main.hs

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}

import Data.Proxy
import Data.Type.Equality

data Void
data Void1 a

class Test (a :: k) b where
  test :: b -> Proxy a
  default test :: (a ~~ Void, Test a b) => b -> Proxy a
  test = undefined

instance Test Void1 b where test = undefined
instance Test Void b