[GHC] #15569: Constant folding optimises 1 into 3

[GHC]

#15569: Constant folding optimises 1 into 3
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        Reporter:  snowleopard       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9136             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by snowleopard):

 Here is a quick fix: https://phabricator.haskell.org/D5109.

 Adding the above example as a single test and closing this ticket seems a
 bit unsatisfactory. I think we need to either 1) do some automated
 correctness checking or 2) refactor the code so that it's more obvious
 that there are no remaining bugs.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15569#comment:4>
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