Cursors on prepared statements: OPEN ... USING does not normalize parameter collation

SET NAMES utf8mb4;

SET @v1 = _latin1 0xE9;

SET @v2 = _koi8r 0xE9;

PREPARE stmt2 FROM 'SELECT (? = ?) AS cmp_result';

EXECUTE stmt2 USING @v1, @v2;

-- succeeds when executing PS directly, both values converted to connection charset first

DELIMITER $$;

CREATE PROCEDURE p3()

BEGIN

  DECLARE v_cmp INT;

  DECLARE c CURSOR FOR stmt2;

  OPEN c USING @v1, @v2;

  FETCH c INTO v_cmp;

  CLOSE c;

  SELECT v_cmp;

END;

$$

DELIMITER ;$$

CALL p3;

-- ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,COERCIBLE) and (koi8r_general_ci,COERCIBLE) for operation '='

-- Expected: should succeed identically to the EXECUTE ... USING 

12.3.3-opt>SET NAMES utf8mb4;

Query OK, 0 rows affected (0.000 sec)

12.3.3-opt>SET @v1 = _latin1 0xE9;

Query OK, 0 rows affected (0.000 sec)

12.3.3-opt>SET @v2 = _koi8r 0xE9;

Query OK, 0 rows affected (0.000 sec)

12.3.3-opt>PREPARE stmt2 FROM 'SELECT (? = ?) AS cmp_result';

Query OK, 0 rows affected (0.000 sec)

Statement prepared

12.3.3-opt>EXECUTE stmt2 USING @v1, @v2;              -- succeeds: cmp_result = 0

+------------+

| cmp_result |

+------------+

|          0 |

+------------+

1 row in set (0.000 sec)

12.3.3-opt>

12.3.3-opt>DELIMITER $$

12.3.3-opt>CREATE PROCEDURE p3()

    -> BEGIN

    ->   DECLARE v_cmp INT;

    ->   DECLARE c CURSOR FOR stmt2;

    ->   OPEN c USING @v1, @v2;

    ->   FETCH c INTO v_cmp;

    ->   CLOSE c;

    ->   SELECT v_cmp;

    -> END;

    -> $$

Query OK, 0 rows affected (0.002 sec)

12.3.3-opt>DELIMITER ;

12.3.3-opt>CALL p3;

ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,COERCIBLE) and (koi8r_general_ci,COERCIBLE) for operation '='

12.3.3-opt>