For what it's worth, a GSL developer later reopened that GitHub issue and stated that they're going to look into fixing the UB. Sutter may have just been stating an assumption.
https://github.com/microsoft/GSL/issues/786#issuecomment-513...
> I'll raise this issue in the next internal GSL sync. I'd agree with y'all that this behavior: https://godbolt.org/z/4Tr1fe9xG is undesirable
But ... surely Sutter ought to know better than to say "because the hardware handles this conversion reasonably, it's a benign case of UB"? Surely he knows that compilers can and will optimize based on the assumption that UB never happens?
The problem isn't, "oh no what if my CPU's float->int conversion instruction traps", that's an extremely naive way to think about UB. Everyone who has thought seriously about UB in C++ for any length of time knows this. It's worrying that this was Sutter's response.