Cast year as datetime should return null

[ichn-hu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (a year);
insert into t values (2021);
select cast(a as datetime) from t;

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

MySQL [test]> select cast(a as datetime) from t;
+---------------------+
| cast(a as datetime) |
+---------------------+
| NULL                |
+---------------------+
1 row in set, 1 warning (0.000 sec)
MySQL [test]> show warnings;
+---------+------+----------------------------------+
| Level   | Code | Message                          |
+---------+------+----------------------------------+
| Warning | 1292 | Incorrect datetime value: '2021' |
+---------+------+----------------------------------+
1 row in set (0.000 sec)

3. What did you see instead (Required)

MySQL [test]> select cast(a as datetime) from t;
+---------------------+
| cast(a as datetime) |
+---------------------+
| 2021-00-00 00:00:00 |
+---------------------+
1 row in set (0.001 sec)

4. What is your TiDB version? (Required)

master

this bug is caused by #20233 , TiDB acts correctly before this PR, namely in e9b11b7

[Howie59]

/assign

[ichn-hu]

@Howie59 Thanks for your interest, but I've already proposed a PR, feel free to look at other issues on #20804

[ichn-hu]

/unassign @Howie59

[ichn-hu]

/assign @ichn-hu