api.structuredClone - fails for {Readable,Writable}Stream in Safari

What type of issue is this?

Browser bug (a bug with a feature that may impact site compatibility)

What information was incorrect, unhelpful, or incomplete?

The Browser Compatibility shows Safari and Safari iOS fully support structuredClone, but the API fails for ReadableStream or WritableStream.

What browsers does this problem apply to, if applicable?

Safari

What did you expect to see?

A clone of the ReadableStream.

Did you test this? If so, how?

Run the following in the console to test:

let rs = new ReadableStream();
structuredClone(rs, {transfer: [rs]})

Chrome et al will log something like this:

▶︎ ReadableStream {locked: false}

Safari will log this:

▶︎ DataCloneError: The object can not be cloned.

Can you link to any release notes, bugs, pull requests, or MDN pages related to this?

No response

Do you have anything more you want to share?

Transferring streams is also broken for postMessage: https://bugs.webkit.org/show_bug.cgi?id=215485

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/Window/structuredClone

MDN metadata

MDN page report details

• Query: api.structuredClone
• Report started: 2024-10-05T13:15:00.407Z

Start by locating the api.structuredClone compatibility entry and read the linked WebKit issue alongside the reported Safari and Safari iOS behavior. Compare the entry with the ReadableStream and WritableStream transfer results; the work is done when the compatibility data no longer claims support that these cases do not provide.