svelte-check/typescript detecting type mismatch where there shouldn't be one

[MathiasFrost]

Describe the bug

svelte-check/typescript detecting mismatch where there shouldn't be one

Reproduction

<script lang="ts">
	import { createEventDispatcher } from "svelte";

	/** */
	type T = $$Generic;

	/** */
	const dispatch = createEventDispatcher<{ change: T }>();

	export let value: T;

	/** */
	function test(): void {
		dispatch("change", value);
	}
</script>

Logs

Error: Argument of type '["resolved", T]' is not assignable to parameter of type '[T] extends [never] ? [type: "resolved", parameter?: null | undefined, options?: DispatchOptions | undefined] : null extends T ? [type: "resolved", parameter?: T | undefined, options?: DispatchOptions | undefined] : undefined extends T ? [type: ...] : [type: ...]'. (ts)
        data.onInvoked = (promise) => dispatch("invoked", promise);
        data.onResolved = (res) => dispatch("resolved", res);
        data.onReject = (e) => dispatch("rejected", e);

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (24) x64 AMD Ryzen 9 5900X 12-Core Processor
    Memory: 15.99 GB / 31.93 GB
  Binaries:
    Node: 18.16.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.18 - ~\AppData\Local\Yarn\bin\yarn.CMD
    npm: 8.4.1 - C:\Program Files\nodejs\npm.CMD
    pnpm: 8.5.1 - ~\AppData\Local\pnpm\pnpm.CMD
  Browsers:
    Edge: Spartan (44.19041.1266.0), Chromium (114.0.1823.58)
  npmPackages:
    svelte: ^4.0.0 => 4.0.0

Severity

blocking an upgrade

[dummdidumm]

Gah, this is tricky. I think the problem is that TypeScript doesn't know whether or not the generic extends never, so it will not narrow the extends clause to any of the possibilities. We probably need to find some type hack to work around this.

[MathiasFrost]

Gah, this is tricky. I think the problem is that TypeScript doesn't know whether or not the generic extends never, so it will not narrow the extends clause to any of the possibilities. We probably need to find some type hack to work around this.

I am not a master of TypeScript generics; is there a way for me to put a constraint on the generic to tell TypeScript that it never extends never?

[VismaTobbe]

Type check in Rider (IntelliJ) is complaining about Argument type "save" is not assignable to parameter type any. It does pass svelte-check though. Maybe this is a separate ticket or even svelte plugin related?

  const eventDispatcher = createEventDispatcher<{ save: boolean }>();
  eventDispatcher("save", true);

[dummdidumm]

Narrowed this down to microsoft/TypeScript#54806, let's see what happens (I fear this is a TS limitation)

[MathiasFrost]

Type check in Rider (IntelliJ) is complaining about Argument type "save" is not assignable to parameter type any. It does pass svelte-check though. Maybe this is a separate ticket or even svelte plugin related?

  const eventDispatcher = createEventDispatcher<{ save: boolean }>();
  eventDispatcher("save", true);

That snippet should be working. JetBrain's Svelte plugin is unfortunately unusable at this moment and I would advise against using it; What you are experiencing is just one of many things that doesn't work.
There seem to be some effort now to focus on it thought, so I'm hoping it'll get better soon.

[MathiasFrost]

Narrowed this down to microsoft/TypeScript#54806, let's see what happens (I fear this is a TS limitation)

Ah I understand now. Yeah, maybe better to see what TypeScript can do about it, since that does seem like something they probably would want to fix as well.

[VismaTobbe]

Type check in Rider (IntelliJ) is complaining about Argument type "save" is not assignable to parameter type any. It does pass svelte-check though. Maybe this is a separate ticket or even svelte plugin related?

  const eventDispatcher = createEventDispatcher<{ save: boolean }>();
  eventDispatcher("save", true);

That snippet should be working. JetBrain's Svelte plugin is unfortunately unusable at this moment and I would advise against using it; What you are experiencing is just one of many things that doesn't work. There seem to be some effort now to focus on it thought, so I'm hoping it'll get better soon.

Interesting, been using that plugin over a year in my Svelte project (not Sveltekit) with not to much drama.

[dummdidumm]

Does that same snippet work within a .ts file in Rider? If not, I fear that Jetbrains uses a different way to do TypeScript checking and doesn't support the specific way the types are written.

[VismaTobbe]

Does that same snippet work within a .ts file in Rider? If not, I fear that Jetbrains uses a different way to do TypeScript checking and doesn't support the specific way the types are written.

Yes, it works in plain .ts file, only a problem inside <script lang="ts">

[dummdidumm]

Then that sounds like another case where the plugin doesn't quite work right - though I remember that they just announced to use the svelte-language-server for diagnostics, which would mean it should work. Mhm.. Either way, nothing we can influence.

[MathiasFrost]

Type check in Rider (IntelliJ) is complaining about Argument type "save" is not assignable to parameter type any. It does pass svelte-check though. Maybe this is a separate ticket or even svelte plugin related?

  const eventDispatcher = createEventDispatcher<{ save: boolean }>();
  eventDispatcher("save", true);

That snippet should be working. JetBrain's Svelte plugin is unfortunately unusable at this moment and I would advise against using it; What you are experiencing is just one of many things that doesn't work. There seem to be some effort now to focus on it thought, so I'm hoping it'll get better soon.

Interesting, been using that plugin over a year in my Svelte project (not Sveltekit) with not to much drama.

Been using it myself at work for about 2 years on and off (have given up and used VSCode last 6 months), and I am sure on smaller projects you have a decent chance of not experiencing much issues, but dive into a larger project and you'll hit wall after wall:

• Like this
• And this
• And this
• And this
• Don't remember if there is an issue for it, but $$Generic or generics="T" is not recognized at all.
• The plugin hasn't been updated much since before SvelteKit 1.0, so open up a standard SvelteKit 1.0 project in JetBrains IDE and you'll see a lot of warnings and errors.
• Working with npm packages packaged with SvelteKit won't work with intellisense at all

If they can manage to use the svelte language server instead of their own it would help a lot, since without it they basically have to make their own from scratch. I'm not sure how they would do it, since JetBrain's language tools uses their own little programming language, but last WebStorm update they said Vue now uses Volar so maybe they figured it out (haven't tried Vue in a while, let alone with WebStorm).
Me and my collogue have opened a bunch of issues on YouTrack for these and have seen little to no activity on them last 2 years, but starting to see more now, so fingers crossed :)

[MathiasFrost]

@dummdidumm Do you mind me asking why in the playground for the TypeScript issue (and in svelte source code for that matter), you use [Type] extends [never] and not Type extends never. I am very curious.

[dummdidumm]

This blog post explains it quite nicely: https://www.zhenghao.io/posts/ts-never#how-to-check-for-never