Method#box

static VALUE
method_box(VALUE obj)
{
    struct METHOD *data;
    const rb_box_t *box;

    TypedData_Get_Struct(obj, struct METHOD, &method_data_type, data);
    box = data->me->def->box;
    if (!box) return Qnil;
    if (box->box_object) return box->box_object;
    rb_bug("Unexpected box on the method definition: %p", (void*) box);
    UNREACHABLE_RETURN(Qnil);
}