[GHC] #8678: Derivin `Functor` complains about existential type

[GHC]

#8678: Derivin `Functor` complains about existential type
------------------------------------+-------------------------------------
       Reporter:  heisenbug         |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 When deriving a functor with !DataKinds enabled
 {{{
 {-# LANGUAGE DataKinds, DeriveFunctor, FlexibleInstances, GADTs,
 KindSignatures, StandaloneDeriving #-}

 data {- kind -} Nat = Z | S Nat

 data NonStandard :: Nat -> * -> * where
   Standard :: a -> NonStandard (S n) a
   Non :: NonStandard n a -> a -> NonStandard (S n) a

 deriving instance Show a => Show (NonStandard n a)
 deriving instance Functor (NonStandard n)
 }}}
 I get following error message
 {{{
 NonStandard.hs:10:1:
     Can't make a derived instance of ‛Functor (NonStandard n)’:
       Constructor ‛Standard’ must not have existential arguments
     In the stand-alone deriving instance for ‛Functor (NonStandard n)’
 }}}
 But the `Standard` constructor is not at all existential in the last type
 argument!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8678>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler