How to fix v8 deprecated GetFunction?

Question

I'm trying to repair node-osmium so that it works with Node 12 as I've got some old code I'd like to run.

v8 has now fully deprecated a lot of APIs that do not signal failure properly. These were previously only warnings of deprecation soon, they're now errors so it will no longer build. I (think I've) fixed most of these by following this CPP STYLE GUIDE.md's use maybe version of v8 APIs section.

But, i'm stuck with this error for GetFunction:

../src/utils.hpp:39:67: error: no matching function for call to ‘v8::FunctionTemplate::GetFunction()’
         Nan::MaybeLocal<v8::Object> maybe_local = Nan::NewInstance(Nan::New(T::constructor)->GetFunction(), 1, &ext);

I assume it's a similar fix as the other functions, but where do I get the context from in this constructor?

extract from node-osmium/src/utils.hpp:

namespace node_osmium {

    template<class T>
    auto unwrap(const v8::Local<v8::Object>& object) -> decltype(Nan::ObjectWrap::Unwrap<T>(object)->get()) {
        return Nan::ObjectWrap::Unwrap<T>(object)->get();
    }

    template<class T, class... Args>
    v8::Local<v8::Object> new_external(Args&&... args) {
        Nan::EscapableHandleScope scope;
        v8::Local<v8::Value> ext = Nan::New<v8::External>(new T(std::forward<Args>(args)...));
        Nan::MaybeLocal<v8::Object> maybe_local = Nan::NewInstance(Nan::New(T::constructor)->GetFunction(context), 1, &ext);
        if (maybe_local.IsEmpty()) Nan::ThrowError("Could not create new Buffer instance");
        return scope.Escape(maybe_local.ToLocalChecked());

    }

    v8::Local<v8::Value> create_js_box(const osmium::Box& box);

    osmium::osm_entity_bits::type object_to_entity_bits(v8::Local<v8::Object> options);

} // namespace node_osmium