[GHC] #4220: EmptyDataDecls + DeriveFunctor == Panic!

[GHC]

#4220: EmptyDataDecls + DeriveFunctor == Panic!
---------------------------------+------------------------------------------
    Reporter:  conal             |       Owner:                    
        Type:  bug               |      Status:  new               
    Priority:  normal            |   Component:  Compiler          
     Version:  6.12.1            |    Keywords:                    
          Os:  Unknown/Multiple  |    Testcase:                    
Architecture:  Unknown/Multiple  |     Failure:  Compile-time crash
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Comment(by conal):

 The following explicit instance works fine, though it's too bad we have to
 make the error case explicit, since Void can be covered exhaustively with
 no clauses.
 {{{
 instance Functor Void where
   fmap _ = error "Void fmap: no void value"  -- so ghc won't complain
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4220#comment:1>
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