Unused variable warnings when deriving (Data)

Hello!

When I compile the following module with "ghc -W -c A.hs" I see several
unnecessary warning messages:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.4
$ ghc -no-recomp -W -c A.hs

<no location info>: Warning: Defined but not used: `k'

<no location info>: Warning: Defined but not used: `k'

<no location info>: Warning: Defined but not used: `k'

Adding parameters to data constructors makes the warnings disappear,
example:
data T = A Int | B Char

---------------- A.hs -------------------

{-# OPTIONS -fglasgow-exts #-}

module A where

import Data.Generics

data T = A | B 
    deriving (Typeable, Data)

-----------------------------------------

Best regards
Tomasz