DROP USER IF EXISTS behavior difference

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

This is a similar problem to #7867 :

DROP USER IF EXISTS idontexist;

2. What did you expect to see?

MySQL:

mysql80> DROP USER IF EXISTS idontexist;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql80> SHOW WARNINGS;
+-------+------+---------------------------------------------------+
| Level | Code | Message                                           |
+-------+------+---------------------------------------------------+
| Note  | 3162 | Authorization ID 'idontexist'@'%' does not exist. |
+-------+------+---------------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> DROP USER IF EXISTS idontexist;
Query OK, 0 rows affected (0.01 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v3.0.0-beta.1-109-g7666f688b
Git Commit Hash: 7666f688b3d91ea84fa6b0a20c2e784fff3c8945
Git Branch: master
UTC Build Time: 2019-04-17 05:09:28
GoVersion: go version go1.12.1 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.00 sec)

[july2993]

I test with 5.7.21, Message is User.... but not Authorization ID..

Server version: 5.7.21-log Homebrew

mysql> DROP USER IF EXISTS idontexist;
Query OK, 0 rows affected, 1 warning (0.03 sec)

mysql> show warnings;
+-------+------+---------------------------------------+
| Level | Code | Message                               |
+-------+------+---------------------------------------+
| Note  | 3162 | User 'idontexist'@'%' does not exist. |
+-------+------+---------------------------------------+
1 row in set (0.00 sec)

mysql>