Faulty Word64 arithmetic if optimized

The following program produces different results depending on optimization:

Checked.hs:

module Checked ((.-.)) where

(.-.) :: (Bounded a, Integral a) => a -> a -> Maybe a
x .-. y | x >= 0 && y < 0 = if z > x  then Just z
                                      else Nothing
        | x >= 0          = if z <= x then Just z
                                      else Nothing
        | x < 0 && y > 0  = if z < x  then Just z
                                      else Nothing
        | otherwise       = Just z
    where z = x - y

Main.hs:

module Main (main) where

import Data.Maybe
import Data.Word
import Numeric
import Checked

main :: IO ()
main = print $ showHex (fromJust ((0xFFFFFFFF00000000 :: Word64) .-. 0x00000000FFFFFFFF)) ""

D:\dev\haskell\ch1>ghc --make Checked.hs Main.hs
[1 of 2] Compiling Checked          ( Checked.hs, Checked.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )
Linking Main.exe ...

D:\dev\haskell\ch1>Main.exe
"fffffffe00000001"

D:\dev\haskell\ch1>del *.hi *.o *.exe

D:\dev\haskell\ch1>ghc --make -O2 Checked.hs Main.hs
[1 of 2] Compiling Checked          ( Checked.hs, Checked.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )
Linking Main.exe ...

D:\dev\haskell\ch1>Main.exe
"fffe00000001"

D:\dev\haskell\ch1>

The bug disappears if definition and usage of (.-.) are in the same module.

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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