toStore not receiving updates from 5.22.3 onwards

[jamesbirtles]

Describe the bug

From 5.22.3 onwards, usage of toStore seems broken.

A simple case of wrapping some state

const count = toStore(() => counter, value => counter = value);

and then using it in the template for example

Count state: {counter}
Count store: {$count}

The count state updates as expected, but the count store never receives a new value.

Reproduction

See the following basic example:
https://svelte.dev/playground/0892822945674a63ad0d61aca0eba214?version=5.22.3

It works when running on 5.22.2:
https://svelte.dev/playground/0892822945674a63ad0d61aca0eba214?version=5.22.2

It also starts to work if you $inspect the store
https://svelte.dev/playground/a1380d57e6c24ab3aff00d8b74cda745?version=5.22.3

Logs

System Info

System:
    OS: Linux 6.11 cpe:/o:nixos:nixos:25.05 25.05 (Warbler)
    CPU: (16) x64 AMD Ryzen 7 7840U w/ Radeon  780M Graphics
    Memory: 41.81 GB / 60.64 GB
    Container: Yes
    Shell: 5.9 - /run/current-system/sw/bin/zsh
  Binaries:
    Node: 22.14.0 - /etc/profiles/per-user/james/bin/node
    Yarn: 1.22.22 - /etc/profiles/per-user/james/bin/yarn
    npm: 10.9.2 - /etc/profiles/per-user/james/bin/npm
  Browsers:
    Chromium: 134.0.6998.88
  npmPackages:
    svelte: ^5.23.2 => 5.23.

Severity

blocking an upgrade

[trueadm]

This is due to #15446. The reason this is happening is because we're now associating the root effect created in toStore to the template effect – as that is what is calling the read on the toStore. When the template effect gets invalidated, it now destroys the root effect each time – killing the store contract entirely.