https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125656
--- Comment #10 from Matt Bentley <mattreecebentley at gmail dot com> --- Oh, so it's a shift-by-bitwidth == undefined behaviour flag in this case. Thanks for that. I do have that in the source code for set_overflow_to_one, I commented it out because it's only visible at C++20 or above - below C++20 I disabled the check because it invoked a (I thought) unnecessary branch, as opposed to doing the check at compile time.
