P0-[4.0-bug-hunting]-[SQL Hint]-Using an index twice in an index hint results in an incorrect result

[mrigger]

Consider the following statements:

CREATE TABLE t0(c0 INT, c1 INT);
CREATE INDEX i0 ON t0(c0, c1);
INSERT INTO t0 VALUES (0, 0);
SELECT /*+ USE_INDEX(t0, i0, i0)*/t0.c0 FROM t0 WHERE t0.c1; -- expected: {}, actual: {0}

Unexpectedly, when using the index hint, the row is fetched:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.25-TiDB- TiDB Server (Apache License 2.0), MySQL 5.7 compatible

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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> CREATE TABLE t0(c0 INT, c1 INT);
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE INDEX i0 ON t0(c0, c1);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t0 VALUES (0, 0);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT /*+ USE_INDEX(t0, i0, i0)*/t0.c0 FROM t0 WHERE t0.c1;
+------+
| c0   |
+------+
|    0 |
+------+
1 row in set (0.00 sec)

When omitting the hint, or specifying i0 only once, the row is not fetched, as expected. I found this bug based on the latest master commit b6fcc157442894f048c1cc65cfcc7776e9ed1a72, and checked that it is also reproducible for the 4.0 RC.

[shuke987]

/bug P2