[GHC] #14768: -O1 changes result at runtime, duplicating __DEFAULT case

[GHC]

#14768: -O1 changes result at runtime, duplicating __DEFAULT case
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        Reporter:  Bodigrim          |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.4.1-alpha3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Incorrect result  |            Test Case:
  at runtime                         |  simplCore/should_run/T14768
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Many thanks to Bodigrim for finding this, with such a tractable test case!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14768#comment:8>
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