Svelte 5: derived store not reactively updated

[mquandalle]

Describe the bug

In the following code snippet I expect the store countChecked to be recalculated whenever checks is modified:

import { derived, writable } from "svelte/store";

const checks = writable([false, false, false])
const countChecked = derived(checks, ($checks) => $checks.filter(Boolean).length)

It works on Svelte 4 but is broken on Svelte 5.

Reproduction

Svelte 5 REPL • Svelte 4 REPL

Logs

No response

System Info

System:
    OS: macOS 14.3.1
    CPU: (8) arm64 Apple M2
    Memory: 128.69 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.16.0 - ~/.nvm/versions/node/v18.16.0/bin/node
    Yarn: 1.22.19 - ~/.nvm/versions/node/v18.16.0/bin/yarn
    npm: 9.5.1 - ~/.nvm/versions/node/v18.16.0/bin/npm
    pnpm: 8.14.1 - ~/.nvm/versions/node/v18.16.0/bin/pnpm
    bun: 1.0.25 - ~/.bun/bin/bun
  Browsers:
    Brave Browser: 114.1.52.126
    Safari: 17.3.1
  npmPackages:
    svelte: next => 5.0.0-next.75

Severity

blocking an upgrade

[rmunn]

Note that in runes mode you get the error "Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. 'array[i] = value' instead of 'entry = value')", and if you rewrite the Svelte 5 repro to that style, it works. E.g. if the each block looks like this:

{#each $checks as checked, i}
	<input type="checkbox" bind:checked={$checks[i]} />
{/each}

Then the derived store is updated correctly, in both runes and non-runes modes.

[adminy]

It sounds to be related to my issue: #10689

I'm trying to reactively update $state variable from a derived store builtin to sveltekit.