Query failed by: `[HY000][1105] DB::Exception: tidb_cast to Int32 is not supported`

reproduce step:

drop table t;
create table t(a int not null);
alter table t set tiflash replica 1;

explain analyze select count(*) from (
    SELECT * from (
        select a from t group by a
        union all
        select a from t group by a
    ) t1
) t2;

error message:

[HY000][1105] DB::Exception: tidb_cast to Int32 is not supported

version:

0608 tiup playground nightly

mysql> select * from information_schema.cluster_info;
+---------+-----------------+-----------------+----------------------------+------------------------------------------+---------------------------+--------------+-----------+
| TYPE    | INSTANCE        | STATUS_ADDRESS  | VERSION                    | GIT_HASH                                 | START_TIME                | UPTIME       | SERVER_ID |
+---------+-----------------+-----------------+----------------------------+------------------------------------------+---------------------------+--------------+-----------+
| tidb    | 127.0.0.1:4000  | 127.0.0.1:10080 | 5.2.0-alpha                | 75388187f3865db18bc70ca1ef30260d85dba2e2 | 2021-06-15T19:07:47+08:00 | 3m8.735728s  |         0 |
| pd      | 127.0.0.1:2379  | 127.0.0.1:2379  | 5.1.0-alpha                | d26c09a3fa4d56ac73f145b3a858432d73cec15f | 2021-06-15T19:07:09+08:00 | 3m46.735732s |         0 |
| tikv    | 127.0.0.1:20160 | 127.0.0.1:20180 | 5.1.0-alpha                | 1c07acbc6477c5623d78db1023318bdd590361a5 | 2021-06-15T19:07:19+08:00 | 3m36.735733s |         0 |
| tiflash | 127.0.0.1:3930  | 127.0.0.1:20292 | v5.2.0-alpha-10-g1078d0b81 | 1078d0b8199a1cfe54d73623ea8da636505f419b | 2021-06-15T19:08:50+08:00 | 2m5.735734s  |         0 |
+---------+-----------------+-----------------+----------------------------+------------------------------------------+---------------------------+--------------+-----------+
4 rows in set (0.00 sec)

Run the supplied SQL reproduction against the listed TiDB/TiFlash versions and confirm where the tidb_cast to Int32 error occurs. Trace the query's TiFlash execution path to identify the unsupported cast handling; done means the query completes without this error and regression coverage verifies the case.