You have to know what the standard allows because every optimizer change tries to be more aggressive without violating it. People have been bitten by error chec

You have to know what the standard allows because every optimizer change tries to be more aggressive without violating it. People have been bitten by error checks whose object code was elided because the error "can't happen". You have to decide whether you need code that will always work, or code that seemed to work for a while.