GHC panic: while printing Non type-variable argument

{-# LANGUAGE GADTs, TypeFamilies, DataKinds, TypeOperators, MultiParamTypeClasses, UndecidableInstances, UndecidableSuperClasses, FlexibleInstances, PolyKinds, KindSignatures #-}

import GHC.Exts (Constraint)

newtype I a = I a

data NP :: (k -> *) -> [k] -> * where
  Nil  :: NP f '[]
  (:*) :: f x -> NP f xs -> NP f (x ': xs)

infixr 5 :*

class (AllF f xs, SListI xs) => All (f :: k -> Constraint) (xs :: [k])
instance (AllF f xs, SListI xs) => All f xs

data SList :: [k] -> * where
  SNil  :: SList '[]
  SCons :: SListI xs => SList (x ': xs)

class SListI (xs :: [k]) where
  -- | Get hold of the explicit singleton (that one can then
  -- pattern match on).
  sList :: SList xs

instance SListI '[] where
  sList = SNil

instance SListI xs => SListI (x ': xs) where
  sList = SCons

-- | Type family used to implement 'All'.
--
type family AllF (c :: k -> Constraint) (xs :: [k]) :: Constraint
type instance AllF _c '[]       = ()
type instance AllF  c (x ': xs) = (c x, All c xs)

semigroup :: All ((~) (Maybe Int)) xs => NP I xs -> NP I xs -> NP I xs
semigroup = undefined

Causes

ghc-failure-all.hs:37:14: error:
    • Non type-variable argumentghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-apple-darwin):
	print_equality ~

If AllF is used directly in the definition of sappend, there is no error whatsoever.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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