create or replace table t1 (a inet6, b int);

insert into t1 (a) values ('::');

update t1 set b = a;

MariaDB [test]> update t1 set b = a;

Query OK, 1 row affected (0.047 sec)

Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [test]> select * from t1;

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

| a    | b    |

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

| ::   |    0 |

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

1 row in set (0.000 sec)

MariaDB [test]> create or replace table t1 (a inet6, b timestamp);

Query OK, 0 rows affected (0.355 sec)

MariaDB [test]> insert into t1 (a) values ('::');

Query OK, 1 row affected (0.045 sec)

MariaDB [test]> update t1 set b = a;

Query OK, 1 row affected (0.032 sec)

Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [test]> select * from t1;

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

| a    | b                   |

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

| ::   | 0000-00-00 00:00:00 |

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

1 row in set (0.001 sec)

MariaDB [test]> create or replace table t1 (a inet6);

Query OK, 0 rows affected (0.321 sec)

MariaDB [test]> insert into t1 (a) values ('::');

Query OK, 1 row affected (0.032 sec)

MariaDB [test]> alter table t1 modify a date;

Query OK, 1 row affected (0.926 sec)               

Records: 1  Duplicates: 0  Warnings: 0

MariaDB [test]> select * from t1;

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

| a          |

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

| 2019-10-11 |

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

1 row in set (0.001 sec)