Cannot drop column after dropping covered index

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t;
create table t (a int ,b int, key idx (a, b));
alter table t drop index idx, drop column b;

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

Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

3. What did you see instead (Required)

ERROR 8200 (HY000): can't drop column b with composite index covered or Primary Key covered now

4. What is your TiDB version? (Required)

405d2c0