[GHC] #4419: dph tests fail with panic

[Simon Peyton-Jones]

|  But don't references to the other Ids in those RHS have to have the right unfolding?
|  That is, in
|  
|  f x = ... g y ...
|  g x = ... f y ...
|  
|  shouldn't the f in g's RHS and the g in f's RHS have the right unfolding info?

In the simplifier, no.  If it chooses to process this rec group top-to-bottom, the 'g' in f's RHS does *not* have an unfolding at all.  After all, g is a loop breaker, so its unfolding will not be used. On the other hand, the 'f' in g's RHS *does* have an unfolding. 

Simon