Expected: if the index's FOR filter remove nulls values,
a candidate index have to not fire a error

Observed: when a NULL value is inserted/updated
VFP can fire a error

SET NULL ON

CREATE CURSOR testDefault (f1 i NULL DEFAULT NULL)

* PUT A NULL
INSERT INTO testDefault VALUES (NULL)
&& NO PROBLEM WITH THE EXISTS NULLS
INDEX ON F1 FOR NOT ISNULL(F1) TAG T1 CANDIDATE

REINDEX

&& no problem with these updates

REPLACE f1 WITH f1
REPLACE f1 WITH 1
REPLACE f1 WITH NULL

BLANK DEFAULT

&& BUG ON THE CHECK ORDER, AND THIS FIRE A ERROR
UPDATE testDefault SET F1 = F1

&& BUG ON THE CHECK ORDER, AND THIS FIRE A ERROR
INSERT INTO testDefault VALUES (NULL)

&& WITH APPEND too
APPEND BLANK

It prevents to the use of keys candidates for not null values,
one great function, absent also in MS SQL 2000.

I hope it is fixed in VFP9 SP1.