TypeScript 5.4 aggressive union type narrowing breaks svelte-check about language-tools HOT 4 OPEN

Just ran into this. This fails for me:

<script lang="ts">
  let period: 'monthly' | 'yearly' = 'yearly'
</script>

{#if period === 'monthly'}
  paid every month
{/if}

With this error:

Error: This comparison appears to be unintentional because the types '"yearly"' and '"monthly"' have no overlap. (ts)

EDIT: To temporarily work around this issue, I changed the typescript dependency to: 5.3.x

from language-tools.

Just ran into this. This fails for me:

<script lang="ts">
  let period: 'monthly' | 'yearly' = 'yearly'
</script>

{#if period === 'monthly'}
  paid every month
{/if}

With this error:

Error: This comparison appears to be unintentional because the types '"yearly"' and '"monthly"' have no overlap. (ts)

EDIT: To temporarily work around this issue, I changed the typescript dependency to: 5.3.x

You can cast the string 'yearly' as 'monthly' | 'yearly' to get around it for now if you wanna stay on 5.4.x.

 let period: 'monthly' | 'yearly' = 'yearly' as 'monthly' | 'yearly' 

from language-tools.

This one is expected because there isn't any reassignment. If your actual case has an assignment somewhere that isn't binding. Can you share it here?

from language-tools.

@dummdidumm Sadly, this is not completely fixed in the latest svelte-check v3.6.8. Maybe it's missing a special case for bind:group? At least that's the problem I'm running into. I built a small reproduction: https://stackblitz.com/edit/vitejs-vite-r64k1v?file=src%2FApp.svelte

If you run pnpm i && pnpm check, you can still see the error.

from language-tools.