If I replace Promises on the global object, what Promises do async/await use?

Question

Say I replace the global Promise object with Bluebird promises (such as to support cancellation). If I then write a function using async/await:

async function foo() {
    const response = await fetch(...);
    const result = doSomethingWith(response);
    return result;
}

What kind of Promise is returned here?

IIRC when the code is compiled down to ES5, it will indeed return a cancellable Bluebird promise, since async/await is translated to use the global Promise object internally. But in ES7, where async/await is a native construct, will it use the Promise constructor in the global object, or is it hard-coded to use the native one?

Answer 1

Native syntax will always create native promise objects from the current realm, no matter what value the global Promise variable in the realm has.

A transpiler should strive to recreate that experience, however when it also includes a promise shim (which might be configured to be a custom library) that needs to live somewhere and for simplicity will probably depend on the global Promise value.