[GHC] #2365: Warn about usage of `OPTIONS_GHC -XLanguageExtension` (was: Warn about suspicious flags in OPTIONS_GHC pragmas)

[GHC]

#2365: Warn about usage of `OPTIONS_GHC -XLanguageExtension`
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              Reporter:  igloo       |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:  7.10.1
              Priority:  lowest      |          Version:  6.8.2
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #1407, #2499
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by thomie):

 * related:  #1407 => #1407, #2499


Comment:

 Warnings for `{-# OPTIONS_GHC -package foo #-}` were added in #2499.

 This ticket is for showing a warning when `{-# OPTIONS_GHC
 -XLanguageExtension #-}` is used. The
 [https://www.haskell.org/ghc/docs/7.8.3/html/users_guide/pragmas.html
 #language-pragma user guide] says: "The LANGUAGE pragma should be used
 instead of OPTIONS_GHC, if possible."

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2365#comment:16>
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