Allow satisfies keyof assertions in computed property names

[weswigham]

Fixes #58800

This implements the proposal in the linked issue, along with the addendum about invalid satisfies keyof property types still getting unique, singular property names (which ensures declaration file output always includes the computed names, even when they don't resolve).

We can still bikeshed the syntax if anyone really wants, but I think this probably fits the bill without actually introducing anything too far off of what we already allow. Though yes, 15 characters is a lot - that's satisfies for you.

The open question I think worth answering is if satisfies keyof should be allowed in all expression positions and not just in computed property names. It's certainly at least useful as a shorthand for satisfies (string | number | symbol) (though the check should be a wee bit stronger than that, since it also checks unit-ness), which has a similar effect on literals to as const, without readonly-ing or const-ing array/object types (and instead erroring on them). It's probably worth noting, while generics are generally forbidden, given T extends "a" | "b", T & "a" simplifies to "a" under current rules, so presents as a unit type - making it the only generic form, afaik, that can pass the unit-type-key check.

[typescript-bot]

Looks like you're introducing a change to the public API surface area. If this includes breaking changes, please document them on our wiki's API Breaking Changes page.

Also, please make sure @DanielRosenwasser and @RyanCavanaugh are aware of the changes, just as a heads up.

[weswigham]

@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 @weswigham, 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/162167/artifacts?artifactName=tgz&fileId=3CD46A7AA8AD3812EBDD66F53B563E5CC213528DD504920028F176DBB3DC40A502&fileName=/typescript-5.6.0-insiders.20240611.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@5.6.0-pr-58829-3".;

[fatcerberus]

Though yes, 15 characters is a lot - that's satisfies for you.

That's not actually my main complaint, it's a more general concern than that. satisfies is documented as not affecting the types of expressions (except via contextual typing, which people already complain about)--it's just a type-checking mechanism--so through that lens, { [x]: "foo" } and { [x satisfies keyof]: "foo" } should be completely equivalent assuming they both typecheck - and yet they may produce completely semantically different declarations!

tl;dr This feels like a confusing overloading of the satisfies keyword.

The open question I think worth answering is if satisfies keyof should be allowed in all expression positions and not just in computed property names. It's certainly at least useful as a shorthand for satisfies (string | number | symbol)

If you do go that route, then I'd suggest to go all the way and just make keyof with no argument basically an alias for PropertyKey.

[lazytype]

I think logically satisfies keyof makes more sense, but aesthetically I think I'd be happy with re-fitting the deferred prefix operator introduced in #58729 if it goes through.

class Foo {
  [deferred x]: ...;
}
vs
class Foo {
  [x satisfies keyof]: ...;
}