Planner doesn't rewrite conflict expressions to dual in utf8mb4_general_ci collation

[Defined2014]

Enhancement

mysql> create table t(a char(1));
Query OK, 0 rows affected (0.01 sec)
mysql> explain select * from t where a = 'C' and a = 'D';
+-------------------------+----------+-----------+---------------+--------------------------------------+
| id                      | estRows  | task      | access object | operator info                        |
+-------------------------+----------+-----------+---------------+--------------------------------------+
| TableReader_7           | 0.00     | root      |               | data:Selection_6                     |
| └─Selection_6           | 0.00     | cop[tikv] |               | eq(test.t.a, "C"), eq(test.t.a, "D") |
|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo       |
+-------------------------+----------+-----------+---------------+--------------------------------------+
3 rows in set (0.00 sec)

mysql> show variables like '%collation%';
+---------------------------------------+--------------------+
| Variable_name                         | Value              |
+---------------------------------------+--------------------+
| collation_connection                  | utf8mb4_general_ci |
| collation_database                    | utf8mb4_bin        |
| collation_server                      | utf8mb4_bin        |
| default_collation_for_utf8mb4         | utf8mb4_bin        |
| tidb_hash_exchange_with_new_collation | ON                 |
+---------------------------------------+--------------------+
5 rows in set (0.00 sec)

mysql> set collation_connection='utf8mb4_bin';
Query OK, 0 rows affected (0.00 sec)

mysql> explain select * from t where a = 'C' and a = 'D';
+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| TableDual_5 | 0.00    | root |               | rows:0        |
+-------------+---------+------+---------------+---------------+
1 row in set (0.01 sec)

I think when collation_connetion equals to utf8mb4_general_ci, the plan should also be TableDual