segfault when casting dictionary with circular reference to `nlohmann::json`

[dbaston]

Demonstrated by the following test case:

TEST(pybind11_caster_tojson, recursive_dict)
{
    py::scoped_interpreter guard;
    py::module m = create_module("test");

    m.def("to_json", &test_fromtojson);

    // Simulate calling this binding from Python with a dictionary as argument
    py::dict obj("number"_a=1234, "hello"_a="world");
    obj["recur"] = obj;
    py::print(obj); // ok

    nl::json j = m.attr("to_json")(obj); // segfault
}

This situation is detected by the json module:

>>> obj = {"number": 1234, "hello" : "world"}
>>> obj["recur"] = obj
>>> json.dumps(obj)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.10/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python3.10/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.10/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
ValueError: Circular reference detected

[martinRenou]

Thanks for opening an issue!

Any help in fixing it would be very much appreciated