[GHC] #2683: Regression with 6.10: panic in xmonad-contrib

[GHC]

#2683: Regression with 6.10: panic in xmonad-contrib
----------------------+-----------------------------------------------------
 Reporter:  dons      |          Owner:                
     Type:  bug       |         Status:  new           
 Priority:  high      |      Milestone:  6.10.1        
Component:  Compiler  |        Version:  6.9           
 Severity:  critical  |     Resolution:                
 Keywords:            |     Difficulty:  Unknown       
 Testcase:            |   Architecture:  x86_64 (amd64)
       Os:  Linux     |  
----------------------+-----------------------------------------------------
Changes (by igloo):

  * priority:  normal => high
  * difficulty:  => Unknown
  * milestone:  => 6.10.1

Comment:

 You need a DEBUG compiler to reproduce this (so it may not be a
 regression, but we should take a look anyway):
 {{{
 {-# LANGUAGE ExistentialQuantification, MultiParamTypeClasses,
              FunctionalDependencies, Rank2Types #-}

 module Q where

 class Transformer t a | t -> a where
     transform :: t -> l a -> (forall l'. l' a -> b) -> b

 data EL a = forall l. EL (l a)

 unEL :: EL a -> (forall l. l a -> b) -> b
 unEL = undefined

 transform' :: (Transformer t a) => t -> EL a -> EL a
 transform' = undefined

 data MultiToggleS ts a = MultiToggleS ts

 data MultiToggle = MultiToggle

 expand :: HList ts a => MultiToggleS ts a -> MultiToggle
 expand (MultiToggleS ts) =
     resolve ts
         (\x mt ->
             let g = transform' x in
             mt
         )
         MultiToggle

 class HList c a where
     resolve :: c -> (forall t. (Transformer t a) => t -> b) -> b
 }}}
 {{{
 $ ghc -fforce-recomp -c q.hs
 ghc: panic! (the 'impossible' happened)
   (GHC version 6.11.20081010 for x86_64-unknown-linux):
         ASSERT failed! file typecheck/TcUnify.lhs line 1000 a{tv ahb}
 [box]
 }}}
 Line 1000 is the second ASSERT here:
 {{{
 unifyType :: TcTauType -> TcTauType -> TcM CoercionI
 -- No boxes expected inside these types
 -- Acutal and expected types
 unifyType ty1 ty2       -- ty1 expected, ty2 inferred
   = ASSERT2( not (isBoxyTy ty1), ppr ty1 )
     ASSERT2( not (isBoxyTy ty2), ppr ty2 )
     addErrCtxtM (unifyCtxt ty1 ty2) $
     uTysOuter True ty1 True ty2
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2683#comment:1>
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