We have a number of places in the code (all in tests and examples) that check the server version (grep for get_server_version).

This code

1. relies on string-comparison (isn't really semver compliant)
2. probably is not what we'd want users to actually do if they have server-version-dependent code-paths

Instead we could support a mongocxx::version value-type or something that a client could expose:

mongocxx::version minimum {3,6,0};
mongocxx::version actual = client.server_version();
if (actual < minimum) {
    throw std::logic_error("Server version " + actual + " < minimum " + minimum);
}

(Just an idea I had - fine to resolve if others don't see the value or if there's a better alternative.)