Protected operator delete

Issue description

I'm trying to wrap a complicated library (that I have no control over). For one reason or another some (base)classes have a protected operator delete. Trying to wrap a class like that results in a compilation error (cf. code below).

What is the canonical way of handling such a situation? NB: I don't want to instantiate the protected operator class I just want to register it as a part of the type system.

BTW: I found the following PR that might be related: #264

Reproducible example code

#include <pybind11/pybind11.h>

namespace py = pybind11;

class A{
  
  protected:
  
  	void operator delete(void* ptr){};
  
};

class B: public A{
  
  public:
  
  	using A::operator delete;
  
}; 

void dummy(A a){};


PYBIND11_MODULE(example, m) {
    
    py::class_<A>(m,"A");
    py::class_<B>(m,"B");
    
    py::implicitly_convertible<B, A>();
    
    m.def("dummy",&dummy);
    
};

For the record this is the error I get

/home/adam/anaconda2/envs/cpp-py-bindgen/include/python3.6m/pybind11/pybind11.h: In instantiation of ‘struct pybind11::detail::has_operator_delete<A, void>’:
/home/adam/anaconda2/envs/cpp-py-bindgen/include/python3.6m/pybind11/pybind11.h:1002:35:   required by substitution of ‘template<class T, typename std::enable_if<((! pybind11::detail::has_operator_delete<T>::value) && pybind11::detail::has_operator_delete_size<T>::value), int>::type <anonymous> > void pybind11::detail::call_operator_delete(T*, pybind11::size_t, pybind11::size_t) [with T = A; typename std::enable_if<((! pybind11::detail::has_operator_delete<T>::value) && pybind11::detail::has_operator_delete_size<T>::value), int>::type <anonymous> = <missing>]’
/home/adam/anaconda2/envs/cpp-py-bindgen/include/python3.6m/pybind11/pybind11.h:1356:41:   required from ‘static void pybind11::class_<type_, options>::dealloc(pybind11::detail::value_and_holder&) [with type_ = A; options = {}]’
/home/adam/anaconda2/envs/cpp-py-bindgen/include/python3.6m/pybind11/pybind11.h:1081:24:   required from ‘pybind11::class_<type_, options>::class_(pybind11::handle, const char*, const Extra& ...) [with Extra = {}; type_ = A; options = {}]’
example.cpp:27:24:   required from here
/home/adam/anaconda2/envs/cpp-py-bindgen/include/python3.6m/pybind11/pybind11.h:994:69: error: ‘static void A::operator delete(void*)’ is protected within this context
 template <typename T> struct has_operator_delete<T, void_t<decltype(static_cast<void (*)(void *)>(T::operator delete))>>
                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
example.cpp:10:9: note: declared protected here
    void operator delete(void* ptr){};

Start with the has_operator_delete and call_operator_delete code in pybind11.h, then reproduce the protected-delete case from example.cpp. Determine the supported behavior for registering A and B without instantiation, and document or test the resulting handling once the intended behavior is established.