Add custom msgpack optional to work around VS bug

net/cpp/include/msgpack_optional.hpp

+//
+// MessagePack for C++ static resolution routine
+//
+// Copyright (C) 2017 KONDO Takatoshi
+//
+//    Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//    http://www.boost.org/LICENSE_1_0.txt)
+//
+#ifndef MSGPACK_V1_TYPE_OPTIONAL_HPP
+#define MSGPACK_V1_TYPE_OPTIONAL_HPP
+
+// #if __cplusplus >= 201703
+
+#include "msgpack/versioning.hpp"
+#include "msgpack/adaptor/adaptor_base.hpp"
+#include "msgpack/adaptor/check_container_size.hpp"
+
+#include <optional>
+
+namespace msgpack {
+
+/// @cond
+MSGPACK_API_VERSION_NAMESPACE(v1) {
+/// @endcond
+
+namespace adaptor {
+
+#if !defined (MSGPACK_USE_CPP03)
+
+template <typename T>
+struct as<std::optional<T>, typename std::enable_if<msgpack::has_as<T>::value>::type> {
+    std::optional<T> operator()(msgpack::object const& o) const {
+        if(o.is_nil()) return std::nullopt;
+        return o.as<T>();
+    }
+};
+
+#endif // !defined (MSGPACK_USE_CPP03)
+
+template <typename T>
+struct convert<std::optional<T> > {
+    msgpack::object const& operator()(msgpack::object const& o, std::optional<T>& v) const {
+        if(o.is_nil()) v = std::nullopt;
+        else {
+            T t;
+            msgpack::adaptor::convert<T>()(o, t);
+            v = t;
+        }
+        return o;
+    }
+};
+
+template <typename T>
+struct pack<std::optional<T> > {
+    template <typename Stream>
+    msgpack::packer<Stream>& operator()(msgpack::packer<Stream>& o, const std::optional<T>& v) const {
+        if (v) o.pack(*v);
+        else o.pack_nil();
+        return o;
+    }
+};
+
+template <typename T>
+struct object<std::optional<T> > {
+    void operator()(msgpack::object& o, const std::optional<T>& v) const {
+        if (v) msgpack::adaptor::object<T>()(o, *v);
+        else o.type = msgpack::type::NIL;
+    }
+};
+
+template <typename T>
+struct object_with_zone<std::optional<T> > {
+    void operator()(msgpack::object::with_zone& o, const std::optional<T>& v) const {
+        if (v) msgpack::adaptor::object_with_zone<T>()(o, *v);
+        else o.type = msgpack::type::NIL;
+    }
+};
+
+} // namespace adaptor
+
+/// @cond
+} // MSGPACK_API_VERSION_NAMESPACE(v1)
+/// @endcond
+
+} // namespace msgpack
+
+// #endif // __cplusplus >= 201703
+
+#endif // MSGPACK_V1_TYPE_OPTIONAL_HPP
+