UPDATE .. FOR PORTION leads to duplicate hash keys in partitioned tables

--source include/have_partition.inc

CREATE TABLE t (a INT, f BLOB, s date, e date, PERIOD FOR p(s,e), UNIQUE KEY f (f))

  ENGINE=MyISAM PARTITION BY KEY (a) PARTITIONS 2;

INSERT INTO t VALUES

  (1,'foo','1980-01-01','2000-01-02'),(1,'bar','1980-00-01','2000-01-02');

--error ER_DUP_ENTRY

UPDATE t FOR PORTION OF p FROM '1990-01-01' TO '2000-01-02' SET a = 100;

SELECT * FROM t;

# Cleanup

DROP TABLE t;

mysqltest: At line 9: query 'UPDATE t FOR PORTION OF p FROM '1990-01-01' TO '2000-01-02' SET a = 100' succeeded - should have failed with error ER_DUP_ENTRY (1062)...

...

SELECT * FROM t;

a	f	s	e

1	foo	1980-01-01	1990-01-01

1	bar	1980-00-01	1990-01-01

100	foo	1990-01-01	2000-01-02

100	bar	1990-01-01	2000-01-02