ReadableStream.values().return() should return undefined, instead of cancellation reason

[compulim]

Version

22.14.0

Platform

Microsoft Windows NT 10.0.26100.0 x64

Subsystem

No response

What steps will reproduce the bug?

(async () => {
  const stream = new ReadableStream();

  const values = stream.values({ preventCancel: true });

  const returnResult = await values.return('Cancellation reason');

  // EXPECT: { done: true, value: undefined }
  // ACTUAL: { done: true, value: 'Cancellation reason' }
  console.log(returnResult);
})()

How often does it reproduce? Is there a required condition?

It always reproduce.

What is the expected behavior? Why is that the expected behavior?

According to WHATWG Streams - Asynchronous iteration. When the iterator return, it should return undefined if preventCancel is set (see point 6.)

The reason (denoted by arg in the spec) is only used for ReadableStreamReaderGenericCancel(reader, arg).

What do you see instead?

The reason is being returned as IteratorResult.

Additional information

This also repro in Chrome 134.0.6998.89, Edge 134.0.3124.93, and Firefox 136.0.4.

[islandryu]

WHATWG Streams - Asynchronous iteration
The asynchronous iterator return mentioned here might not refer to the return method itself.

According to the definition of asynchronous iterator return, its return value is ignored.
https://webidl.spec.whatwg.org/#asynchronous-iterator-return

It must return a Promise; if that promise fulfills, its fulfillment value will be ignored, but if it rejects, that failure will be passed on to users of the async iterator API.

The actual definition of the return method might be here.

If an asynchronous iterator return algorithm is defined for the interface, then the asynchronous iterator prototype object must have a return data property with attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true } and whose value is a built-in function object, taking one argument value, that behaves as follows: