[GHC] #13263: cant derive functor on function newtype with unboxed tuple result

[GHC]

#13263: cant derive functor on function newtype with unboxed tuple result
-------------------------------------+-------------------------------------
           Reporter:  carter         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 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:


 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13263>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler