stream/iter: arrayBuffer consumers return SharedArrayBuffer for SAB-backed chunks

[trivikr]

Version

main

Platform

macOS 26.5.2

Subsystem

stream

What steps will reproduce the bug?

import { arrayBuffer, arrayBufferSync } from 'node:stream/iter';

const shared = new SharedArrayBuffer(4);
const chunk = new Uint8Array(shared);

const asyncResult = await arrayBuffer([chunk]);
const syncResult = arrayBufferSync([chunk]);

console.log('arrayBuffer():', asyncResult.constructor.name);
console.log('arrayBufferSync():', syncResult.constructor.name);
console.log('async instanceof ArrayBuffer:', asyncResult instanceof ArrayBuffer);
console.log('sync instanceof ArrayBuffer:', syncResult instanceof ArrayBuffer);

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

Always

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

arrayBuffer(): ArrayBuffer
arrayBufferSync(): ArrayBuffer
async instanceof ArrayBuffer: true
sync instanceof ArrayBuffer: true

Spec §11.3 says

1. Let bytes be the result of bytes() with source and options.

Spec §11.1 says

8. If chunks has exactly one element whose buffer is not shared, return that element.
9. Otherwise, concatenate all chunks into a new Uint8Array and return it.

What do you see instead?

arrayBuffer(): SharedArrayBuffer
arrayBufferSync(): SharedArrayBuffer
async instanceof ArrayBuffer: false
sync instanceof ArrayBuffer: false

Additional information

No response