[GHC] #15586: Compilation panic! (the 'impossible' happened)

[GHC]

#15586: Compilation panic! (the 'impossible' happened)
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        Reporter:  subaruru          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 There are quite a few other tickets about `piResultTy`, now fixed.  I
 wonder if someone could check if the example here, `STree`, is already
 fixed?  Thanks

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15586#comment:1>
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