[Bug libstdc++/64399] g++ does not diagnose when upcasting owning pointer (e.g. unique_ptr) with non-virtual destructor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64399

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It might be valid with a custom deleter, but the example shown has undefined
behaviour.