Svelte 5: `$props()` isn't generic anymore

[Aloso]

Describe the bug

After updating to 5.0.0-next.79, I got type errors in all my components:

<script lang="ts">
  let { foo, bar, baz } = $props<SomeType>()
  //                             ^^^^^^^^^
  // Expected 0 type arguments, but got 1. ts(2558)
</script>

Instead, I need to write this:

  let { foo, bar, baz }: SomeType = $props()

Note that other runes, such as $state(), still support generics.

Reproduction

See above (the error appears in VS Code, but not in the web editor)

Logs

No response

System Info

System:
    OS: Linux 6.6 Manjaro Linux
    CPU: (12) x64 AMD Ryzen 5 5500U with Radeon Graphics
    Memory: 1.77 GB / 7.11 GB
    Container: Yes
    Shell: 5.2.26 - /bin/bash
  Binaries:
    Node: 21.6.1 - /usr/bin/node
    npm: 10.4.0 - /usr/bin/npm
    pnpm: 8.15.4 - /usr/bin/pnpm
  npmPackages:
    svelte: 5.0.0-next.79 => 5.0.0-next.79 
    svelte-check: ^3.6.7 => 3.6.7 
    tslib: ^2.6.2 => 2.6.2 
    typescript: ^5.4.2 => 5.4.2 
    vite: ^5.1.6 => 5.1.6

Severity

annoyance

[trueadm]

This is intentional, see #10694

[Aloso]

@trueadm thanks.

This issue applies to $state() as well, if I'm not mistaken.

[trueadm]

@Aloso $state has never been a generic AFAIK. Just type it like you do any other declaration.

let counter: number = $state(0);

[brunnerh]

Still looks generic:

svelte/packages/svelte/src/main/ambient.d.ts

Lines 17 to 18 in 474fc7e

	declare function $state<T>(initial: T): T;
	declare function $state<T>(): T | undefined;

[trueadm]

Sorry, I meant that it's never needed to be based a discrete generic. The inference system should play ball with the type annotation on the declaration – which is important as the initial value needs to align, thus generic is needed.

[Aloso]

The generic is often required. For example:

let s = $state<'idle' | 'loading' | 'done'>('idle')

[trueadm]

Why can't you do?

let s: 'idle' | 'loading' | 'done' = $state('idle')

[Aloso]

@trueadm of course this works, but the function still needs to be generic. If you made it

function $state(initial: any): any

Then it wouldn't be type-safe.

[trueadm]

@Aloso We are not going to make it any, in fact we plan on keeping it a generic as my reply above states. Why do you think we plan on changing it?

[sysmat]

• uff shit, you want to ts infer type
• you need to create wrappers types for components, boilerplate