feat(16656): as tuple type assertion

[graphemecluster]

Fix #16656
Fix #27179
Fix #29163
Fix #48052
Fix #49414

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #16656. If you can get it accepted, this PR will have a better chance of being reviewed.

[Jack-Works]

I expect tuple used as the primitive type for tc39 proposal https://github.com/tc39/proposal-record-tuple

[graphemecluster]

@Jack-Works Yea but I can’t think of a word better than this. After all, at least for the time being, the term tuple is used exclusively to mean array with length boundary or mixed types in the current TypeScript codebase.

And there are actually no conflicts at all:

const a = ["foo", 42, true] as tuple; // [string, number, boolean]
const b = #["foo", 42, true]; // ESTuple<["foo", 42, true]>

[jakebailey]

There's no conflict as of now, but if tuple is defined as a primitive type, it would need to be able to be written in type assertions like foo as unknown as tuple, and that is definitely a conflict.

[graphemecluster]

@jakebailey But I guess you need some type parameters for the tuple type right?

… Anyway I’d like to ask what keyword would the Team prefer then?

[sandersn]

To help with PR housekeeping, I'm going to close this PR since there's still design work to be done on the original issue.