[GHC] #11520: GHC falls into a hole

31 Jan 2016


      #11520: GHC falls into a hole
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE RankNTypes, PolyKinds, TypeInType, GADTs,
 UndecidableSuperClasses #-}

 module Play where

 import GHC.Types hiding (TyCon)

 data TyCon (a :: k) = TyCon

 data TypeRep (a :: k) where
     TypeCon :: forall (a :: k). TyCon a -> TypeRep k -> TypeRep a
     TypeApp :: forall k1 k2 (a :: k1 -> k2) (b :: k1).
                TypeRep a
             -> TypeRep b
             -> TypeRep (a b)

 class Typeable k => Typeable (a :: k) where
     typeRep :: TypeRep a

 data Compose (f :: k1 -> *) (g :: k2 -> k1) (a :: k2) = Compose (f (g a))

 composeTyCon :: TyCon Compose
 composeTyCon = TyCon Fingerprint "Compose"

 instance (Typeable f, Typeable (g :: k), Typeable k) => Typeable (Compose
 f g) where
     typeRep = TypeApp (TypeApp (TypeCon composeTyCon typeRep) typeRep)
 typeRep

 instance (Typeable f, Typeable g, Typeable a) => Typeable (Compose f g a)
 where
     typeRep = TypeApp (TypeApp (TypeApp (TypeCon composeTyCon typeRep)
 typeRep) typeRep) typeRep
 }}}
 fails with
 {{{
 λ> :load Bug.hs
 [1 of 1] Compiling Play             ( Bug.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20160122 for x86_64-unknown-linux):
         fvProv falls into a hole {abet}

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11520
GHC http://www.haskell.org/ghc/
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