The following program doesn't compile:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PolyKinds        #-}
{-# LANGUAGE TypeFamilies     #-}

module Test where

class Back t

class Back (FrontBack t) => Front t where
  type FrontBack t :: k

instance Back Bool

instance Front Int where
  type FrontBack Int = Bool

with the error message:

    • No instance for (Back (FrontBack Int))
        arising from the superclasses of an instance declaration
    • In the instance declaration for ‘Front Int’

The example successfully compiles if the kind annotation on FrontBack is removed.