Instant ALTER allows to violate column check constraint

--source include/have_innodb.inc

create table t (a int) engine=InnoDB;

insert into t values (0);

alter table t add b int default 0 check(b!=a);

show create table t;

select * from t;

# Cleanup

drop table t;

alter table t add b int default 0 check(b!=a);

show create table t;

Table	Create Table

t	CREATE TABLE `t` (

  `a` int(11) DEFAULT NULL,

  `b` int(11) DEFAULT 0 CHECK (`b` <> `a`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci

select * from t;

a	b

0	0

create table t (a int) engine=InnoDB;

insert into t values (0);

alter table t add b int check(a!=a);