Derived Ord instance for enumerations with more than 8 elements seems to be incorrect

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:

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

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.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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