;

Query OK, 0 rows affected (0.14 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> show create table t2;

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

| Table | Create Table                                                                                                                                                                            |

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

| t2    | CREATE TABLE `t2` (

  `f2` int(11) DEFAULT NULL,

  `f3` int(11) DEFAULT NULL,

  CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`f2`) REFERENCES `t1` (`f1`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

MariaDB [test]> insert into t2 values (1,1);

Query OK, 1 row affected (0.06 sec)

MariaDB [test]> SET foreign_key_checks = 1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> insert into t2 values (2,2);

Query OK, 1 row affected (0.02 sec)

MariaDB [test]> select * from t2;

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

| f2   | f3   |

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

|    1 |    1 |

|    2 |    2 |

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

2 rows in set (0.00 sec)

MariaDB [test]> select * from t1;

Empty set (0.00 sec)

MariaDB [test]> alter table t2 add key  t(f2);

Query OK, 0 rows affected (0.19 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> insert into t2 values (3,3);

Query OK, 1 row affected (0.07 sec)

MariaDB [test]> show create table t2;

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

| Table | Create Table                                                                                                                                                                                              |

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

| t2    | CREATE TABLE `t2` (

  `f2` int(11) DEFAULT NULL,

  `f3` int(11) DEFAULT NULL,

  KEY `t` (`f2`),

  CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`f2`) REFERENCES `t1` (`f1`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.00 sec)

MariaDB [test]> select * from t2;

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

| f2   | f3   |

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

|    1 |    1 |

|    2 |    2 |

|    3 |    3 |

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

3 rows in set (0.00 sec)

MariaDB [test]> select * from t1;

Empty set (0.00 sec)

create table t1(f1 int,primary key(f1))engine=innodb;

create table t2(f2 int,f3 int,key t(f2,f3),foreign key(f2) references t1(f1))engine=innodb;

SET foreign_key_checks=0;

--error ER_DROP_INDEX_FK

drop index t on t2;

drop table t2;

drop table t1;