different behavior with mysql when cast json string to unsigned

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

> create table tj(a json);
> insert into tj values('"-1"');
> select cast(a as unsigned) from tj

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

In MySQL 8.0:

mysql> select cast(a as unsigned) from tj;
+----------------------+
| cast(a as unsigned)  |
+----------------------+
| 18446744073709551615 |
+----------------------+
1 row in set (0.01 sec)

3. What did you see instead (Required)

In tidb:

mysql> select cast(a as unsigned) from tj;
+---------------------+
| cast(a as unsigned) |
+---------------------+
|                   0 |
+---------------------+
1 row in set, 1 warning (0.01 sec)

mysql> show warnings;
+---------+------+-----------------------------------------------+
| Level   | Code | Message                                       |
+---------+------+-----------------------------------------------+
| Warning | 1690 | BIGINT UNSIGNED value is out of range in '-1' |
+---------+------+-----------------------------------------------+
1 row in set (0.00 sec)

tidb returns 0 and an extra warning.

4. What is your TiDB version? (Required)

master, but I think it affects all versions with json