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This bug was filed before the changes to Bugzilla on 2011-10-16. Thus it
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Reproduced on master with the following code:

REM  *****  BASIC  *****
Option VBASupport 1
Option Explicit

Sub Main()

MsgBox(func1())
MsgBox func1("Par1_SetValue")

End Sub

Function func1(Optional MyPar1 As String = "Par1_DefaultValue") as String
func1 = MyPar1
End Function

----------

Result: An empty message box pops up followed by a message box that says "Par1_SetValue"

Expected Result: A message box that says "Par1_DefaultValue" followed by a message box that says "Par1_SetValue"

It looks to me like this is in some vba-specific part of the code because I get syntax errors if I remove the Option VBASupport 1.

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Your bug was confirmed at least 1 year ago and has not had any activity on it for over a year. Your bug is still set to NEW which means that it is open and confirmed. It would be nice to have the bug confirmed on a newer version than the version reported in the original report to know that the bug is still present -- sometimes a bug is inadvertently fixed over time and just never closed.

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I was about to open a very similar bug report and can provide a simple test case to demonstrate the problem still occurs of LO 4.3.0.3

VBA allows an optional parameter to be defined as - optional str as string = "abc" - the variable is not initialized to "abc" in LO

Alternatively if no default is provided - optional mstr as string mstr  - should not be initialized and isMissing(mstr) should return True it actually returns false and mstr is a null string

LO works correctly for data type Double and Variant,  I have not tested other data types

This error is particularly insidious when porting from Excel since no error is raised and differences in function execution may not be readily apparent.

I used the following function to show the problem:

function TestOpt(optional num as double =123,  optional str as string = "abc", optional numv  =456,  optional strv = "def", optional miss as double, optional mstr as string) as string 
Dim printstr as string 

printstr =              "num: >>" &num &"<<  isMissing " & isMissing(num) 
printstr = printstr & "; str: >>" &str &"<<  isMissing " & isMissing(str) 

printstr = printstr & "; numv: >>" &numv &"<<  isMissing " & isMissing(numv) 
printstr = printstr & "; strv: >>" &strv &"<<  isMissing " & isMissing(strv)
rem print printstr
printstr = printstr & "; miss: >>" &miss &"<<  isMissing " & isMissing(miss)
printstr = printstr & "; mstr: >>" &mstr &"<<  isMissing " & isMissing(mstr)
print printstr
TestOpt = printstr
end function 

Output is 
num: >>123<<  isMissing False; str: >><<  isMissing False; numv: >>456<<  isMissing False; strv: >>def<<  isMissing False; miss: >>Error ǀ<<  isMissing True; mstr: >><<  isMissing False

str and mstr are handled incorrectly, Double and Variant formats are correct

Created attachment 103446 [details]
Function to demonstrate problem