[2.0 rc.14 – rc.19 / next] A memo born in a newly opened <Match> that reads latest() throws a stale <Match> read and halts reactivity #4341

Describe the bug

Since 2.0.0-rc.14, a memo created inside a newly opened <Match> crashes the whole app if it reads latest() of an unrelated memo. The update that opens the <Match> also starts a sibling async memo under its own <Loading>. The engine then reads the <Match> callback's accessor after the branch has been closed, which throws "Attempting to access a stale value from that could possibly be undefined". Nothing catches the throw, so the reactive system halts (REACTIVITY_HALTED) and the screen stays on the previous branch for good.

Reading the same memo without latest() works.

Your Example Website or App

Inline below: a self-contained vitest + jsdom test, solid-js / @solidjs/web only.

Steps to Reproduce the Bug or Issue

repro.test.jsx:

import { render } from "@solidjs/web";
import { createMemo, createSignal, latest, Loading, Match, Switch } from "solid-js";
import { expect, it } from "vitest";

const wait = (ms) => new Promise((r) => setTimeout(r, ms));

it("a memo created in a newly opened <Match> may read latest()", async () => {
  const el = document.createElement("div");
  const [ready, setReady] = createSignal(false);
  const zone = createMemo(() => (ready() ? "ready" : "loading"));
  const format = createMemo(() => "ru");
  function Picker(props) {
    // replacing latest(format) with format() makes the test pass
    const label = createMemo(() => `${latest(format)}:${zone()}:${props.value()}`);
    return <span>{label()}</span>;
  }
  render(() => {
    const data = createMemo(async () => {
      if (zone() !== "ready") return "none";
      await wait(30);
      return "data";
    });
    return (
      <>
        <Switch>
          <Match when={zone() === "loading"}><i>loading</i></Match>
          <Match when={zone() === "ready" && "sel"}>{(s) => <Picker value={s} />}</Match>
        </Switch>
        <Loading fallback={<b>wait</b>}><b>{data()}</b></Loading>
      </>
    );
  }, el);
  setTimeout(() => setReady(true), 10);
  await wait(150);
  expect(el.textContent).toBe("ru:ready:seldata");
});

{
  "private": true,
  "type": "module",
  "dependencies": {
    "solid-js": "2.0.0-rc.19", "@solidjs/web": "2.0.0-rc.19", "@solidjs/signals": "2.0.0-rc.19",
    "@solidjs/babel-plugin": "2.0.0-rc.19", "@babel/core": "7.28.5",
    "vitest": "3.2.4", "vite": "7.1.12", "jsdom": "26.1.0"
  },
  "pnpm": { "overrides": { "solid-js": "2.0.0-rc.19", "@solidjs/web": "2.0.0-rc.19", "@solidjs/signals": "2.0.0-rc.19" } }
}

// vitest.config.mjs
import { transformAsync } from "@babel/core";
import solid from "@solidjs/babel-plugin";
const conditions = ["browser", "development"];
export default {
  plugins: [{
    name: "solid-jsx", enforce: "pre",
    async transform(code, id) {
      if (!id.endsWith(".jsx")) return null;
      const r = await transformAsync(code, { filename: id, babelrc: false, configFile: false, sourceMaps: true,
        plugins: [[solid, { generate: "dom", hydratable: false }]] });
      return { code: r.code, map: r.map };
    },
  }],
  resolve: { conditions },
  ssr: { resolve: { conditions, externalConditions: conditions }, noExternal: [/solid-js/, /@solidjs\//] },
  test: { environment: "jsdom", server: { deps: { inline: [/solid-js/, /@solidjs\//] } } },
};

Expected behavior

ru:ready:seldata. The <Match> for ready opens, Picker shows ru:ready:sel, and the <Loading> shows data once it lands. This is what rc.13 renders.

Actual on rc.14 through rc.19 and on next@17d4d1b:

• textContent is loadingnone;
• the console shows Attempting to access a stale value from <Match> …, then [REACTIVITY_HALTED] An uncaught error halted the reactive system.

On rc.14 and rc.15, vitest also aborts the worker during teardown (Abort trap: 6).

Screenshots or Videos

No response

Platform

• OS: macOS
• Runtime: Node 25.8.2, vitest 3.2.4, jsdom 26.1.0, @solidjs/babel-plugin (generate dom). Originally seen in Chromium: a period picker inside a <Switch> halted the page.
• Version: solid-js / @solidjs/web / @solidjs/signals 2.0.0-rc.19. Also reproduces on next@17d4d1b (signals built from source).

Additional context

Bisect. I built packages/signals from source at each signals commit between the rc.13 and rc.14 release commits (35e0651ab7 … c421e199fa), with solid-js and @solidjs/web held at rc.14:

• first bad: 1b9ceb6 signals: the hold model (L2) — core rebuilt from its rulings (#3774);
• last good signals commit: 4f676973d3.

rc.19's #4281 (#4275) and #4304 (#4284) change how latest() behaves, but neither fixes this case.

Usage. Each piece is documented:

• latest(fn): "Reads the freshest in-flight value available inside fn, falling back to the current settled value."
• <Match> with a function child receiving the narrowed accessor.
• createMemo returning a promise, read under <Loading>.

The component reads the accessor only from its own memo and does not keep it past unmount (no timers or async).