Support for base class of derived types

[randomizedthinking]

Understand that proxy is for member functions, but not member variables. Yet, in practice, there is a common theme of using proxy for T types derived from a common TBase.

class T : public TBase {};

pro::proxy<Facade, T> proxy;

Is there any way that proxy can accommodate such a pattern? For instance, define a new class with_base_class for the facade_builder.

struct Facade
    : pro::facade_builder
      ::with_base_class<TBase>
      ::build {};

pro::proxy<Facade, T> proxy;  // T is ensured to derived from TBase
proxy.base()->memvar;          // proxy.base() would return the pointer to TBase