[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:  bgamari
            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 ekmett):

 Including `-fwarn-redundant-constraints` but not `-Wcompat` in `-Wall` to
 avoid contradictory suggestions, does indeed seem like a reasonable
 compromise.

 Going through all of the code I've updated for GHC 8, `-fwarn-redundant-
 constraints` actually gave useful information in 18/20 cases, and had to
 be manually disabled because there was no appropriate "fix" in 2 places,
 putting it on similar footing with `-fwarn-orphans`, where most times you
 can slide the instance around to live with the data type or class in
 question, but sometimes you get boxed in.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11370#comment:33>
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