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

    class D a where { op :: (F a b) => a -> b } 
    instance (TypeCast Bool b') => D Int where { op _ = typeCast True }

With `TypeCast` defined as for HList.
That dangling type variable `b'` in the constraint is weird.
But GHC still rejects it; and rejects a version with a `(~)` constraint instead of the `TypeCast`.

AntC