The following program, extracted from the test exceptionsrun001, should exit with exitcode 100. Instead, when compiled with -O1, it never gets past the ioTest and somehow manages to exit with exitcode 0.

{-# LANGUAGE ScopedTypeVariables #-}
module Main where

import Control.Exception
import System.IO.Error
import System.Exit

main = do
  ioTest
  exitWith (ExitFailure 100)

ioTest :: IO ()
ioTest = (catch (ioError (userError "wibble"))
                (\(e::IOException) -> return ())

I think this will require a git bisect:

• last known good commit: 34bb4605
• first known bad commit: f83aab95.