[GHC] #14626: No need to enter a scrutinised value

[GHC]

#14626: No need to enter a scrutinised value
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        Reporter:  heisenbug         |                Owner:  heisenbug
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  performance
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13861            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by heisenbug):

 Hmm, I made a run with traces about which `Id`s the condition holds for:
 https://circleci.com/api/v1.1/project/github/ghc/ghc/951/output/108/0?file=true

 you have to `grep getCallMethod` to find them. If I treat those values as
 tagged, and simply return them, the `ghc-stage2` crashes. :-(

 So something is definitely fishy here. For completeness, this is what I
 changed: https://github.com/ghc/ghc/compare/92f6a671a6a8...8f627c9f25b6

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14626#comment:3>
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