This module fails to compile with ghc head (with optimizations and using a 'perf' build):

module Bug where

import Ix

newtype U = U Int deriving (Eq, Ord)

instance Ix U where
  index (U from, U to) (U idx) = index (from, to) idx

The output is as follows - the underlined part is the identifier that it fails to find. (I added a ++ show id to the message.)

# ghc -c -fforce-recomp -O Bug.hs
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 6.11.20090801 for i386-unknown-linux):
        expectJust chooseExternalIds { GHC.Arr.$windex2 }
                                       ^^^^^^^^^^^^^^^^

Here's a stripped down version of Ix.hs that still exhibits the bug, to make the example more self-contained. It has to be compiled with -O as well:

module Ix2 where

class Ix a where
    index :: (a, a) -> a -> Int

instance Ix Int where
    index (m, n) i
        | m <= i && i <= n = m - i
        | otherwise        = indexError i

{-# NOINLINE indexError #-}
indexError :: a -> b
indexError _ = undefined