[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):  Phab:D5109
       Wiki Page:                    |
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Comment (by snowleopard):

 Thanks @sgraf! I think we can proceed with the testing-based solution for
 now, but I hope we will revisit constant folding rules in future and
 reimplement them in a more type-safe way. I can volunteer to do that once
 most of the urgent work on Hadrian is complete.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15569#comment:17>
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