createConnection does not return a Promise, as it should

[trycoon]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Running createConnection() gives the following error "TypeError: mongoose.createConnection(...).then is not a function"

If the current behavior is a bug, please provide the steps to reproduce.

import mongoose from 'mongoose';
mongoose.createConnection('mongodb://localhost:27017/db', {}).then((db) => {});

What is the expected behavior?
That the function would return a Promise, according to the documentation:
@return {Connection} the created Connection object. Connections are thenable, so you can do await mongoose.createConnection()

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Mongoose 6.3.0
The problem is this line: https://github.com/Automattic/mongoose/blob/master/lib/index.js#L286
In Mongoose 5.12.3 its working, and then the code reads:

 if (arguments.length > 0) {
    return conn.openUri(uri, options, callback);
  }

  return conn;

It returns the Promise created by openUri()

[trycoon]

Could be related to #8706

[trycoon]

Ok, I missed this: https://mongoosejs.com/docs/migrating_to_6.html#the-aspromise-method-for-connections
So then this is the expected behavior... But this line should then be removed: "Connections are thenable, so you can do await mongoose.createConnection()"