There is a warning against using bsoncxx::types::b_timestamp in user code:

> This BSON type is used internally by the MongoDB server - use by clients is discouraged.

However the options::change_stream::start_at_operation_time API requires you to pass in this type as input. Please either:

• Remove the warning from b_timestamp or
• Use an std type like std::chrono::system_clock::time_point

(second option seems better to me)