GHC panic! Template variable unbound in rewrite rule

When compiling following code with -O:

{-# 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

Trac metadata

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