Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

select 1 union ((select 1));

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

MySQL > select 1 union ((select 1));

+---+
| 1 |
+---+
| 1 |
+---+

3. What did you see instead (Required)

release-5.0(v5.0.3) & release-5.1(v5.1.1) execute correctly
release-5.2(v5.2.0, v5.2.1） got syntax error.

4. What is your TiDB version? (Required)

v5.2.0, v5.2.1, master(6ebfe8a)