Allow signature help argument count to be equal to argument index

[Andarist]

fixes the issue reported here #58191 (comment)
a regression from #56372 , not fixed by #57637

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Andarist]

While it might look weird that this allows what usually isn't allowed - it all depends on the definition of those values. I think it makes sense to treat them as follows:

• argument index - a min argument index at the position
• argument count - a minimum count of known arguments

A spread element doesn't always increase such an argument count but the argument index at its positions gets increased

[DanielRosenwasser]

But what should an editor do when the argument index exceeds the count? The index has to be capped, doesn't it?

[jakebailey]

Out of bounds indexes are just ignored; on Python I did this intentionally, too (previously used a negative number until some rust-based LSP clients started getting picky). See: microsoft/language-server-protocol#1271

But, I'm more confused by how a spread element would change the number here; we're definitely indexing them correctly left to right, correct? Like if we have (x, {y, z}), we don't go out of bounds on z because it's "at index 2" but the number of real params is 2? I assume not.

[Andarist]

But what should an editor do when the argument index exceeds the count? The index has to be capped, doesn't it?

The added baseline here shows how we can still highlight the parameter in the signature - at least in this case 😉

But, I'm more confused by how a spread element would change the number here

function test(...args: any[]) {}

const twoNums = [2, 3] as const
test(1, ...twoNums)

In the situation above the rest adds 2 to the count (and not 1). See the PR that changed this: #56372

[fatcerberus]

Nitpick: That's a spread, not a rest. Rest is what you do when destructuring, spread is what you do at construction ("rest" doesn't make sense in that context because you can have multiple).

[jakebailey]

@typescript-bot pack this

[typescript-bot]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
pack this	✅ Started	✅ Results

[typescript-bot]

Hey @jakebailey, 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/161380/artifacts?artifactName=tgz&fileId=BCF59F43A5EC9B506AB89789B11D216B6577E1C070792765F4BBC471CF9C056502&fileName=/typescript-5.5.0-insiders.20240419.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@5.5.0-pr-58203-3".;