[Bug sanitizer/65769] [UBSAN] qt-4.6 and qt-4.7 applications using qobject_cast may crash

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65769

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this is a missed diagnostic in the C++ frontend if it doesn't warn
about this?