[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 RyanGlScott):

 Here's an example which doesn't depend on any code from `containers`. It
 also makes the derived `Ord` code explicit:

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 module Main where

 import qualified Data.Foldable as Foldable
 import GHC.Exts (dataToTag#, tagToEnum#, (==#), (<#))

 main :: IO ()
 main | not_ordered a b = print $ Foldable.foldl' (flip wumbo) (singleton
 a) b
      | otherwise       = pure ()
   where
     {-# NOINLINE f #-}
     f () = T2
     {-# NOINLINE a #-}
     a = f ()
     {-# NOINLINE b #-}
     b = [f ()]

 data T = T1 | T2 | T3 | T4 | T5 | T6 | T7 | T8 | T9
   deriving (Eq, Show)

 instance Ord Main.T where
   compare a b
     = case dataToTag# a of
         a' -> case dataToTag# b of
                 b' -> if tagToEnum# (a' <# b') :: Bool then
                           LT
                       else
                           if tagToEnum# (a' ==# b') :: Bool then
                               EQ
                           else
                               GT

 data Set a = Bin !a !(Set a) !(Set a)
            | Tip
   deriving Show

 not_ordered :: Ord a => a -> [a] -> Bool
 not_ordered _ [] = False
 not_ordered x (y : _) = x >= y

 wumbo :: Ord a => a -> Set a -> Set a
 wumbo x0 = go x0 x0
   where
     go :: Ord a => a -> a -> Set a -> Set a
     go orig _ Tip = singleton orig
     go orig x t@(Bin y l r) = case compare x y of
         LT -> error "not used here"
         GT -> Bin y l (go orig x r)
         EQ -> t
 {-# INLINE wumbo #-}

 singleton :: a -> Set a
 singleton x = Bin x Tip Tip
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696#comment:6>
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