[GHC] #11370: Redundant superclass warnings being included in -Wall destroys the "3 Release Policy"

[GHC]

#11370: Redundant superclass warnings being included in -Wall destroys the "3
Release Policy"
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        Reporter:  ekmett            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11369, #11429    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonmar):

 @gershomb - re the missing discussion, this came up during the GHC Skype
 meeting yesterday, I was concerned that we had stopped adding new warnings
 to `-Wall` which is exactly what I wanted to avoid (see
 [https://mail.haskell.org/pipermail/ghc-devs/2016-January/010955.html]).

 I've also argued that we should put `-Wcompat` into `-Wall` for the same
 reason.  However, I wasn't aware that there would be a conflict between
 the compat warnings and `-Wredundant-constraints`, nor was I aware that we
 planned to have compat warnings that were very difficult to act upon.  So
 in the light of that, I suggest that we don't add warnings to `-Wall` that
 are not actionable or that conflict with other warnings in `-Wall`.  Sadly
 that means that a lot of people won't see some of the compat warnings, but
 maybe that's ok.

 My apologies for diving into the discussion with partial information.  But
 I do feel strongly about one thing: that `-Wall` should always be the set
 of all warnings that we believe it is both possible and sensible for
 programmers to act upon.  We shouldn't try to freeze `-Wall` under a
 misguided desire for stability.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11370#comment:29>
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