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const type parameters don't work with mutable array constraints #51931
Description
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]
Activity
MartinJohns commentedon Dec 17, 2022
Just pointing out that
readonly any[]does work.ahejlsberg commentedon Dec 17, 2022
This is working as intended. See the last part of the description here for the rationale.
Andarist commentedon Dec 17, 2022
Oh, that... it is somewhat unexpected at the first glance but in retrospect, I understand this reasoning. But in such a case -
constmodifier 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?
Should the second one benefit from
const? Thereadonlymodifier seems to be always lost when spreading (existing behavior predatingconst) 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 commentedon Dec 20, 2022
This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
[-]`const` doesn't work with `extends`[/-][+]`const` doesn't work mutable array constraints[/+][-]`const` doesn't work mutable array constraints[/-][+]`const` type parameters don't work with mutable array constraints[/+]