invalid abstract return type when wrapping operator overloading on an abstract base class

Issue description

It appears that execute_cast in pybind11's operators.h introduces an artificial requirement that the wrapped type be concrete and copyable (because execute_cast returns a plain instance).

pybind11/include/pybind11/operators.h:84:14: error: invalid abstract return type ‘Abstract’
     static B execute_cast(const L &l, const R &r) { return B(expr); }                  \

Reproducible example code

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

class Abstract
{
  bool operator==(const Abstract &other) const
  {
    return true;
  }
  virtual void doit() = 0;
};

class Concrete : public Abstract
{
  virtual void doit()
  {
  }
};

namespace py = pybind11;

PYBIND11_MODULE(mod, m)
{
  py::class_<Abstract>(m, "Abstract")
    .def(py::self == py::self)
    ;
}

compiled with

 c++ -Ipybind11/include -I/usr/include/python3.6 t.cpp

gives

In file included from t.cpp:2:
pybind11/include/pybind11/operators.h: In instantiation of ‘struct pybind11::detail::op_impl<(pybind11::detail::op_id)25, (pybind11::detail::op_type)0, Abstract, Abstract, Abstract>’:
pybind11/include/pybind11/operators.h:59:24:   required from ‘void pybind11::detail::op_<id, ot, L, R>::execute(Class&, const Extra& ...) const [with Class = pybind11::class_<Abstract>; Extra = {}; pybind11::detail::op_id id = (pybind11::detail::op_id)25; pybind11::detail::op_type ot = (pybind11::detail::op_type)0; L = pybind11::detail::self_t; R = pybind11::detail::self_t]’                                                                                                      
pybind11/include/pybind11/pybind11.h:1103:9:   required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const pybind11::detail::op_<id, ot, L, R>&, const Extra& ...) [with pybind11::detail::op_id id = (pybind11::detail::op_id)25; pybind11::detail::op_type ot = (pybind11::detail::op_type)0; L = pybind11::detail::self_t; R = pybind11::detail::self_t; Extra = {}; type_ = Abstract; options = {}]’                                                      
t.cpp:25:30:   required from here
pybind11/include/pybind11/operators.h:130:67: error: ‘bool Abstract::operator==(const Abstract&) const’ is private within this context
 PYBIND11_BINARY_OPERATOR(eq,        eq,           operator==,   l == r)
                                                                 ~~^~~~
pybind11/include/pybind11/operators.h:83:61: note: in definition of macro ‘PYBIND11_BINARY_OPERATOR’
     static auto execute(const L &l, const R &r) -> decltype(expr) { return (expr); }   \
                                                             ^~~~
t.cpp:6:8: note: declared private here
   bool operator==(const Abstract &other) const
        ^~~~~~~~
In file included from t.cpp:2:
pybind11/include/pybind11/operators.h:84:14: error: invalid abstract return type ‘Abstract’
     static B execute_cast(const L &l, const R &r) { return B(expr); }                  \
              ^~~~~~~~~~~~
pybind11/include/pybind11/operators.h:130:1: note: in expansion of macro ‘PYBIND11_BINARY_OPERATOR’
 PYBIND11_BINARY_OPERATOR(eq,        eq,           operator==,   l == r)
 ^~~~~~~~~~~~~~~~~~~~~~~~
t.cpp:4:7: note:   because the following virtual functions are pure within ‘Abstract’:
 class Abstract
       ^~~~~~~~
t.cpp:10:16: note:      ‘virtual void Abstract::doit()’
   virtual void doit() = 0;

Start with pybind11/include/pybind11/operators.h, especially execute_cast and the operator implementation used by .def(py::self == py::self). Compile the provided abstract-class example first, then inspect existing operator tests for comparable coverage. Done means the example no longer requires an invalid abstract return type when wrapping the operator.