Add check for delete expression must be optional

[Wenlu Wang (Kingwl)]

Fixes#13783

[Jordi Oliveras Rovira (j-oliveras)]

Is needed a test with an index type? Like:

interfaceBar{[id: string]: string;}declareconsta: Bardeletea["a"];deletea.b;

[Wenlu Wang (Kingwl)]

Is needed a test with an index type? Like:

Good point.

TypeScript Bot (@typescript-bot) pack this.

[Wenlu Wang (Kingwl)]

Hey, How are you? TypeScript Bot (@typescript-bot)

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 6e4b381. 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/71131/artifacts?artifactName=tgz&fileId=48169EB321B4086E6AE66D6A286AB7E7D6AA2CCF9BF40D7869FAD1ED36C17C9602&fileName=/typescript-3.9.0-insiders.20200413.tgz"
}
}

and then running npm install.

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There is also a playground for this build.

[Daniel Rosenwasser (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.

[Simon Hanna (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)

[Daniel Rosenwasser (DanielRosenwasser)]

Because delete doesn't leave behind a null.

[Rohit Falor (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

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