[GHC] #10635: -fwarn-redundant-constraints should not be part of -Wall

[GHC]

#10635: -fwarn-redundant-constraints should not be part of -Wall
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        Reporter:  Lemming           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.11
  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:  #9939, #9973,     |  Differential Rev(s):
  #10100, #10183, #11370             |
       Wiki Page:                    |
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Comment (by mboes):

 Replying to [comment:15 acfoltzer]:
 > I also wonder if it might make sense to distinguish between certain
 types of redundancy: for example, in our code a redundancy arising from a
 superclass relationship (e.g., `(Functor f, Applicative f)`) is almost
 always something we want to fix, but unused classes are almost always an
 API design choice rather than a bug we wish to avoid.

 Our particular bug (and I think there are many situations like it), fell
 in the latter category (it was a mistakenly unused constraint). If only
 the former category is included in -Wall, then -Wall will be useless to us
 and others to catch similar bugs in the future. We'll have to remember to
 `-Wall -Wsome-other-warning-flag` in all projects.

 I think what would be nice though is if there existed a pragma to mark a
 constraint as explicitly redundant, for those who want that. In general,
 it's nicer to be able to specify things at fine granularity - disabling
 redundant constraint checking at the module level is too coarse a
 granularity. Probably a matter for a separate ticket.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10635#comment:17>
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