[GHC] #1033: ghc-6.7: panic! (the 'impossible' happened) -- typechecker getting confused by Data.Generics?

[GHC]

#1033: ghc-6.7: panic! (the 'impossible' happened) -- typechecker getting confused
by Data.Generics?
-------------------------+--------------------------------------------------
    Reporter:  int-e     |       Owner:         
        Type:  bug       |      Status:  new    
    Priority:  normal    |   Milestone:         
   Component:  Compiler  |     Version:  6.7    
    Severity:  normal    |    Keywords:         
  Difficulty:  Unknown   |    Testcase:         
Architecture:  Unknown   |          Os:  Unknown
-------------------------+--------------------------------------------------
This bug is triggered by
 [http://www.cse.unsw.edu.au/~dons/lambdabot/Lib/Pointful.hs]. A reduced
 test case follows.
 {{{
 {-# OPTIONS_GHC -fglasgow-exts #-}
 module Pointful' where

 import Language.Haskell.Syntax
 import Data.Generics
 import Control.Monad.State

 -- rename :: () -> HsExp -> State (HsName, [HsName]) HsExp
 rename = \_ -> everywhereM (mkM (\e -> case e of HsWildCard -> return e))
 {-
 Pointful.hs:9:0:ghc-6.7: panic! (the 'impossible' happened)
   (GHC version 6.7 for i386-unknown-linux):
         dictPred
     ic{v a1nF} :: {forall a{tv a1iH} [sk].
                    (base:Data.Generics.Basics.Data{tc 29} a{tv a1iH} [sk])
 =>
                    <pred>base:GHC.Base.Monad{tc 28} m{tv a1nq} [tau]}
                   Refinement {[]}
 -}

 -- rename _ = everywhereM (mkM (\e -> case e of  HsWildCard -> return e))
 {-
 Pointful.hs:30:17:
     Couldn't match expected type `Monad m' against inferred type `()'
     In the first argument of `rename', namely `()'
     In the expression: rename () undefined
     In the definition of `uncomb'': uncomb' = rename () undefined
 -}

 uncomb' :: State (HsName, [HsName]) HsExp
 uncomb' = rename () undefined
 }}}
 The errors can be reproduced by uncommenting the line immediately
 preceding it (and commenting out the other one). In both cases the error
 can be fixed by uncommenting the type signature for {{{rename}}}.

 Tested with ghc-6.7 as of 2006-11-27. ghc-6.6 and ghc-6.4.2 are both fine.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1033>
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