#725 vtable lookups don't work-around sidefx, static method calls don't preserve sidefx

[fxm (freebasic.net)]

When a virtual member procedure or a static member procedure (or variable) is called (or accessed) on an object returned from a function, the function is called respectively twice or not at all!

Code to highlight the bug:

Type UDT Extends Object
  Declare Virtual Sub info_virtual ()
  Declare Static Sub info_static ()
  Static As Integer int_static
End Type
Dim As Integer UDT.int_static = 1
Virtual Sub UDT.info_virtual ()
  Print " UDT.info_virtual()"
  Print UDT.int_static
End Sub
Static Sub UDT.info_static ()
  Print " UDT.info_static()"
  Print UDT.int_static
End Sub

Function inc (Byref u As UDT) Byref As UDT
  Print " inc()"
  UDT.int_static += 1
  Return u
End Function


Dim As UDT u

Print "'Print UDT.int_static':"
Print UDT.int_static
Print
Print "'inc(u).info_virtual()':"
inc(u).info_virtual()      '' *** Bug : function inc() called twice!
Print
Print "'inc(u).info_static()':"
inc(u).info_static()      '' *** Bug : function inc() not called at all!
Print
Print "'Print inc(u).int_static':"
Print inc(u).int_static   '' *** Bug : function inc() not called at all!

Sleep

Output:

'Print UDT.int_static':
 1

'inc(u).info_virtual()':
 inc()
 inc()
 UDT.info_virtual()
 3

'inc(u).info_static()':
 UDT.info_static()
 3

'Print inc(u).int_static':
 3

---

Referring at forum from post http://www.freebasic.net/forum/viewtopic.php?p=196522#p196522

[dkl]

To clarify, there are two problems:

• vtable lookups re-use (duplicate) the This argument expression without handling/working-around side-effects in it. Thus any functions in the This expression are being called twice, but shouldn't be.

• static method calls delete the This argument expression without handling side-effects. Thus any functions in the This expression aren't being called but probably should be.