[GHC] #10195: GHC forgets constraints when matching on GADTs

[GHC]

#10195: GHC forgets constraints when matching on GADTs
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        Reporter:  crockeea          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by crockeea):

 goldfire: In what sense does the `Bar m m'` constraint overlap with the
 instance? As far as I was aware, it would be impossible to have
 "overlapping" instances with just a single instance. This is likely just a
 misunderstanding of the term on my part.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10195#comment:5>
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