union fails 'is_base_of' assert

[danpf]

I'm trying to bind a union with pybind11 and I'm getting a strange error. I'm not sure if i'm hitting a limitation of pybind11, or i'm implementing this incorrectly. Could anyone let me know if I'm implementing this incorrectly or if this is a limitation of pybind11? It looks like it is failing on

cl.def_readwrite("boolean", &union_type::boolean);

I'll note that I'm getting a similar error with

Issue description

union fails 'is_base_of' assert

Reproducible example code

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

namespace py = pybind11;

union union_type {
	 bool boolean;
	 uint64_t u64;
	 int64_t  i64;
	 double   f64;
};

PYBIND11_MODULE(example, m) {
	union_type a;
	py::class_<union_type> cl(m, "union_type", "");
	cl.def( pybind11::init( [](union_type const &o){ return new union_type(o); } ) );
	cl.def( pybind11::init( [](){ return new union_type(); } ) );
	cl.def_readwrite("boolean", &union_type::boolean);
	cl.def_readwrite("u64", &union_type::u64);
	cl.def_readwrite("i64", &union_type::i64);
	cl.def_readwrite("f64", &union_type::f64);
}

Error

pybind11/pybind11.h: In instantiation of ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_readwrite(const char*, D C::*, const Extra& ...) \
[with C = union_type; D = bool; Extra = {}; type_ = union_type; options = {std::shared_ptr<union_type>}]’:
mytest.cpp:35:59:   required from here
python3.7m/pybind11/pybind11.h:1161:9: error: static assertion failed: def_readwrite() requires a class member (or base class member
)
         static_assert(std::is_base_of<C, type>::value, "def_readwrite() requires a class member (or base class member)");
         ^~~~~~~~~~~~~

compiled with:

c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` mytest.cpp -o example`python3-config --extension-suffix`

[rolandd]

I wonder if the assertion in pybind11 should be

static_assert(std::is_same<C, type>::value || std::is_base_of<C, type>::value, ...

the C++ standard says that is_base_of will be true if the two types are the same non-union type.

All the pybind stuff will work with unions, so I think the assertion is just being too strict.