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Consider this well-typed code:

{-# LANGUAGE NoImplicitPrelude, TypeInType, TypeFamilies,
             UndecidableSuperClasses, RankNTypes, TypeOperators,
             FlexibleContexts, TypeSynonymInstances, FlexibleInstances,
             UndecidableInstances #-}
module Monolith where

import Data.Kind (Type)
import GHC.Exts (Constraint)

type family (~>) :: c -> c -> Type

type instance (~>) = (->)
type instance (~>) = ArrPair

type family Fst (p :: (a, b)) :: a where
  Fst '(x, _) = x

type family Snd (p :: (a, b)) :: b where
  Snd '(_, y) = y

data ArrPair a b = ArrPair (Fst a ~> Fst b) (Snd a ~> Snd b)

type family Super c :: Constraint where
  Super Type = ()
  Super (c, d) = (Category c, Category d)

class Super cat => Category cat where
  id :: forall (a :: cat). a ~> a

instance Category Type where
  id = \x -> x

instance (Category c, Category d) => Category (c, d) where
  id = ArrPair id id

class Category (c, d) => Functor (f :: c -> d) where
--class (Category c, Category d) => Functor (f :: c -> d) where
  map :: (a ~> b) -> (f a ~> f b)

data OnSnd f a b = OnSnd (f '(a, b))

instance Functor (f :: (c, d) -> Type) => Functor (OnSnd f a) where
  map f (OnSnd x) = OnSnd (map (ArrPair id f) x)

The compiler accepts it. But if I change the definition of 'Functor' for the commented one, I get an error:

super.hs:39:10: error:
    • Could not deduce (Category d)
        arising from the superclasses of an instance declaration
      from the context: Functor f
        bound by the instance declaration at super.hs:39:10-61
      Possible fix:
        add (Category d) to the context of the instance declaration
    • In the instance declaration for ‘Functor (OnSnd f a)’
   |
39 | instance Functor (f :: (c, d) -> Type) => Functor (OnSnd f a) where
   |

I can reproduce this on 8.0.1, 8.2.0.20170704 and HEAD.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonpj
Operating system
Architecture