[GHC] #13766: Confusing "redundant pattern match" in 8.0, no warning at all in 8.2

[GHC]

#13766: Confusing "redundant pattern match" in 8.0, no warning at all in 8.2
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        Reporter:  edsko             |                Owner:  (none)
            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 ezyang):

 This looks related to some changes to inaccessibility which happened for
 GHC 8.2. There's more context at #12466, #11066 and #12694, and possibly
 more (do a search for "inaccessible")

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13766#comment:4>
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