fix(signals): settle async iterators on empty completion and sync rej…

[yumemi-thomas]

Closes #2868
Closes #2869

Summary

Two settlement gaps in the async-iterator branch of the client async runtime, both of which leave the node STATUS_PENDING forever — the <Loading> fallback never leaves and <Errored> never renders:

1. Empty completion (2.0.0-beta.16: async iterable that completes without yielding leaves <Loading> pending forever #2868): an async iterable that completes ({done: true}) asynchronously without ever yielding never settles. The sync completion path already commits undefined, so a stream that legitimately produces zero items is indistinguishable from a hung request.
2. Synchronous rejection (2.0.0-beta.16: async-iterator rejection delivered synchronously is silently dropped (pends forever) #2869): a next() thenable that calls onRejected before .then() returns is dropped with no error handling. The promise branch handles the exact same thenable correctly — only the iterator branch loses it.

Repros: https://stackblitz.com/edit/axbsw9wp-5do9ntby?file=src%2FApp.tsx (#2868), https://stackblitz.com/edit/3etmyz9r-pn3hs9cj?file=src%2FApp.tsx (#2869)

Root cause

Both are missing halves of the sync/async duality the promise branch handles a few lines above. The fulfillment handler settles yielded values but not an empty completion (completed = true; schedule(); flush() — no write, no status change), and the rejection handler is guarded by if (!isSync && …), so a rejection delivered during the synchronous window is ignored entirely with nothing else recording it.

Fix

Mirror the promise branch:

• Empty completion: when {done: true} arrives asynchronously and no value was ever yielded (hadSyncValue broadened to hadValue, which now also tracks async yields), settle through the standard path with asyncWrite(undefined) — matching the immediately-done sync iterator. Completion after a yield keeps the existing schedule/flush (the last value is already committed).
• Synchronous rejection: record rejected / syncError during the synchronous window, then route through handleError(syncError) once the window closes, exactly like the promise branch — including the rethrow that unwinds the in-progress synchronous read. The rethrow is gated to the initial read: later iterate() calls run from asyncWrite continuations, where rethrowing would produce an unhandled rejection; there handleError settles the node and iteration stops.

No change to the per-yield hot path beyond one boolean assignment; stale-iterator writes remain guarded by the existing el._inFlight checks.

Solid 1.x note

Not applicable — async-iterable memo bodies are a new 2.0 API with no 1.x counterpart. Same hardening class as the server-side sync-thenable fixes in #2858, client iterator branch.

Tests

Seven new tests in tests/syncThenable.test.ts, five red-first (confirmed failing on the unfixed source) and two passing behavior-pins:

• empty async iterator settles to undefined (red)
• sync rejection on the initial read reaches the boundary with the original error (red)
• sync rejection after synchronously yielded values (red)
• nullish sync rejection preserves undefined through the boundary (red; composes with 2.0.0-beta.16: Errored fallback receives the internal StatusError wrapper for bare rejections #2866)
• sync rejection after an asynchronous yield settles without an unhandled rejection (red)
• completion after async yields preserves the last value (pin)
• async completion after a synchronous yield preserves it (pin)

Both issue repros confirmed reproducing against the published 2.0.0-beta.16 npm package. Full solid-signals / solid-js / @solidjs/web suites pass.

Full disclosure: I wrote this with the help of an AI assistant (Claude Fable 5) and reviewed every line before pushing. All red-first tests were confirmed failing on the unfixed code, and the bugs were verified against the published beta.16 npm package, not just this repo.

[changeset-bot]

🦋 Changeset detected

Latest commit: c46b34e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@solidjs/signals	Patch
test-integration	Patch
solid-js	Patch
babel-preset-solid	Patch
@solidjs/web	Patch
@solidjs/html	Patch
@solidjs/h	Patch
@solidjs/universal	Patch
@solidjs/element	Patch

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[codspeed-hq]

Merging this PR will improve performance by 25.66%

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 118 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	merge	266.2 µs	372.6 µs	-28.56%
⚡	merge	166.9 µs	75.5 µs	×2.2

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing yumemi-thomas:fix/async-iterator-settlement (c46b34e) with next (a99d6c8)}