`const` type parameters don't work with mutable array constraints

[diegohaz]

Bug Report

I'm not sure if this is intended behavior. If this is not a bug, I'd suggest supporting this as a feature.

🔎 Search Terms

const, args, generic args, extends

🕗 Version & Regression Information

v5.0.0

⏯ Playground Link

Playground link with relevant code

💻 Code

declare function fn<const T extends any[]>(...args: T): T;

const a = fn("a", false);

🙁 Actual behavior

The return type is any[]

🙂 Expected behavior

The return type should be ["a", false]

[MartinJohns]

Just pointing out that readonly any[] does work.

[ahejlsberg]

This is working as intended. See the last part of the description here for the rationale.

[Andarist]

Oh, that... it is somewhat unexpected at the first glance but in retrospect, I understand this reasoning. But in such a case - const modifier with a mutable array constraint doesn't make sense. Could we add a diagnostic about this?

Those two are also interesting but I don't have a clear understanding of what a spread does to a type parameter, so maybe both here are exped. Could you take a look?

declare function fn<const T extends any[]>(...args: [...T]): T;

const a = fn("a", false);
//    ^? [string, boolean]

declare function fn2<const T extends readonly any[]>(...args: [...T]): T;

const a = fn2("b", true);
//    ^? [string, boolean]

Should the second one benefit from const? The readonly modifier seems to be always lost when spreading (existing behavior predating const) and that's probably why the first one doesn't "fallback" to the constraint type but perhaps the inferred elements for the second one should be narrowed down here?

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.