Make node asynchronous promise bluebird

Question

In Node.js v7.8.0 I have a module foo.js:

class Foo {
    async bar() { return 'bar' } 
}

In the main module, app.jsI assign Bluebird a global promise:

global.Promise = require('bluebird').

And in `app.js' I have:

const Foo = require('./foo);
let foo = new Foo();
foo.bar().tap(res => console.log(res));

I assumed that since I replaced the Bluebird global promise, calling the async function will return the Bluebird promise (which has .tap), but apparently I'm wrong because I get the error:

TypeError: foo.bar(...).tap is not a function

Is there a way to achieve what I want?