message: Fix for template issue to fix build error on macOS with clang
diff: @@ -81,13 +81,13 @@ template <class W, class B> std::string JsonVisitor<W, B>::write() {
  auto p = kv.second;
  switch (p.which()) {
  case 0:
  - writer->Int(p.as<int>());
  + writer->Int(p.template as<int>());
  break;
  case 1:
  - writer->Double(p.as<double>());
  + writer->Double(p.template as<double>());
  break;
  case 2:
  - writer->String(p.as<std::string>());
  + writer->String(p.template as<std::string>());
  break;
  default:
  writer->String(p.toString());
  