Missing error cases in GHC.Internal.IO.Windows.Handle

In GHC.Internal.IO.Windows.Handle I see:

hwndReadNonBlocking hwnd ptr offset bytes
  = do mngr <- Mgr.getSystemManager
       val <- withOverlappedEx mngr "hwndReadNonBlocking" (toHANDLE hwnd)
                               (isAsynchronous hwnd) offset (startCB ptr)
                               completionCB
       return $ ioValue val

where ioValue is a record selector:

data IOResult a
  = IOSuccess { ioValue :: a }
  | IOFailed  { ioErrCode :: Maybe Int }

So what happens if val is IOFailed? Just a runtime crash.

The same thing happens in hwndWriteNonBlocking. Another runtime crash.

This seems outright wrong. So far as I can see IOFailed is a real possibility.

As part of !13308 (closed) I am expunging incomplete record selectors, which is what brought this up. I have adjusted the code so that, while it still crashes, it gives a more localised error message, and includes the error code. But it needs fixing properly by someone who understands what is going on.