[Bug c++/115160] Enabling undefined behaviour sanitizer causes or'ed bit shift to report wrong result

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115160

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Specifically, std::vector<uint8_t>::iterator::operator++(int) is "just a
function", so the compiler doesn't "know" that it modifies the iterator every
time it's called. To the compiler, the code looks like:

(deref(inc(b)) | (deref(inc(b)) << 8) | (deref(inc(b)) << 16) | (deref(inc(b))
<< 24)

And this isn't clear that this modifies b and that the result depends on the
unspecified order that each deref(inc(b)) subexpression is evaluated.

But that doesn't really matter anyway. I don't think there is actually any
undefined behaviour here at all, so UBsan should not give any errors.
Evaluation of each *b++ subexpression happens before the evaluation of the next
one. But the standard doesn't specify which one is "the next one". It's not
undefined, but it's unspecified. So it's valid for the compiler to evaluate it
left-to-right, but also valid to evaluate it right-to-left, or any other order
as long as the b++ evaluations do not interleave.