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diff --git a/testsuite/tests/simplCore/should_compile/simpl005.hs b/testsuite/tests/simplCore/should_compile/simpl005.hs
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+++ b/testsuite/tests/simplCore/should_compile/simpl005.hs
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+-- !!! CPR on newtype with polymorphic argument
+
+{-# OPTIONS -O #-}
+
+module ShouldCompile where
+
+data StateM m s a = STM (s -> m (a,s)) 
+
+instance Functor m => Functor (StateM m s) where
+  fmap f (STM xs) =  STM (\s -> fmap (\ (x,s') -> (f x, s')) 
+                    		     (xs s)                                
+		         )
+{-	With GHC 4.04 (first release) this program gave:
+
+  panic! (the `impossible' happened):
+          mk_cpr_let: not a product
+      forall a{-ruq-} b{-rur-}.
+      (a{-ruq-} -> b{-rur-})
+      -> MonadLibrary.StateM{-r2o,x-} m{-a30Y-} s{-a30Z-} a{-ruq-}
+      -> MonadLibrary.StateM{-r2o,x-} m{-a30Y-} s{-a30Z-} b{-rur-}
+
+ The reason: 'Functor' is a newtype, whose element is a for-all type.
+
+	newtype Functor f = Functor (forall a,b.  (a->b) -> f a -> f b)
+-}