VFP version: 6SP5,7SP1,8SP1,9Beta

Preface:

When VFP VM execute a line like:
StringVariable = StringVariable + AnotherString

it uses a optimized concat routine:
if in the heap,
after the value of StringVariable, exists suffiente space for AnotherString, then it copy AnotherString and updates the length in StringVariable.

The VFP VM parser have a bug,
and under particular conditions,
it confuses a object's property name with that one of the variable one,
and,
it completely mistakes in copying the concatenate string.

That is a probable example where this can happen:

CLEAR
SET MEMOWIDTH TO 150
CREATE CURSOR myCursor ( myField M)
INSERT INTO myCursor VALUES (REPLICATE('?',101))
SCATTER MEMO NAME myRecord
INSERT INTO myCursor VALUES (PADR('My name is Fabio',101,'-'))
SCATTER MEMO MEMVAR
myRecord.myField = m.myField +'I add this'
? 'Property'
? '*********************************'
? 'Expected  =', myCursor.myField +'I add this'	AT 20
? 'Effective =', myRecord.myField		AT 20
?
?
? 'Variable'
? '*********************************'
? 'Expected  =', myCursor.myField 	AT 20
? 'Effective =', m.myField 		AT 20

This is a repro code with comments:

* YOU CAN CHANGE THESE CONSTANTS
#define PROPERTYNAME 	someName
#define SUFFIXSTRING	null

CLEAR

oSome=CREATEOBJECT("Empty")
WITH m.oSome
	ADDPROPERTY(m.oSome,[someName],NULL)
	FOR N=1 TO 50
		
		* Set the property to some variant value
		randomSet = RAND()
		DO CASE
			CASE m.randomSet < 0.2						&& A NULL
				.propertyName = .NULL.
			CASE m.randomSet < 0.3						&& A LOGICAL
				.propertyName = RAND()<0.5
			CASE m.randomSet < 0.5						&& A NUMBER
				.propertyName = RAND()
			CASE m.randomSet < 0.6						&& A CURRENCY
				.propertyName = NTOM(RAND()*1000000)
			CASE m.randomSet < 0.7						&& A DATETIME
				.propertyName = DATETIME()
			OTHERWISE
				.propertyName = REPLICATE(CHR(RAND()*256),RAND()*10)
		ENDCASE
		* save it
		propertyPush  = .propertyName

		* Now: build a variable WITH THESE CONDITIONS:
		* - the variable's name is equal to the property's name
		* - fill this variable with a string
		
		propertyName = REPLICATE(CHR(RAND()*256),RAND()*200)
		
		* Now, copy the variable into the property, but add a suffix string ( it can to be empty or null ),
		* with this copy command:
		* --------------------
		* <someobjectreference>.<propertyName> = !m.!<propertyName> !+! SUFFIXSTRING
		* --------------------
		* IF LEN(m.propertyName) IS UPPER TO 100,
		* THEN THE COPY HAVE NOT EFFECT, AND THE PROPERTY VALUE STILL UNCHANGED !
		* IF the added string have LEN(SUFFIXSTRING) upper to zero, VFP put the result value into the source variable !
		* This is correlate with the VFP's "Variable string add optimization alghoritm"

		* !m.! and !+! they are important for the apparition of the bug
		
		* ######### BUG LINE #################
		.propertyName = m.propertyName + SUFFIXSTRING
		* ######### BUG LINE #################
		
		********** with these commands the bug it is not appear
*		<someobjectreference>.propertyName = m.propertyName
*		<someobjectreference>.propertyName = propertyName + SUFFIXSTRING
*		<someobjectreference>.propertyName = '' + m.propertyName + SUFFIXSTRING
*		STORE 	m.propertyName + SUFFIXSTRING TO this.propertyName

		IF VARTYPE(.propertyName,.T.)==VARTYPE(m.propertyPush,.T.) AND .propertyName == m.propertyPush
			? 'property is unchanged',' type :', VARTYPE (.propertyName),' variable lenght :',LEN(m.propertyName)
			?? ' variable is changed to ',m.propertyName
		ENDIF
	NEXT
ENDWITH
? ">>>>>>>>>>  Expected this text line to be printed, only"

Notes:

It is from some weeks that i do not shipment bugs.

Naturally, during this time I have found many other bugs and issues in VFP9Beta; some are serious why they regard SQL SELECT that they generate mistakes results.
But, for me this demands time, and if I do not see resolved the bugs or issues that to more they interest me, I do not see the reason to continue in this way.

From now, without news, this is the last bug that I will send.

I will send to all the accumulated bugs after to have it tries them in the VFP9 final release.

Sorry.
Fabio