[Bug c/102427] -Woverflow only works in constant expressions

[david at westcontrol dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102427

--- Comment #2 from David Brown <david at westcontrol dot com> ---
Runtime detection is good - compile-time detection is much, much better when it
is possible.  (IMHO, of course.)