Non-allocatable types with holders

[zooba]

I'm wrapping up a type that is non-allocatable, but trivially copyable. (Why the authors did this I have no idea, but I'm not in a position to change their mind.)

Approximated:

struct Object {
    int my_value;
    void* operator new(std::size_t) = delete;
};

Because it cannot be allocated on the heap directly, pybind11 won't project it properly. So I create a holder for it:

template <typename T> struct Holder {
    T object;
    holder(T o) : object(o) { }
    inline void *get() const { return (void *)&object; }
};
PYBIND11_DECLARE_HOLDER_TYPE(T, Holder<T>);

...
py::class_<Object, Holder<Object>>(m, "Object")
    .def(py::init([](Object o) { return Holder<Object>(o); });

But now when I pass it into a Python function, it raises a runtime exception "return_value_policy = copy, but type Object is non-copyable!".

void NativeFunction(Object o, Object2 o)
{
    mod.attr("py_function")(o, o2);
}

The exception is coming from here in the stack (I have two different types in this call - there are actually hundreds of Object for me to worry about).

pybind11::detail::type_caster_generic::cast(const void * _src, pybind11::return_value_policy policy, pybind11::handle parent, const pybind11::detail::type_info * tinfo, void *(*)(const void *) copy_constructor, void *(*)(const void *) move_constructor, const void * existing_holder) Line 553
pybind11::detail::type_caster_base<Object>::cast(const Object * src, pybind11::return_value_policy policy, pybind11::handle parent) Line 1111
pybind11::detail::type_caster_base<Object>::cast(const Object & src, pybind11::return_value_policy policy, pybind11::handle parent) Line 1077
pybind11::make_tuple<1,Object &,Object2 &>(Object & <args__0>, Object2 & <args__1>) Line 1255
pybind11::detail::simple_collector<1>::simple_collector<1><Object &,Object2 &>(Object & <values_0>, Object2 & <values_1>) Line 1493
pybind11::detail::collect_arguments<1,Object &,Object2 &,void>(Object & <args_0>, Object2 & <args_1>) Line 1646
pybind11::detail::object_api<pybind11::detail::accessor<pybind11::detail::accessor_policies::str_attr>>::operator()<1,Object &,Object2 &>(Object & <args_0>, Object2 & <args_1>) Line 1672
NativeFunction(Object o, Object2 o2)

Any tips on how to make sure my types get converted property when passing into a Python call?

[zooba]

I've worked around it for now by explicitly creating the holder when passing an object back into a Python call. This should be fine for my purposes, since I'm doing code generation anyway, but it seems like it's either not working as intended or not clearly documented?

(I'll add that I did eventually find the third argument to PYBIND11_DECLARE_HOLDER_TYPE - the code snippets look like headings in the doc so I kept missing the line before the example - but setting it to true didn't change anything.)