Missing FK constraint after changing column type under disabled FK checks

--source include/have_innodb.inc

call mtr.add_suppression(

    "has or is referenced in foreign key constraints");

CREATE TABLE t1 (pk INT AUTO_INCREMENT PRIMARY KEY, a INT, KEY(a))

                 ENGINE=InnoDB;

CREATE TABLE t2 (pk INT PRIMARY KEY, b INT,

                 FOREIGN KEY(b) REFERENCES t1(a)) ENGINE=InnoDB;

SET FOREIGN_KEY_CHECKS= OFF;

ALTER TABLE t2 MODIFY COLUMN b YEAR;

SHOW CREATE TABLE t2;

--connect (con1,localhost,root,,test)

INSERT INTO t2 values (1, 1);

ALTER TABLE t2 ADD INDEX ind (b);

SHOW CREATE TABLE t2;

--connection con1

INSERT INTO t2 values (2, 2);

#Cleanup

DROP TABLE t2, t1;

SET FOREIGN_KEY_CHECKS= OFF;

ALTER TABLE t2 MODIFY COLUMN b YEAR;

SHOW CREATE TABLE t2;

Table	Create Table

t2	CREATE TABLE `t2` (

  `pk` int(11) NOT NULL,

  `b` year(4) DEFAULT NULL,

  PRIMARY KEY (`pk`),

  KEY `b` (`b`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci

connect  con1,localhost,root,,test;

INSERT INTO t2 values (1, 1);

ALTER TABLE t2 ADD INDEX ind (b);

SHOW CREATE TABLE t2;

Table	Create Table

t2	CREATE TABLE `t2` (

  `pk` int(11) NOT NULL,

  `b` year(4) DEFAULT NULL,

  PRIMARY KEY (`pk`),

  KEY `ind` (`b`),

  CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`b`) REFERENCES `t1` (`a`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci

connection con1;

INSERT INTO t2 values (2, 2);

mysqltest: At line 21: query 'INSERT INTO t2 values (2, 2)' failed: ER_NO_REFERENCED_ROW_2 (1452): Cannot add or update a child row: a foreign key constraint fails (`test`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`b`) REFERENCES `t1` (`a`))