-- A type class with a single instance
class C a where
    c :: a -> a
instance D a => C a where
    c = d

-- Asking for the type of c will give me a function with the constraint D a:
$ :t c
> D a => a -> a

-- However, I want the type to be C a => a -> a
-- One way to solve this is by adding another instance that should never match

data Bottom
instance C Bottom where
   c = undefined

-- Now the type will have the correct constraint
$ :t c
> C a => a -> a

-- Is there another way of solving this?