IN(NULL) causes "cannot convert <nil>(type <nil>) to string"

[kolbe]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t ( id bigint(10) primary key, f varchar(191) default null, unique key `idx_f` (`f`));
select * from t where f in (null);

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

This query should return no results, but it should complete without any error.

3. What did you see instead (Required)

ERROR 1105 (HY000): cannot convert <nil>(type <nil>) to string

4. Affected version (Required)

All 4.0 releases are affected.

TiDB v3.0.16 is not affected.

5. Root Cause Analysis

[glorv]

/label component/expression