Interesting! This must be new. Bluebird didn't have upstream cancelation last time I checked. Now it might actually be viable. Should update the readme. Thanks!

Unfortunately it's still not viable in the browser due to its size, promises too easily get converted into non-cancelables, and worst of all, async/await forces native promises. If you want cancelable coroutines, you're forced to roll a generator-based implementation such as what Posterus provides.