py::cast on non-polymorphic base class pointer does not find existing derived instance

Issue description

I'm not sure if this is a bug or just a limitation of what pybind11 can do, but it could merit at least a mention in the docs or the FAQ.

When using py::cast to convert a C++ pointer to a py::object, if the pointer has a base class type but actually points to a derived object that was created from Python, it fails to recognise that this is an existing instance and instead creates a second Python wrapper.

If the base class is made polymorphic, then everything works. However, I have a base class I don't want to be polymorphic for performance reasons.

The use case is that I need to get a Python handle to this inside a method that is being exposed to Python, for lifetime management purposes (it's a case that won't fit into the standard call policies).

Tested on v2.2.0-38-g6a81dbb.

Reproducible example code

#include <pybind11/pybind11.h>

namespace py = pybind11;

class Base {};
class A : public Base {};

PYBIND11_MODULE(variant, m) {
    py::class_<Base>(m, "Base")
        .def("show", [] (const Base &base)
        {
            py::print("I am", py::cast(&base));
        });

    py::class_<A, Base>(m, "A");

    m.def("make_a", []() -> py::object
    {
        py::object obj = py::cast(A());
        py::print("Created", obj);
        return obj;
    });
}

from variant import make_a
a = make_a()
a.show()

Output:

Created <variant.A object at 0x7fc7288b3ce0>
I am <variant.Base object at 0x7fc7288b3d18>

Expected output:

Created <variant.A object at 0x7fc7288b3ce0>
I am <variant.A object at 0x7fc7288b3ce0>

Start by running the supplied C++ binding and Python example to reproduce the duplicate wrapper for a non-polymorphic base pointer. Trace the py::cast path involved in converting the base pointer and compare it with the polymorphic case. Done means the existing A instance is returned, or the limitation is documented in the docs or FAQ.