The flen of conv function should be 65

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SELECT CONV(123, 10, 10);

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

The Length should be 260.

Field   1:  `CONV(123, 10, 10)`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8mb4_0900_ai_ci (255)
Length:     260
Max_length: 3
Decimals:   31
Flags:


+-------------------+
| CONV(123, 10, 10) |
+-------------------+
| 123               |
+-------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> SELECT CONV(123, 10, 10);
Field   1:  `CONV(123, 10, 10)`
Catalog:    `def`
Database:   ``
Table:      ``
Org_table:  ``
Type:       VAR_STRING
Collation:  utf8mb4_bin (46)
Length:     256
Max_length: 3
Decimals:   31
Flags:      NOT_NULL


+-------------------+
| CONV(123, 10, 10) |
+-------------------+
| 123               |
+-------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

master