[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 osa1):

 I'm probably missing the big picture, but I think it makes sense to give
 evaluated things `evaldUnfolding` and if that breaks something else then
 we should fix that something else. In (3) of comment:60, perhaps the thing
 to fix is the binder-swapping transform rather than the id unfolding, so
 that it avoids swapping binders with different evaluated-ness (or maybe
 only avoid it if doing so will break the let/app invariant -- not sure how
 hard would it be to check this though).

 I don't know enough about demand analysis to comment on Secondly. I'll try
 it.

 Re Thirdly, the demand type does not make too much sense to me (how can an
 id have strict demand but unused cardinality? Sounds like a hack to make
 some transformations possible) but I can see how the current code changes
 cardinality incorrectly. I'll update.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:78>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler