Okay, I submitted it as a GHC feature request. Thanks for the feedback.

On Wed, Mar 11, 2009 at 5:16 PM, Creighton Hogg <wchogg@gmail.com> wrote:
2009/3/11 Peter Verswyvelen <bugfact@gmail.com>:
> When I put
> {-# OPTIONS_GHC -Wall -Werror #-}
> in my source file, I don't get compiler (GHC) warnings about redundant
> language extensions that I enabled.
> 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?
> What do you think?

So you mean something like if you put {-# LANGUAGE
GeneralizedNewtypeDeriving #-} in a file, but never do newtype
deriving, it would warn you?

I have no idea how hard that'd be to implement, but that sounds kind
of cool.  Useful for both refactoring and when you've inherited old
code.

Cheers,
C