Throw types

[Jack Works (Jack-Works)]

Please move to #40468

[Ali Taheri Moghaddar (alitaheri)]

This is great, it can help develop and debug complicated type systems much more efficiently. I really hope it lands along with it's base PR on 4.1 😍

[Amin Pakseresht (aminpaks)]

Why would you use typeof keyword in

type MustNumber<T> = T extends number ? T : throw `Expected, but found "${typeof T}"`

T already is a type in that expression.

[Jack Works (Jack-Works)]

It's can be changed to another keyword if there is a better alternative

[Misha Kaletsky (mmkal)]

Jack Works (@Jack-Works) this looks awesome. Is there a playground link we can try? I have a type assertions library (expect-type) and I'd like to see if I can use this to improve the error messages.

[Jack Works (Jack-Works)]

Wenlu Wang (@Kingwl) hi can you let the bot to pack this?

[Wenlu Wang (Kingwl)]

sure. TypeScript Bot (@typescript-bot) pack this

[TypeScript Bot (typescript-bot)]

Heya Wenlu Wang (@Kingwl), I've started to run the tarball bundle task on this PR at 198754f. You can monitor the build here.

[Jack Works (Jack-Works)]

Oh seems like the bot won't pack this until no test error, I'll handle this later

Now it can pack without error, sorry Wenlu Wang (@Kingwl) can you call the bot again?

[Wenlu Wang (Kingwl)]

As your wish. :XD

TypeScript Bot (@typescript-bot) pack this.

[TypeScript Bot (typescript-bot)]

Heya Wenlu Wang (@Kingwl), I've started to run the tarball bundle task on this PR at f1b195d. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Hey Wenlu Wang (@Kingwl), I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/84410/artifacts?artifactName=tgz&fileId=89E67B25A073B99041FC4982F2230A04CEECE579CB68839CB8E82E1316023AB202&fileName=/typescript-4.1.0-insiders.20200908.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build.

[Orta Therox (orta)]

I'm gonna re-pack, a bunch of PRs got merged on monaco / monaco-ts last night which broke the playground build - I think I fixed it.

TypeScript Bot (@typescript-bot) pack this.

[Orta Therox (orta)]

Third time lucky ( see #40445 )

TypeScript Bot (@typescript-bot) pack this.

[TypeScript Bot (typescript-bot)]

Heya Orta Therox (@orta), I've started to run the tarball bundle task on this PR at f1b195d. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Hey Orta Therox (@orta), I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/84651/artifacts?artifactName=tgz&fileId=F1E73F036BD00B0E703FB5AC26800759E4D8AAA8CB6B9FA6479472D55EF1703002&fileName=/typescript-4.1.0-insiders.20200909.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build.

[Misha Kaletsky (mmkal)]

Jack Works (@Jack-Works) trying this out on my library now, and it's working beautifully. I did notice one small issue, though. Throw types for generic functions inside generic interfaces can get replaced with Unknown, if the throw type uses both generics. There may be other ways to repro it, but here's a reasonably simple illustrative example:

interface Example<T> {
  tOnly: <U>() => throw `T: ${typeof T}`
  uOnly: <U>() => throw `U: ${typeof U}`
  tAndU: <U>() => throw `T: ${typeof T}, U: ${typeof U}`
}

declare const ex: Example<{foo: string}>

ex.tOnly<{bar: string}>() // Error looks good: Type instantiated results in a throw type saying: T: { foo: string; }

ex.uOnly<{bar: string}>() // Error looks good: Type instantiated results in a throw type saying: U: { bar: string; }

ex.tAndU<{bar: string}>() // Error looks bad: Type instantiated results in a throw type saying: Unknown

playground link

[Jack Works (Jack-Works)]

Thanks Misha Kaletsky (@mmkal) , I'll try to investigate the bug later

[Jack Works (Jack-Works)]

I found another potential usage. This PR might be able to resolve #18433 (Dangerous "name" global).
If we mark it as declare var name: throw "Error-prone deprecated global variable" and emit type error once it referenced, the problem is gone.

[Jack Works (Jack-Works)]

hi everyone please move to #40468 cause the base branch has been merged so this PR is closed automatically.

[Jack Works (Jack-Works)]

Misha Kaletsky (@mmkal) the bug has been fixed!