def_buffer doesn't accept pointer-to-member-function

[bmerry]

Issue description

The implementation of def_buffer directly calls the passed function or function object, so a pointer-to-member-function causes compilation errors.

I'll provide a PR to fix it (I've got some code that should work, just need to add a regression test).

Reproducible example code

#include <pybind11/pybind11.h>
#include <cstdint>

namespace py = pybind11;

class A {
public:
    int32_t data = 0;

    py::buffer_info get_buffer_info()
    {
        return py::buffer_info(
            &data, sizeof(data),
            py::format_descriptor<decltype(data)>::format(),
            1);
    }
};

PYBIND11_PLUGIN(_buffer) {
    py::module m("_buffer");

    py::class_<A>(m, "A", py::buffer_protocol())
        .def(py::init<>())
        .def_readwrite("data", &A::data)
        .def_buffer(&A::get_buffer_info);

    return m.ptr();
}

Output (GCC 5.4):

    In file included from buffer.cpp:1:0:
    /home/bmerry/work/sdp/env/include/site/python2.7/pybind11/pybind11.h: In instantiation of ‘pybind11::class_<type_, options>::def_buffer(Func&&)::<lambda(PyObject*, void*)> [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}; PyObject = _object]’:
    /home/bmerry/work/sdp/env/include/site/python2.7/pybind11/pybind11.h:1014:31:   required from ‘struct pybind11::class_<type_, options>::def_buffer(Func&&) [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}]::<lambda(PyObject*, void*)>’
    /home/bmerry/work/sdp/env/include/site/python2.7/pybind11/pybind11.h:1014:9:   required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_buffer(Func&&) [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}]’
    buffer.cpp:25:40:   required from here
    /home/bmerry/work/sdp/env/include/site/python2.7/pybind11/pybind11.h:1018:67: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘((pybind11::class_<type_, options>::def_buffer(Func&&) [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}]::capture*)ptr)->pybind11::class_<type_, options>::def_buffer(Func&&) [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}]::capture::func (...)’, e.g. ‘(... ->* ((pybind11::class_<type_, options>::def_buffer(Func&&) [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}]::capture*)ptr)->pybind11::class_<type_, options>::def_buffer(Func&&) [with Func = pybind11::buffer_info (A::*)(); type_ = A; options = {}]::capture::func) (...)’
                 return new buffer_info(((capture *) ptr)->func(caster));