[GHC] #11000: Implement `-Wcompat`

[GHC]

#11000: Implement `-Wcompat`
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:  Design/Warnings
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 Currently we have only the following generic warning "levels":

 {{{
    -W     enable normal warnings
    -w     disable all warnings
    -Wall  enable almost all warnings (details in )
 }}}

 It would be useful to be able to refer collectively to all currently
 implemented forward-compatibility warnings (e.g. about upcoming changes in
 `base`, like `-fwarn-amp`) via a symbolic `-Wcompat` flag.

 This way, users have more flexibility in opt-in or out of such warnings.
 So e.g. the following settings are possible in `.cabal` files:

   `ghc-options: -Wall` :: enable "almost all warnings" as before. Some
 compat warnings maybe part of `-Wall`, some won't (yet).
   `ghc-options: -Wall -Wcompat` :: enable "almost all warnings" as well as
 all implemented warnings. E.g. if GHC 8.0 implements `-fwarn-mfp-compat`
 but doesn't include it in `-Wall`, this allows users to get notified asap
 about warnings to opt into that
   `ghc-options: -Wall -Wno-compat` :: This enables `-Wall` **minus** any
 future-compat warnings. This is for users who want to be warned at all
 about any future compat warnings.
   `ghc-options: -Wcompat` :: Of course, `-W(no-)compat` can be used also
 w/o `-Wall`, and then is equivalent to turning on/off the respective
 individual `-fwarn-*-compat` flags.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11000>
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