FWIW, I'm still looking at this on and off.  I beleive the pure function  
was a red herring, as this example also does the wrong thing:

extern int z;

int foo(int *y)
{
     z = 1;
     *y = 0;
     return z;  // returns 1, even though y could point to z :(
}

Not much joy tracking it down as yet.  I've attached the alias information  
tree dump in case anyone fancies a look.  It looks ok to me, but I don't  
really understand it fully.

As a reminder: this bug is fixed from r119502 on trunk. I'd like to fix it  
on 4.2.x.


Ned.


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