I've got a feeling that https://github.com/mongodb/node-mongodb-native/commit/f6a8ceb0aa84a50a7aed14cd4d4d847a1b8e3f72#diff-af7f90364fd34f588f43f92c13a92ab6R217 has broken the promiseLibrary option.

For example, if I call the static MongoClient.connect with options including promiseLibrary set to bluebird, the promise returned is not a bluebird promise. I looks like maybePromise (https://github.com/mongodb/node-mongodb-native/blob/master/lib/utils.js#L699) does not honour the promiseLibrary option.