[GHC] #11494: Add -Wcompat to -Wall

[GHC]

#11494: Add -Wcompat to -Wall
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
  https://ghc.haskell.org/trac/ghc/wiki/Design/Warnings|
  #To-Wcompat-Wallornot11494         |
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Comment (by bgamari):

 Edward has said that

 Relevant discussion,
   * November 2015:
 [[https://mail.haskell.org/pipermail/libraries/2015-November/026512.html|"3
 release policy and -Wno-compat"]] on libraries@
   * January 2016: [[https://mail.haskell.org/pipermail/ghc-
 devs/2016-January/010955.html|"Warnings, -Wall, and versioning policy"]]
 on ghc-devs@, ghc-users@
   * #11370: Redundant superclass warnings being included in -Wall destroys
 the "3 Release Policy"
   * The [[https://prime.haskell.org/wiki/Libraries/3-Release-Policy|Three-
 Release Policy]] on Haskell Prime wiki
   * October 2015:
 [[https://mail.haskell.org/pipermail/libraries/2015-October/026348.html|3
 release policy]] on libraries@

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11494#comment:2>
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