[Bug ipa/62051] [4.9/5 Regression] Undefined reference to vtable with -O2 and -fdevirtualize-speculatively

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62051

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
So what's happening here is that the compiler sees that *m is a Derived, so it
can devirtualize the destructor call to ~Derived, and ~Derived refers to the
vtable.

One solution would be to give ~Derived default visibility and move its
definition out of line.  But calls to virt_func would probably produce direct
calls and thus undefined symbol errors as well.

It seems like your code is designed for calling through the vtable, and
devirtualization breaks that design, so turning off devirtualization is the
right approach.

It might be possible to make the compiler more clever about recognizing
patterns that don't work well with devirtualization and automatically
suppressing it in such cases.  Such as, in this case, seeing explicit default
visibility on a constructor.