The result of expression nullif is not correct

[lilinghai]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int primary key);
insert into t values(1),(2);
select * from t where nullif(a,a) is null;

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

+---+
| a |
+---+
| 1 |
| 2 |
+---+

3. What did you see instead (Required)

Empty set

4. What is your TiDB version? (Required)

Release Version: v4.0.0-beta.2-2287-g280d07c4a-dirty
Edition: Community
Git Commit Hash: 280d07c
Git Branch: master
UTC Build Time: 2021-03-05 11:03:56
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[lilinghai]

The result of statement select * from t where if(a,null,a) is null; if also not correct.

[wjhuang2016]

Relate to #23102, @guo-shaoge Could you take a look?

[guo-shaoge]

Relate to #23102, @guo-shaoge Could you take a look?

Looks like same reason as #23102. Trying to fix it.

[ti-srebot]

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5. Affected versions

[4.0.0:v4.0.11],master

6. Fixed versions

master