Fix missing pythonic type hints for native_enum

[dyollb]

Description

Fixes #5618

Suggested changelog entry:

Fix signature for functions with a native_enum in the signature

[dyollb]

I also tested the following, however, I am unsure if the return type is correct pybind11_tests.native_enum.in_class. Should it not be pybind11_tests.native_enum.class_with_enum.in_class?

struct class_with_enum {
    enum class in_class { one, two };

    in_class value = in_class::one;
};

TEST_SUBMODULE(native_enum, m) {
    py::class_<class_with_enum> py_class_with_enum(m, "class_with_enum");
    py::native_enum<class_with_enum::in_class>(py_class_with_enum, "in_class", "enum.IntEnum")
        .value("one", class_with_enum::in_class::one)
        .value("two", class_with_enum::in_class::two)
        .finalize();

    py_class_with_enum.def(py::init()).def_readwrite("value", &class_with_enum::value);

def test_property_type_hint():
    prop = m.class_with_enum.__dict__["value"]
    assert isinstance(prop, property)
    assert prop.fget.__doc__.startswith(
        "(self: pybind11_tests.native_enum.class_with_enum) -> pybind11_tests.native_enum.in_class"
    )

[dyollb]

Digging a bit deeper, it looks like maybe the native_enum has the "wrong" __qualname__. If I patch it like so

inline void native_enum_data::finalize() {
    disarm_finalize_check("DOUBLE finalize");
    if (hasattr(parent_scope, enum_name)) {
        pybind11_fail("pybind11::native_enum<...>(\"" + enum_name_encoded
                      + "\"): an object with that name is already defined");
    }
    auto py_enum_type = import_or_getattr(native_type_name, " (native_type_name)");
    auto py_enum = py_enum_type(enum_name, members);
    object module_name = get_module_name_if_available(parent_scope);
    if (module_name) {
        py_enum.attr("__module__") = module_name;
    }
    # START PATCH
    if (hasattr(parent_scope, "__qualname__")) {
        const auto parent_qualname = parent_scope.attr("__qualname__").cast<std::string>();
        py_enum.attr("__qualname__") = str(parent_qualname + "." + enum_name.cast<std::string>());
    }
    # END PATCH
    parent_scope.attr(enum_name) = py_enum;
   ...

I get the expected class type in the property.fget signature above. Interestingly it also "fixes" the pickle test test_pickle_roundtrip. The nested enum no longer raises a TypeError and actually can be pickled.

EDIT: I commited the changes shown in the comments for you to review.

[rwgk]

@dyollb Thanks for the fixes, that's really awesome!

I added commit 247a1d4, could you please review? — This changes only the test code. See commit message for the motivation.

[rwgk]

I forgot to add: enum class func_sig_rendering now doubles as a test for the corner case that there are no members.

[dyollb]

Thanks, LGTM

[dyollb]

@rwgk thanks for the fast review process! It's really appreciated