Eigen Matrix + buffer_protocol

What is correct way to give Eigen matrix buffer protocol?

Doing this

py::class_<Eigen::MatrixXd>(m, "MatrixXd", pybind11::buffer_protocol())
  .def_buffer([](Matrix<double> &m) { return py::buffer_info { ... }});

will cause segfault if pybind11/eigen.his included as in def_buffer

install_buffer_funcs([](PyObject *obj, void *ptr) -> buffer_info* {
            detail::make_caster<type> caster;
            if (!caster.load(obj, false)) {
              return nullptr;
            }

null pointer is returned

Start with the custom def_buffer snippet and the install_buffer_funcs path in pybind11/eigen.h, focusing on why detail::make_caster::load returns a null pointer. Reproduce the reported Eigen MatrixXd buffer-protocol case and determine the expected behavior when the header is included; done means the case no longer segfaults and exposes a valid buffer.