When I compile this with 7.8.3, it gives an error message saying that type variables a and m are not in scope. If I add them to the forall, it tells me I need FlexibleContexts. If I add that, then it gives me an error about an ambiguous type variable. Clearly, something crashed ghc-4.04proto, but there's no indication of what that was. If this test is still at all relevant, it probably needs to be updated to target something more narrowly. The program: {-# LANGUAGE MultiParamTypeClasses, ExplicitForAll #-} module ShouldFail where -- !!! For-all with parens -- This one crashed ghc-4.04proto; the parens after the for-all fooled it class Monad m => StateMonad s m where getState :: m s setState0 :: forall b. (StateMonad (a,b) m => m a) setState0 = getState >>= \ (l,_r) -> return l David Feuer