An OO GC'd language "class" is a mechanism for run-time polymorphism. That's what it's for.
C++ has no "runtime" concerns of these kinds, so virtualization sits on top systems with compile-time semantics (ie., c++ classes).
If the GC language properly supported interior pointers (and first class pointers) for the GC there wouldn't be a need to force boxing in all cases. You would be able to make the decision at the point of instantiation of the object. It would make the GC more complex, but I think these days most GCs support interior pointers for other reason anyway, right?
Or am I missing something?
I feel like this is every wart in the C++ language!