For this to work you would need stable bridge for calling between old and new version, and with c++ lacking stable ABI and high dependency to header-template in

For this to work you would need stable bridge for calling between old and new version, and with c++ lacking stable ABI and high dependency to header-template interfaces (which is part of the bad c++ one would want deprecated), this could be an issue. Sure, extern C already exists but it's a leaky abstraction and it won't be added to existing libraries overnight. Without this you can look at python 2 to 3 migration to see what will happen.