[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:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Sometimes.

 Here is the thread that explains the bug:

 https://github.com/haskell/containers/issues/568

 I originally reported this as a bug on `containers` issue tracker, but we
 seem to have concluded that this is probably a bug in the GHC optimizer
 itself.

 I think the shortest repro so far is this:

 {{{#!hs
 import qualified Data.Set as S

 main = print $
   let {-# noinline f #-}
       f () = T2
   in  S.fromList [f (), f ()]

 data T = T1 | T2 | T3 | T4 | T5 | T6 | T7 | T8 | T9
   deriving (Show, Read, Eq, Ord, Bounded, Enum)
 }}}

 which prints

 {{{#!hs
 fromList [T2,T2]
 }}}

 The person who derived this from my original repro says:

 > And as I said earlier, comment out the T9 constructor => prints fromList
 [T2] as it should.

 Another interesting quote:

 > Can confirm. Tested with ghc-8.6.1, containers-0.6.0.1 and
 leancheck-0.7.5 (so it does not seem to depend on the testing framework).
 Error occurs:
 >
 > * with ghc -O1 and -O2 (but not with -O0)
 > * and if data type has at least 9 elements
 >
 > So, likely a bug in ghc's optimizer.
 >
 > in some cases, input has duplicates, but not always.

 This is a bad one, makes GHC 8.6.1 totally unusable for me.

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