Failed to modify column's default value when has expression index

[wd0517]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> create table t1 (a char(20) not null, index((lower(a))));
Query OK, 0 rows affected (0.09 sec)

mysql> alter table t1 modify a char(20) default null;

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

Success.

3. What did you see instead (Required)

ERROR 3106 (HY000): '[ddl:3837]Column 'a' has an expression index dependency and cannot be dropped or renamed' is not supported for generated columns.

This error message does not match the operation and has a poor format.

I can reproduce this error in both nightly and v7.5.1, but in serverless (5.7.28-TiDB-v7.1.1-serverless) it works fine.

4. What is your TiDB version? (Required)

Release Version: v8.1.0-alpha-24-g555ce02352
Edition: Community
Git Commit Hash: 555ce0235229dc370cc8e63574437cf9ee60bdd0
Git Branch: HEAD
UTC Build Time: 2024-04-03 14:18:26
GoVersion: go1.21.6
Race Enabled: false
Check Table Before Drop: false
Store: tikv

From Django tests migrations.test_operations.OperationTests.test_alter_field_with_func_index

[aytrack]

introduced by #43350

[kennedy8312]

/type regression

[zimulala]

In #24321 (v4.0.0), I hope this kind of DDL error raised an issue, Because the generated column also didn't GA, and the generated column's user doc also said not to support (https://docs.pingcap.com/tidb/stable/generated-columns#limitations).

You cannot modify the expression of a stored generated column through the ALTER TABLE statement.

This PR (#43350) will change the behavior, to become an error. But v7.1 (https://docs-archive.pingcap.com/tidb/v7.2/release-7.1.0) to start generating function has the GA.

So what are our options now? Still let it report an error or let it execute successfully?
@bb7133 Help judge how to deal with this behavior.
If we still choose to report errors, we may need to change the error message and change the bug level.

[tiancaiamao]

I think the current behavior confirm to our previous choice #43455 (comment)

[kennedy8312]

@tiancaiamao Could please check if anything need to do with the error message for this case?