It seems to me that it would not be unreasonable. If you need that detection, you need it at any price, on any platform. If you don't need it, meaning, the code doesn't test it, the compiler has still the right to optimize it away.
So it would be implementation defined behavior. I'm fine with that; personally, I think the standard was too quick to assign "undefined" here. But it can't mandate a certain behavior in the standard across all platforms.
