Exceptions thrown by start() are not rethrown from the TransformStream / ReadableStream / WritableStream constructors #7693

Which Cloudflare product(s) does this pertain to?

Workers Runtime (workerd).

What versions & operating system are you using?

workerd 1.20261011.1 and 1.20260730.1, compatibility date 2026-10-11 and
2026-07-30, no compatibility flags. macOS 26.7.1 (arm64). Same result at
compatibility date 2022-11-30, with streams_enable_constructors +
transformstream_enable_standard_constructor at 2021-11-03, and with
pedantic_wpt.

Please provide a minimal reproduction

const start = () => {
  throw new RangeError("boom");
};

function construct(make) {
  try {
    make();
    return "constructor returned";
  } catch (e) {
    return `constructor threw ${e.name}: ${e.message}`;
  }
}

export default {
  fetch() {
    return Response.json({
      TransformStream: construct(() => new TransformStream({ start })),
      ReadableStream: construct(() => new ReadableStream({ start })),
      WritableStream: construct(() => new WritableStream({ start })),
    });
  },
};

Describe the bug

When start() throws synchronously, the constructor returns normally and the
stream is errored with the exception instead (for the TransformStream, a
later readable read rejects with it). Code that validates its arguments in start() therefore never throws
from new.

The start algorithms run through maybeRunAlgorithm() in
src/workerd/api/streams/standard.c++,
which turns a synchronous throw into a rejected promise
(ReadableImpl::start L1070, the writable start L1702, the transformer start
L4710). That is right for pull(), write() and the others, but not for
start().

Hit by Deno std's ByteSliceStream, which throws a RangeError from
start() for a negative start index. Its test (ByteSliceStream,
assertThrows(() => createStream(-1, Infinity))) fails with "Expected
function to throw."

Expected

All three constructors throw RangeError: boom, as in Chromium 152 and Node
24.18.

• new TransformStream():
  "resolve startPromise with the result of invoking transformerDict["start"]";
  invoking a Web IDL callback function rethrows its exception.
• SetUpReadableStreamDefaultController,
  SetUpReadableByteStreamController
  and SetUpWritableStreamDefaultController:
  "Let startResult be the result of performing startAlgorithm. (This might
  throw an exception.)"

Actual

{"TransformStream":"constructor returned","ReadableStream":"constructor returned","WritableStream":"constructor returned"}

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