Sorry: a my error, the active order filter the records!

Infact it was strange to me, that VFP use two update routines.

Then:

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
****************** VARIATION **********************
SET ORDER TO 

REINDEX

* now any update fail

REPLACE f1 WITH 1  && THIS FAIL BECAUSE vfp Compare and check the old and new value
REPLACE f1 WITH f1
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