[Bug tree-optimization/121280] [15/16 Regression] False positive array-bounds warning with O3 and std::vector.back() of a local vector copy

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121280

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note this code is undefined if vect.empty() is true due to front being
undefined in that case.
Adding that check makes the warning go away too; either for
`vect_copy.empty()`or `vect.empty()`.

So this is definitely NOT a false positive (there is a missed optimization with
respect size()==0 case but that is fixed up later on).

Also see https://gcc.gnu.org/pipermail/gcc-patches/2025-August/691467.html .