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 😍

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.

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

@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.

@Kingwl hi can you let the bot to pack this?

sure. @typescript-bot pack this

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

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 @Kingwl can you call the bot again?

As your wish. :XD

@typescript-bot pack this.

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

Hey @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.

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 pack this.

Third time lucky ( see #40445 )

@typescript-bot pack this.

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

Hey @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.

@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

Thanks @mmkal , I'll try to investigate the bug later

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.

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

@mmkal the bug has been fixed!