ghc 6.6 impossible happened

ghc-6.6: panic! (the 'impossible' happened)
  (GHC version 6.6 for i386-unknown-linux):
        cgPanic
    tpl{v sVs} [lid]
    static binds for:
    main:Util.RWS.runRWS'{v rpz} [gid]
    main:Util.RWS.$f4{v rv4} [gid]
    main:Util.RWS.$f5{v rva} [gid]
    local binds for:
    SRT label main:Util.RWS.$f3{v ruN}_srt

the file which caused it is:

-- modification of Control.Monad.RWS to make it strict in its fields, as well as use unboxed tuples internally

module Util.RWS (
	RWS,
	module Control.Monad.Reader,
	module Control.Monad.Writer,
	module Control.Monad.State,
  ) where

import Prelude

import Control.Monad
import Control.Monad.Fix
import Control.Monad.Trans
import Control.Monad.Reader
import Control.Monad.Writer
import Control.Monad.State
import Data.Monoid



newtype RWS r w s a = RWS { runRWS' :: r -> s -> (# a, s, w #) }

runRWS x r s = case runRWS' x r s of
    (# a, b, c #) -> (a,b,c)

instance Functor (RWS r w s) where
	fmap f m = RWS $ \r s -> let
		(# a, s', w #) = runRWS' m r s
		in s' `seq` w `seq` (# f a, s', w #)

instance (Monoid w) => Monad (RWS r w s) where
	return a = RWS $ \_ s -> (# a, s, mempty #)
	m >>= k  = RWS $ \r s -> case runRWS' m r s of
		(# a, s',  w #) -> case runRWS' (k a) r s' of
                    (# b, s'', w' #) -> let w'' = w `mappend` w'
                        in w'' `seq` (# b, s'', w'' #)

--instance (Monoid w) => MonadFix (RWS r w s) where
--	mfix f = RWS $ \r s -> let (a, s', w) = runRWS (f a) r s in (a, s', w)

instance (Monoid w) => MonadReader r (RWS r w s) where
	ask       = RWS $ \r s -> (# r, s, mempty #)
	local f m = RWS $ \r s -> let r' = f r in r' `seq` runRWS' m r' s

instance (Monoid w) => MonadWriter w (RWS r w s) where
	tell   w = RWS $ \_ s -> (# (), s, w #)
	listen m = RWS $ \r s -> case runRWS' m r s of
            (# a, s', w #) -> (# (a, w), s', w #)
	pass   m = RWS $ \r s -> case runRWS' m r s of
		(# (a, f), s', w #) -> let w' = f w in w' `seq` (# a, s', w' #)

instance (Monoid w) => MonadState s (RWS r w s) where
	get   = RWS $ \_ s -> (# s, s, mempty #)
	put s = RWS $ \_ _ -> s `seq` (# (), s, mempty #)
                                                               

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown