Problem exposing inherited member function of derived class to R through RCPP_MODULE

mod.cpp

#include <Rcpp.h>
using namespace Rcpp;

class B {
    public:
        B (SEXP b);
};
 
template <class T> class A {
    public:
        double var1;
        void fun1();
};
 
class B_derived : public B {
    public:
        B_derived(SEXP b);
};
 
 
B::B(SEXP b) {};
 
template <class T> void A<T>::fun1(){
    Rcpp::Rcout << "Object of Type= " << DEMANGLE(A) << "\n";
    Rcpp::Rcout << "T= " << DEMANGLE(T) << "\n";
}
 
B_derived::B_derived(SEXP b) : B::B(b) {};
       

typedef A<B_derived> A_derived ;
RCPP_MODULE(testing) {
    class_< A_derived >("A_derived")
        .default_constructor()
        .field( "var1", &A_derived::var1)
        .method("fun1", &A_derived::fun1)
    ;
}                                                                  

/*** R
    derived <- new( A_derived )
    derived$fun1()
*/

result.txt

> sourceCpp( "mod.cpp" )

> derived <- new(A_derived)

> derived$fun1()
Object of Type= A<B_derived>
T= B_derived