[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:              |            Owner:
  PVerswyvelen                       |           Status:  new
                  Type:  feature     |        Milestone:  7.10.1
  request                            |          Version:  6.10.1
              Priority:  normal      |         Keywords:  warnings
             Component:  Compiler    |  extensions language
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #9757
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 Though it seems impossible to draw an obvious boundary between this idea
 and the one posed in #9757, I tend to think the warning proposed here
 ''does'' belong within the domain of GHC. I don't have an obvious way to
 implement this in mind, though, and I'm struggling to explain ''why'' I
 feel this is something GHC should do.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3085#comment:35>
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