bug: Error after upgrading svelte from 5.12.0 to >5.13.0 for customElement

[MelleD]

Describe the bug

I have a small hobby project and am currently having a problem upgrading the version. Since I'm not an expert, I don't know whether the behavior is intentional or a bug.

If it's not a bug, please just relabel it.

No one in Discord could help me straight away and say I should open a ticket here.

Problem 1
When I switched from svelte 5.12.0 to 5.13.0 I got the following error:

"error during build:
[vite-plugin-svelte] [plugin vite-plugin-svelte] Not implemented type annotation EmptyStatement"

Problem 2 is different since Svelte >5.15.0 I get a different error

See build: https://github.com/MelleD/lovelace-expander-card/actions/runs/13476126898/job/37655546958#step:7:44

x Build failed in 153ms
error during build:
src/ExpanderCard.svelte (27:15): Unexpected token `config`. Expected * for generator, private key, identifier or async (Note that you need plugins to import files that are not JavaScript)
file: /home/runner/work/lovelace-expander-card/lovelace-expander-card/src/ExpanderCard.svelte:27:15

[25](https://github.com/MelleD/lovelace-expander-card/actions/runs/13476126898/job/37655546958#step:7:26):     extend: (customElementConstructor) => class extends customElementConstructor {
[26](https://github.com/MelleD/lovelace-expander-card/actions/runs/13476126898/job/37655546958#step:7:27):         // re-declare props used in customClass.
27:         public config!: ExpanderConfig;
                   ^
28: 
29:         public setConfig(conf = {}) {

    at getRollupError (file:///home/runner/work/lovelace-expander-card/lovelace-expander-card/node_modules/.pnpm/rollup@4.34.8/node_modules/rollup/dist/es/shared/parseAst.js:397:41)
    at ParseError.initialise (file:///home/runner/work/lovelace-expander-card/lovelace-expander-card/node_modules/.pnpm/rollup@4.34.8/node_modules/rollup/dist/es/shared/node-entry.js:14112:[28](https://github.com/MelleD/lovelace-expander-card/actions/runs/13476126898/job/37655546958#step:7:29))
    at convertNode (file:///home/runner/work/lovelace-expander-card/lovelace-expander-card/node_modules/.pnpm/rollup@4.34.8/node_modules/rollup/dist/es/shared/node-entry.js:16006:10)
    at convertProgram (file:///home/runner/work/lovelace-expander-card/lovelace-expander-card/node_modules/.pnpm/rollup@4.[34](https://github.com/MelleD/lovelace-expander-card/actions/runs/13476126898/job/37655546958#step:7:35).8/node_modules/rollup/dist/es/shared/node-entry.js:15249:12)
    at Module.setSource (file:///home/runner/work/lovelace-expander-card/lovelace-expander-card/node_modules/.pnpm/rollup@4.34.8/node_modules/rollup/dist/es/shared/node-entry.js:16993:24)
    at async ModuleLoader.addModuleSource (file:///home/runner/work/lovelace-expander-card/lovelace-expander-card/node_modules/.pnpm/rollup@4.34.8/node_modules/rollup/dist/es/shared/node-entry.js:20902:13)
 ELIFECYCLE  Command failed with exit code 1.

Problem is here with the public config
https://github.com/MelleD/lovelace-expander-card/blob/main/src/ExpanderCard.svelte#L27

This works before and since svelte >5.15.0 there is an issue.

Thanks for your input

Reproduction

https://github.com/MelleD/lovelace-expander-card/

Logs

x Build failed in 144ms
error during build:
src/ExpanderCard.svelte (27:15): Unexpected token `config`. Expected * for generator, private key, identifier or async (Note that you need plugins to import files that are not JavaScript)
file: /home/runner/work/lovelace-expander-card/lovelace-expander-card/src/ExpanderCard.svelte:27:15

[25](https://github.com/MelleD/lovelace-expander-card/actions/runs/13756074590/job/38463662107?pr=429#step:7:26):     extend: (customElementConstructor) => class extends customElementConstructor {
[26](https://github.com/MelleD/lovelace-expander-card/actions/runs/13756074590/job/38463662107?pr=429#step:7:27):         // re-declare props used in customClass.
27:         public config!: ExpanderConfig;
                   ^
28: 
29:         public setConfig(conf = {}) {

    at getRollupError (file:///home/runner/work/lovelace-expander-card/lovelace-expander-

System Info

System:
    OS: macOS 12.7.6
    CPU: (8) x64 Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
    Memory: 102.78 MB / 16.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 22.8.0 - /usr/local/bin/node
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 10.4.1 - /usr/local/bin/pnpm
  npmPackages:
    svelte: 5.20.0 => 5.20.0 

But also on github action see build
See build: https://github.com/MelleD/lovelace-expander-card/actions/runs/13476126898/job/37655546958#step:7:44

Severity

blocking an upgrade

[aplotor]

also happening on a new install of bits-ui because its svelte version isn't pinned. just importing the bits-ui lib and starting dev server, immediately crashes with err Not implemented type annotation EmptyStatement [plugin vite-plugin-svelte:optimize-svelte]. i had to npm ci on an old package-lock.json to get it working for now. this seems to be quite serious, im surprised there isnt more activity on this issue