[GHC] #9953: Pattern synonyms don't work with GADTs

[GHC]

#9953: Pattern synonyms don't work with GADTs
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        Reporter:  simonpj           |                   Owner:  cactus
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by cactus):

 I almost have this working in the `wip/T9953` branch. However, there is
 one tricky question, very much related to #9954: what do we do with
 required constraints that mention   some GADT indices?

 For example, if `pattern C` also happened to have a required constraint,
 e.g. `Eq a`, where would we put that, if `a` is no longer a universally-
 quantified type argument of `C`? What would be the type of `C`'s matcher?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9953#comment:6>
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