stream/iter: pipeTo() crashes when synchronous writev() returns undefined

[trivikr]

Version

26.2.0

Platform

macOS 26.5.0

Subsystem

stream

What steps will reproduce the bug?

import { pipeTo } from 'node:stream/iter';

async function* source() {
  yield [new Uint8Array([65]), new Uint8Array([66])];
}

const chunks = [];
const writer = {
  write(chunk) {
    chunks.push(chunk);
  },
  writev(batch) {
    chunks.push(...batch);
    // Synchronous success: returns undefined.
  },
  end() {},
};

try {
  const bytes = await pipeTo(source(), writer);
  console.log('resolved:', bytes, Buffer.concat(chunks).toString());
} catch (err) {
  console.log('rejected:', `${err.name}: ${err.message}`);
}

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

Always

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

resolved: 2 AB

What do you see instead?

rejected: TypeError: Method Promise.prototype.then called on incompatible receiver undefined

piping into a custom writer that supports batched writes fails even though the writer successfully accepts the data synchronously. The same kind of synchronous success works for single writes, so batched writev() writes should complete normally instead of rejecting with a TypeError.

Additional information

No response