+ auto to_read = static_cast<std::uint64_t>(buffer_.size());

~ if (remaining_length_ < to_read)
~   to_read = remaining_length_;
+ adaptor_.socket().async_read_some(boost::asio::buffer(buffer_, static_cast<std::size_t>(to_read))
- size_t to_read = buffer_.size();
~ if (remaining_length_ < to_read)
~   to_read = remaining_length_;
- adaptor_.socket().async_read_some( boost::asio::buffer(buffer_, to_read)

~ Changed 'to_read' from [std::size_t] to [std::uint64_t],
  hence when 'remaining_length_' [std::uint64_t] is assigned in 'to_read' no data is lossed.
  When boost::asio::buffer is created 'to_read' is casted back to [std::size_t] explicitly truncating data.
  It seems to be the correct behavior to choose fixed 'network side' types.

include/crow/websocket.h

@@ -408,10 +408,10 @@ namespace crow
                             break;
                         case WebSocketReadState::Payload:
                             {
-                                size_t to_read = buffer_.size();
+                                auto to_read = static_cast<std::uint64_t>(buffer_.size());
                                 if (remaining_length_ < to_read)
                                     to_read = remaining_length_;
-                                adaptor_.socket().async_read_some( boost::asio::buffer(buffer_, to_read), 
+                                adaptor_.socket().async_read_some(boost::asio::buffer(buffer_, static_cast<std::size_t>(to_read)), 
                                     [this](const boost::system::error_code& ec, std::size_t bytes_transferred)
                                     {
                                         is_reading = false;