[GHC] #15696: Derived Ord instance for enumerations with more than 8 elements seems to be incorrect

[GHC]

#15696: Derived Ord instance for enumerations with more than 8 elements seems to be
incorrect
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        Reporter:  mrkkrp            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.6.2
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5196
       Wiki Page:                    |
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Comment (by simonpj):

 > I get this STG with GHC HEAD, -O0:

 That's what you get for monoidal's code in comment:18. Note the missing
 eval before the `dataToTag#`.

 But it is ''not'' what you get for the code in comment:13.  The evals are
 there, as comment:14 shows.  And yet we get the wrong answer.

 But why?  That is the second bug. Can you see why that happens?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:32>
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