Can't find column using case when function with union.

[wshwsh12]

Bug Report

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1. Minimal reproduce step (Required)

Need open newcollation

use test;
create table t1(a varchar(36) NOT NULL) ENGINE = InnoDB DEFAULT CHARSET = utf8 COLLATE = utf8_general_ci;

set names utf8;  #  utf8mb4 will also get error. But binary is ok.
desc select
    t2.b from
    (
        select t1.a as b
        from t1
        union all
        select t1.a as b
        from t1
    ) t2
where
    case
        when (t2.b is not null) then t2.b
        else ''
    end > '1234567';

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

query successfully.

3. What did you see instead (Required)

ERROR 1105 (HY000): Can't find column Column#5 in schema Column: [test.t1.a] Unique key: []

4. What is your TiDB version? (Required)

v4.0.12, master

[ti-srebot]

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