[GHC] #12401: GHC panic! Template variable unbound in rewrite rule

[GHC]

#12401: GHC panic! Template variable unbound in rewrite rule
-------------------------------------+-------------------------------------
           Reporter:  vagarenko      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Windows
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 When compiling following code with `-O`:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts, TypeFamilies, TypeSynonymInstances #-}

 module Bug where

 import Data.Word
 import Foreign.Storable
 import qualified Data.Vector.Storable as VS

 data Image a = Image
     { imageWidth  :: !Int
     , imageHeight :: !Int
     , imageData   :: VS.Vector (PixelBaseComponent a)
     }

 class (Storable (PixelBaseComponent a)) => Pixel a where
     type PixelBaseComponent a :: *

 instance Pixel Pixel8  where type PixelBaseComponent Pixel8  = Word8
 instance Pixel Pixel16 where type PixelBaseComponent Pixel16 = Word16

 data DynamicImage =
       ImageY8  (Image Pixel8)
     | ImageY16 (Image Pixel16)

 type Pixel8 = Word8
 type Pixel16 = Word16

 -- |
 imageMirrorY :: DynamicImage -> DynamicImage
 imageMirrorY dynImg = case dynImg of
     ImageY8  img -> ImageY8  $ mirror img 1
     ImageY16 img -> ImageY16 $ mirror img 1
     where
         mirror img channels = img { imageData = VS.concat $ reverse $ map
 (\y -> VS.slice (y * rowLen) rowLen (imageData img)) [0 .. imageHeight img
 - 1] }
             where
                 rowLen = channels * imageWidth img
 }}}

 I get error:
 {{{
 > ghc Bug -O
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-mingw32):
         Template variable unbound in rewrite rule
   Variable: cobox_s5Rv
   Rule "SPEC mirror @ Pixel8 @ Pixel8"
   Rule bndrs: [cobox_s5Rv, $dStorable_s5Rw]
   LHS args: [TYPE: Pixel8, TYPE: Pixel8,
              CO: <PixelBaseComponent Pixel8>_N, $dStorable_s5Rw]
   Actual args: [TYPE: Pixel8, TYPE: Pixel8,
                 CO: <PixelBaseComponent Pixel8>_N,
                 $fStorableWord8
                 `cast` ((Storable (Sym D:R:PixelBaseComponentWord8[0]))_R
                         :: (Storable Word8 :: Constraint)
                            ~R#
                            (Storable (PixelBaseComponent Pixel8) ::
 Constraint)),
                 img_a4IS, lvl_s7o4]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12401>
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