[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
-------------------------------------+-------------------------------------
        Reporter:  mrkkrp            |                Owner:  osa1
            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:  #14677, #15155    |  Differential Rev(s):  Phab:D5196,
       Wiki Page:                    |  Phab:D5201
-------------------------------------+-------------------------------------

Comment (by simonpj):

 > What's wrong with seeing a literal when floating stuff inwards? Why not
 just introduce case/let around it?

 Because the float-in pass floats bindings inwards ''towards their use
 sites''. If there is no use of the variable bound by a floating binding,
 then we should not be floating that binding into that part of the code.
 Literals are an extreme case.

 > There should be some code somewhere that's supposed to ensure that this
 assertion is not triggered, but I can't find it.

 It's `FloatIn.sepBindsByDropPoint`.

 > Why did simplifier not remove this case expression?

 That is indeed a mystery.  Is the code you are showing the output of the
 simplifier?

 Can you give instructions to reproduce (maybe make a `wip/` branch)?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:71>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler