Different beheviors when performing add column in adding expression default values feature

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table tt(a int);
alter table tt add column b datetime default (date_format(now(),'%Y-%m'));


create table t(a int);
insert into t value(1);
alter table t add column b datetime default (date_format(now(),'%Y-%m'));

2. What did you expect to see? (Required)

mysql> create table tt(a int);
Query OK, 0 rows affected (0.01 sec)
mysql> alter table tt add column b datetime default (date_format(now(),'%Y-%m'));
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> create table t(a int);
Query OK, 0 rows affected (0.02 sec)
mysql> insert into t value(1);
Query OK, 1 row affected (0.00 sec)
mysql> alter table t add column b datetime default (date_format(now(),'%Y-%m'));
ERROR 1292 (22007): Incorrect datetime value: '2024-04' for column 'b' at row 1

3. What did you see instead (Required)

tidb> create table tt(a int);
Query OK, 0 rows affected (0.01 sec)
tidb> alter table tt add column b datetime default (date_format(now(),'%Y-%m'));
ERROR 1292 (22007): Incorrect datetime value: '2024-04'

tidb> create table t(a int);
Query OK, 0 rows affected (0.01 sec)
tidb> insert into t value(1);
Query OK, 1 row affected (0.00 sec)
tidb> alter table t add column b datetime default (date_format(now(),'%Y-%m'));
ERROR 1292 (22007): Incorrect datetime value: '2024-04'

4. What is your TiDB version? (Required)

master

[zimulala]

At the moment I don't think this is a bug, but the behavior is inconsistent with MySQL:
This is just for the record. If you have other ideas, you can also make comments.