[GHC] #12702: Don't warn about redundant constraints for necessary

[GHC]

#12702: Don't warn about redundant constraints for necessary
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > So: if the constraint is required to make foo compile, then it is most
 certainly not redundant, and there shouldn't be a warning suggesting its
 removal.

 Correct.

 But I can't reproduce this error either with HEAD or with the 8.0 branch
 (which has zillions of bugs fixed wrt 8.0.1).

 Could you see if you get the warning with either of those?  You'll need to
 ask for `-Wredundant-constraints` explicitly, because it's no longer in
 `-Wall`.

 Thanks

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12702#comment:1>
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