nb::typed for unions

[Ahajha]

For example, if a function returns object, but it's really a union of int and str, for example. Perhaps something like nb::typed<nb::union_, int, nb::str>. Ideally this could nest with other types, like nb::typed<nb::list, nb::typed<nb::union_, int, nb::str>>.

Is this possible already? I was wondering if we could do this with std::variant but ideally we don't include an extra header if we're not using it for anything else.

[oremanj]

nb::typed<nb::object, std::variant<int, nb::str>> should work for this as of nanobind 2.5.0, including the nesting part. There is a special case where nb::typed<nb::object, T> is typed like T rather than like object[T].

If you don't want to include the variant headers, you can define a stub type plus a type caster for it that exposes the correct name:

template <class... Ts> struct union_ {};
template <class... Ts> struct nanobind::detail::type_caster<union_<Ts...>> {
    NB_TYPE_CASTER(union_<Ts...>, union_name(make_caster<Ts>::Name...))
};
// do stuff with nb::typed<nb::object, union_<int, nb::str>>