This piece of code fails when run with ghci -ignore-dot-ghci -fdefer-type-errors -dcore-lint bug.hs, maybe same as my previous #14584 (closed).

{-# Language DerivingStrategies #-}
{-# Language GeneralizedNewtypeDeriving #-}
{-# Language InstanceSigs #-}
{-# Language KindSignatures #-}
{-# Language PolyKinds #-}
{-# Language ScopedTypeVariables #-}
{-# Language TypeApplications #-}
{-# Language TypeFamilies #-}
{-# Language TypeInType #-}
{-# Language TypeOperators #-}
{-# Language UndecidableInstances #-}

import Data.Kind
import Data.Functor.Identity
import Data.Functor.Product

type a <-> b = a -> b -> Type

class Iso (iso :: a <-> b) where
  iso :: a -> b
  osi :: b -> a

data Iso_Bool :: Either () () <-> Bool

instance Iso Iso_Bool where

class Representable f where
  type Rep f :: Type

  index    :: f a -> (Rep f -> a)
  tabulate :: (Rep f -> a) -> f a

class Comonad w where
  extract   :: w a -> a
  duplicate :: w a -> w (w a)

newtype Co f a = Co (f a) deriving newtype (Functor, Representable)

instance (Representable f, Monoid (Rep f)) => Comonad (Co f) where
  extract = (`index` mempty)

newtype WRAP (iso::old <-> new) f a = WRAP (f a)

instance (Representable f, Rep f ~ old, Iso iso) => Representable (WRAP (iso :: old <-> new) f) where
  type Rep (WRAP (iso :: old <-> new) f) = new

  index :: WRAP iso f a -> (new -> a)
  index (WRAP fa) = index fa . osi @old @new @iso

  tabulate :: (new -> a) -> WRAP iso f a
  tabulate gen = WRAP $ 
    tabulate (gen . iso @old @new @iso)

newtype PAIR a = PAIR (Co (WRAP Iso_Bool (Product Identity Identity)) a)
  deriving newtype
    Comonad 

I unfortunately don't have time to find a more minimal example. Core linter vomits a lot of errors on 8.2.1 & 8.3.20171208.