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

["mw_triad at users dot sourceforge.net" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #7 from Matthew Woehlke <mw_triad at users dot sourceforge.net> ---
(In reply to Thiago Macieira from comment #3)
> Because it's not a bug.
> 
> This is a totally valid scenario.

Valid in what way? I constructed a Y but arranged, probably by accident, that
its dtor is never called. I fail to see how that's not likely a bug in my code
that reasonably warrants a diagnostic. (Note that I'm talking about a
*warning*, and possibly one that isn't even on by default, not an error.)

(In reply to Marc Glisse from comment #6)
> It might pedantically be illegal, but it is useful, and I believe some people
> would like to avoid the warning when the two destructors are equivalent.

However, the compiler doesn't know that here, because I didn't provided a
definition thereof; Y's dtor, even in this example, could have important side
effects. Even if the compiler *can* prove equivalence, I'd be suspicious
whether this was intended, but I'd be okay with a different (i.e. more
pedantic) warning in that case. (I'd also point out that it's not unreasonable
to require the user to somehow annotate if this is intentional if they care
about avoiding the warning when it's enabled.)

Anyway, I still get no warning if Y has members that need to be destroyed,
which definitely causes bad behavior when its dtor isn't called.