TiDB 7.5 Cannot convert string with utf8mb4

[shawn0915]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set names utf8mb4;
select cast(compress('b') as char);

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

in mysql 8, we get this:

mysql> set names utf8mb4;
Query OK, 0 rows affected (0.00 sec)

mysql> select cast(compress('b') as char);
+-----------------------------+
| cast(compress('b') as char) |
+-----------------------------+
| NULL                        |
+-----------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+--------------------------------------------+
| Level   | Code | Message                                    |
+---------+------+--------------------------------------------+
| Warning | 1300 | Invalid utf8mb4 character string: '9C4B02' |
+---------+------+--------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

but in tidb, we get error:

MySQL [test]> select version();
+--------------------+
| version()          |
+--------------------+
| 8.0.11-TiDB-v7.5.0 |
+--------------------+
1 row in set (0.001 sec)

MySQL [test]> set names utf8mb4;
Query OK, 0 rows affected (0.000 sec)

MySQL [test]> select cast(compress('b') as char);
ERROR 3854 (HY000): Cannot convert string '

4. What is your TiDB version? (Required)

TiDB-v7.5.0

reference:
https://asktug.com/t/topic/1018862

[kennedy8312]

Seeing the issue on v5.4.0 as well.