[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 crockeea):

 If you can't reproduce with 8.0 branch, then I'd say it has been fixed by
 some other change. I'll let you close just in close just in case you want
 to add a regression test.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12702#comment:2>
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