[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 nomeata):

 > this is the first warning in -Wall that sometimes just absolutely can
 not be worked around without explicitly turning it off

 Does this sentence have an implicit “without losing 3-release
 compatibility”? Or is there really code that triggers this warning and
 cannot be worked around for a single set of GHC and library versions?

 We should stay explicit about these things. I conjecture that most users
 of GHC do not attempt to write 3-release-compatible code and are just
 interested in the most tidy code for their current target.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11370#comment:26>
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