pybind11 doesn't allow same name nested class defined in different scope

[jslee02]

Given the following code, pybind11 generates binding code correctly and builds with no errors.

class Base
{
public:
    Base() = default;
    class Option
    {
    public:
        Option() = default;
    };
};

class Derived : public Base
{
public:
    Derived() = default;
    class Option : public Base::Option
    {
    public:
        Option() = default;
    };
};

// Binding code
{
    auto sm = m.def_submodule("common");
    auto attr = sm;
    ::pybind11::class_<chimera_test::common::Base >(attr, "Base")
        .def(::pybind11::init<>());
}
{
    auto sm = m.def_submodule("common");
    auto attr = sm.attr("Base");
    ::pybind11::class_<chimera_test::common::Base::Option >(attr, "Option")
        .def(::pybind11::init<>());
}
{
    auto sm = m.def_submodule("common");
    auto attr = sm;
    ::pybind11::class_<chimera_test::common::CommonDerivedA, chimera_test::common::Base >(attr, "CommonDerivedA")
        .def(::pybind11::init<>());
}
{
    auto sm = m.def_submodule("common");
    auto attr = sm.attr("CommonDerivedA");
    ::pybind11::class_<chimera_test::common::CommonDerivedA::Option, chimera_test::common::Base::Option >(attr, "Option")  // works if changed to "Option2"
        .def(::pybind11::init<>());
}

However, module loading fails saying:

ImportError: generic_type: cannot initialize type "Option": an object with that name is already defined

Boost.Python works fine.

---

Additional findings:

1. This issue happens even Base and Derived is in different namespace.
2. This issue only happens when Option is at the same level. For example, this issue disappear in the following case:

class Base
{
public:
    Base() = default;
    class Option
    {
    public:
        Option() = default;
    };
};

class Derived : public Base
{
public:
    Derived() = default;
    class NestedClass
    {
	public:
        class Option : public Base::Option
        {
        public:
            Option() = default;
        };
    };
};

[jslee02]

We can avoid this by specifying the name in the configuration YAML, but it would be great to provide options to automatically handle this issue.

Possible options:

• O1: Prepend the parent class name to the nested class

  base_option = Base.BaseOption()
  derived_option = Derived.DerivedOption()
• O2: Same as O1 but move the nested class to the parent scope

  base_option = BaseOption()
  derived_option = DerivedOption()
• O3: Same as O2 but put underscore between class names

  base_option = Base_Option()
  derived_option = Derived_Option()

Additionally, we may want to detect the name confliction to catch this error before generating bindings and either not generating the binding or renaming it to a unique name like appending characters (e.g., multiple underscores or numbers).

[jslee02]

@psigen Would you have some suggestions on this? This is another name conflict issue that might need the scope-changing method or/and name mangling method.

[jslee02]

Just in case if this is a pybind11 issue: pybind/pybind11#2187