[GHC] #3085: warn about language extensions that are not used

[GHC]

#3085: warn about language extensions that are not used
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Reporter:  PVerswyvelen                  |          Owner:                  
    Type:  feature request               |         Status:  new             
Priority:  normal                        |      Component:  Compiler        
 Version:  6.10.1                        |       Severity:  normal          
Keywords:  warnings extensions language  |       Testcase:                  
      Os:  Unknown/Multiple              |   Architecture:  Unknown/Multiple
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 When I put

 `{-# OPTIONS_GHC -Wall -Werror #-}`

 in my source file, I don't get compiler warnings about redundant language
 extensions that I enabled in that file.

 For example if I write

 `{-# LANGUAGE GeneralizedNewtypeDeriving #-}`

 in the file, but I never do newtype deriving, the compiler could give me a
 warning.

 It would be nice if the compiler gave warnings about this, since after
 refactoring, some language extensions might not be needed anymore, and
 hence should be removed since fewer language extensions mean more stable
 and portable code no?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3085>
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