[GHC] #13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2

[GHC]

#13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2
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        Reporter:  deech             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      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 deech):

 Does one of these other flags [1] imply `-O` because I don't explicitly
 use it in the example?

 [1] https://github.com/deech/821rc2-panic-bug/blob/master/821rc2-panic-
 bug.cabal#L22

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13708#comment:6>
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