FWIW: I've had your comment at the back of my mind for the past 5 days, and I haven't yet come up with any clever way to work around it. I think you're simply right: to "fix" this in "new code" would break ABI for anyone who passed e.g. `vector<unordered_set<T>>` across an ABI boundary. (I.e., anywhere that "new code" might construct an unordered_set object according to the "new ABI," which object might later be destroyed by "old code" which assumed the "old ABI," leading to heap corruption.)

So yeah, it is probably physically impossible for libstdc++ to "get on the ball" in the way I'd hoped. Darn.