Comment by quotemstr

Ah. Dyno (https://github.com/ldionne/dyno) is good at this stuff. If you have types Base, Child1, Child2, you can just return a Dyno object that can be any of these, expressed as a tagged union and not a Box-equivalent, and then do regular vtable-based or otherwise polymorphic dispatch into the object. You can also arrange it so that if you have a Child3 that can't fit in the (Base, Child1, Child2) union, the Child3 can be heap-allocated and invoked transparently as well. It's open-world type erasure.

C++ is so freakishly powerful is that it can not only solve this problem, but it can solve it via a regular library and not a language extension.