Add check for delete expression must be optional

[Kingwl]

Fixes #13783

[j-oliveras]

Is needed a test with an index type? Like:

interface Bar {
    [id: string]: string;
}

declare const a: Bar

delete a["a"];
delete a.b;

[Kingwl]

Is needed a test with an index type? Like:

Good point.

@typescript-bot pack this.

[Kingwl]

Hey, How are you? @typescript-bot

@typescript-bot pack this.

[typescript-bot]

Heya @Kingwl, I've started to run the tarball bundle task on this PR at 6e4b381. You can monitor the build here.

[typescript-bot]

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/71131/artifacts?artifactName=tgz&fileId=48169EB321B4086E6AE66D6A286AB7E7D6AA2CCF9BF40D7869FAD1ED36C17C9602&fileName=/typescript-3.9.0-insiders.20200413.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build.

[DanielRosenwasser]

This is a breaking change that was added in post beta with no test run on it. I think we should have had more due diligence before pulling it in. I think we should back it out and add it in for 4.0.

[simhnna]

What's the reasoning behind excluding null from this list?

the operand must now be any, unknown, never, or be optional (in that it contains undefined in the type)

[DanielRosenwasser]

Because delete doesn't leave behind a null.

[rohitf]

Seeing a possible bug for the following case - even though TS recognizes that key in delete p[key] can be either 'foo' or 'bar', it does not error.

interface Props {
    foo: string;
    bar: string;
}
const p: Props = {
    foo: 'hello',
    bar: 'world'
}

const keys = ['foo', 'bar'] as const;
keys.forEach(key => delete p[key]) // Should error since foo and bar are required props

Playground Link