The collation of JSON is inconsistent with MySQL when process with builtin func

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a char(20), b blob(100), c binary(100), d json, e timestamp, f set('a一','b二','c三','d四'), g text, h enum('a一','b二','c三','d四') default 'c三');
insert into t values ('abcABC一二三abcABC', 'abcABC一二三abcABC', 0xe2e2, '{"abcABC一二三abcABC": "abcABC一二三abcABC"}', '2018-10-13', 1, 'abcABC一二三abcABC', 'a一');
select lpad(d, 5, 0xE4B880) from t;
select rpad(d, 5, 0xE4B880) from t;

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

mysql> select lpad(d, 5, 0xE4B880) from t;
+----------------------+
| lpad(d, 5, 0xE4B880) |
+----------------------+
| {"abc                |
+----------------------+
1 row in set (0.00 sec)

mysql> select rpad(d, 5, 0xE4B880) from t;
+----------------------+
| rpad(d, 5, 0xE4B880) |
+----------------------+
| {"abc                |
+----------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> select lpad(d, 5, 0xE4B880) from t;
+--------------------------------------------+
| lpad(d, 5, 0xE4B880)                       |
+--------------------------------------------+
| 0x7B22616263                               |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> select rpad(d, 5, 0xE4B880) from t;
+--------------------------------------------+
| rpad(d, 5, 0xE4B880)                       |
+--------------------------------------------+
| 0x7B22616263                               |
+--------------------------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

[xiongjiwei]

lpad(f, 5, 0xE4B880), lpad(h, 5, 0xE4B880) is bug of mysql
see https://bugs.mysql.com/bug.php?id=105668

[Defined2014]

The root cause is retType of builtin func with json is not same as MySQL

SQLs

create table t(a json);
insert into t values ('{"a":"b"}');
select collation(upper(a)) from t;
select collation(a) from t;

mysql

mysql> select collation(upper(c)) from t;
+---------------------+
| collation(upper(a)) |
+---------------------+
| utf8mb4_bin         |
+---------------------+
1 row in set (0.01 sec)

mysql> select collation(c) from t;
+--------------+
| collation(a) |
+--------------+
| binary       |
+--------------+
1 row in set (0.00 sec)

TiDB

mysql> select collation(upper(a)) from t;
+---------------------+
| collation(upper(a)) |
+---------------------+
| binary              |
+---------------------+
1 row in set (0.00 sec)

mysql> select collation(a) from t;
+--------------+
| collation(a) |
+--------------+
| binary       |
+--------------+
1 row in set (0.00 sec)

Ref:
https://github.com/mysql/mysql-server/blob/3290a66c89eb1625a7058e0ef732432b6952b435/sql/item.h#L1534