Comment 15 for bug 305460

(In reply to Jonathan Wakely from comment #3)
> The linker error alone doesn't make the root cause obvious, but it's a
> fairly well known and well documented problem:
> http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.10

A better link is now:
https://gcc.gnu.org/wiki/VerboseDiagnostics#undefined_reference_to_vtable_for_X