--- /dev/null
+--!!! 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)
+-}