class F a b | a -> b
    instance F Int Bool

    class D a where { op :: F a b => a -> b }
    instance D Int where { op _ = True }

True that doesn't compile as given. Hugs says: 'Inferred type is not general enough'. 

GHC says 'Couldn't match expected type `b' with actual type `Bool''/ '`b' is a rigid type variable'. (So essentially the same failure of typechecking.)

With a little help for the type inference, this compiles in Hugs. 
    class C a b | a -> b
    instance C Int Bool

    f :: (C Int b, TypeCast b Bool) => b -> Bool
    f x  = typeCast x  
With `TypeCast` defined as for HList.
But GHC still rejects it; and rejects a version with a `(~)` constraint instead of the `TypeCast`.

AntC