QList/QArrayDataPointer is missing std::launder()

See epic for more information.

C++17 needs the launder() when placing new objects, just in case the value_type contains const or reference members, and we emplace_back() over a previously pop_back()'ed element.

Only salvation would be if QList so far didn't support such value_types. But then we should add support for them, because they're at least copy-constructible, so in principle work with CoW.