[GHC] #3833: Panic on standlone deriving without GeneralizedNewtypeDeriving

[GHC]

#3833: Panic on standlone deriving without GeneralizedNewtypeDeriving
---------------------------------+------------------------------------------
    Reporter:  Khudyakov         |       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
---------------------------------+------------------------------------------
 I found that GHC panics on the following program if
 GeneralizedNewtypeDeriving pragma is removed. Otherwise it works just
 fine.

 {{{
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-} -- Removal causes crash
 import Data.Monoid

 newtype DecodeMap e = DecodeMap [e]

 deriving instance Monoid (DecodeMap e)
 }}}

 Here is error message it produce
 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 6.12.1 for i386-unknown-linux):
         genDerivBinds: bad derived class base:Data.Monoid.Monoid{tc rha}

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

 This bug appears only in GHC6.12.1. GHC6.10.4 rightfully complain that I
 should enable GeneralizedNewtypeDeriving unstead of crashing.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3833>
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