[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:  new
        Priority:  highest           |            Milestone:  8.6.1
       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):
       Wiki Page:                    |
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Comment (by simonpj):

 That's terrible!

 The following all produce the same (wrong) restul
 {{{
  S.fromList (map f [ () | _ <- [1..10] ])    -- T2,T2
  S.fromList [f (), f ()]                     -- T2,T2
  S.fromList [f (), f (), f()]                -- T2,T2
 }}}
 However this works fine
 {{{
  f () `S.insert` (f () `S.insert` S.empty)
 }}}
 The comparison operations generated by `deriving` look ok to me.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:2>
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