cant derive functor on function newtype with unboxed tuple result

I'm not sure if this is a bug or a feature request, but i have a simple example that I think should work with deriving functor but falls over

{-# LANGUAGE ScopedTypeVariables, BangPatterns, UnboxedTuples, MagicHash, GADTs #-}
{-# LANGUAGE DeriveFunctor #-}

module Test where 

newtype Eww a = Ew# (Int -> (# a, Int #))
  deriving (Functor)

the error i get is

src/System/Random/SplitMix/Internal.hs:88:13: error:
    • The constructor ‘(#,#)’ should have 2 arguments, but has been given 4
    • In the pattern: (#,#) a1 a2 a3 a4
      In a case alternative:
          ((#,#) a1 a2 a3 a4)
            -> (#,#)
                 ((\ b2 -> b2) a1) ((\ b3 -> b3) a2) (f a3) ((\ b4 -> b4) a4)
      In the expression:
        case b5 of {
          ((#,#) a1 a2 a3 a4)
            -> (#,#)
                 ((\ b2 -> b2) a1) ((\ b3 -> b3) a2) (f a3) ((\ b4 -> b4) a4) }
      When typechecking the code for ‘fmap’
        in a derived instance for ‘Functor Eww’:
        To see the code I am typechecking, use -ddump-deriv

and the dumped deriving is

[1 of 1] Compiling System.Random.SplitMix.Internal ( src/System/Random/SplitMix/Internal.hs, /Users/carter/WorkSpace/projects/active/random-hs/dist-newstyle/build/random-2.0.0.0/build/System/Random/SplitMix/Internal.o )

==================== Derived instances ====================
Derived instances:
  instance GHC.Base.Functor System.Random.SplitMix.Internal.Eww where
    GHC.Base.fmap f_a4Vn (System.Random.SplitMix.Internal.Ew# a1_a4Vo)
      = System.Random.SplitMix.Internal.Ew#
          ((\ b6_a4Vp b7_a4Vq
              -> (\ b5_a4Vr
                    -> case b5_a4Vr of {
                         ((#,#) a1_a4Vs a2_a4Vt a3_a4Vu a4_a4Vv)
                           -> (#,#)
                                ((\ b2_a4Vw -> b2_a4Vw) a1_a4Vs)
                                ((\ b3_a4Vx -> b3_a4Vx) a2_a4Vt)
                                (f_a4Vn a3_a4Vu)
                                ((\ b4_a4Vy -> b4_a4Vy) a4_a4Vv) })
                   (b6_a4Vp ((\ b1_a4Vz -> b1_a4Vz) b7_a4Vq)))
             a1_a4Vo)


GHC.Generics representation types:

Trac metadata

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