Relax requirements on index signatures to 'any' when a type also contains a string index signature to 'any'

[DanielRosenwasser]

Fixes #43064

---

When checking compatibility between two types, TypeScript should not require a corresponding source index signature if the target side index signature maps to any and the target side has a string index signature that maps to any.

So an index signature like in

{ [x: number]: any }

is still required of a source type, but neither index signature in

{ [x: number]: any, [x: string]: any; }

should be required; however, the number index signature in

{ [x: number]: number, [x: string]: any; }

should always be required because the number index signature doesn't map to any.

---

TL;DR:

Given type SomeObject = { hello: string, world: number }:

• ✅ SomeObject -> { [x: string]: any }
• ✅ SomeObject -> { [x: string]: any, [x: number]: any }
• ❌ SomeObject -> { [x: number]: any }
• ❌ SomeObject -> { [x: string]: any, [x: number]: string }

[DanielRosenwasser]

@typescript-bot pack this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the tarball bundle task on this PR at bb8f30c. You can monitor the build here.

[typescript-bot]

Hey @DanielRosenwasser, something went wrong when looking for the build artifact. (You can check the log here).

[DanielRosenwasser]

@typescript-bot pack this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the tarball bundle task on this PR at bb8f30c. You can monitor the build here.

[typescript-bot]

Hey @DanielRosenwasser, 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/97536/artifacts?artifactName=tgz&fileId=2157C5B3D6404A7C57C808E06F23B23DF3A70958FF7A37FE60209F4EA88D2ED102&fileName=/typescript-4.3.0-insiders.20210303.tgz"
    }
}

and then running npm install.

[DanielRosenwasser]

One thing I just thought of - when #26797 goes in, I think it would be best not to relax the requirements to other index signatures since this is just meant to unbreak code.

[DanielRosenwasser]

This might be questionable. This used to work in 4.2, but this PR doesn't un-break this code.

[DanielRosenwasser]

This is the example that's been un-broken.