REGEXP_LIKE shouldn't accept arguments with mixed collation

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run the SQL SELECT REGEXP_LIKE(BINARY 'ABC', 'abc', 'i');

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

mysql> SELECT REGEXP_LIKE(BINARY 'ABC', 'abc', 'i');
ERROR 3995 (HY000): Character set 'binary' cannot be used in conjunction with 'utf8mb4_bin' in call to regexp_like.

3. What did you see instead (Required)

mysql> SELECT REGEXP_LIKE(BINARY 'ABC', 'abc', 'i');
+---------------------------------------+
| REGEXP_LIKE(BINARY 'ABC', 'abc', 'i') |
+---------------------------------------+
|                                     1 |
+---------------------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

[YangKeao]

Also for the following SQLs:

SELECT BINARY 'Abc' REGEXP '^abc';

[zanmato1984]

/severity moderate

[zanmato1984]

I think this is charset/collation related, handing over to sig/sql-infra.