[Haskell-cafe] Possible bug in GHC 7.0.3

Alberto G. Corona agocorona at gmail.com
Tue Jun 28 12:43:45 CEST 2011

I have an "'impossible' happened" error.

The code may look a little bit convoluted but it is part of my real code.:

--------- begin of code------

{-# LANGUAGE   FlexibleInstances, UndecidableInstances
               , MultiParamTypeClasses

class Serializable a b

class IResource a     --The rest of the instance definitions does not matter
for the error

instance  Serializable a b => IResource a

data DBRef a=  DBRef String   a

instance  (IResource a) => Read (DBRef a)

data   Votation a= Votation{
  content         :: DBRef a
} deriving (Read)

------------------------------- end of code ---

gives the following error at compilation time:

tests>runghc impossiblelloop.hs
ghc: panic! (the 'impossible' happened)
  (GHC version 7.0.3 for i386-unknown-mingw32):
        solveDerivEqns: probable loop
    (impossiblelloop.hs:20:13-16 main:Main.$fReadVotation{v rhI} [a{tv abB}
] base:GHC.Read.Read{tc 2d} [main:Main.Votation{tc rbo}

                               a{tv abB} [tv]] = [base:GHC.Read.Read{tc 2d}

                                                    (main:Main.DBRef{tc rbu}

                                                       a{tv abB} [tv])])
    [[main:Main.Serializable{tc rbA} a{tv abB} [tv] b{tv ajE} [tcs]]]

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