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: