Type definition with two rest operations for two arrays doesn't validated correctly

[KingMario]

🔎 Search Terms

rest array

🕗 Version & Regression Information

N/A

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAggTnAhiA6gS2ACwHIFEA2EAthAHbADOAPACoA0U2UEAHsGQCYVSkCuRAIwhwAfFAC8UGgG0AulABkUAN6FSAcywAuRgF8A3AChDoSLATJ0WbAAV8vCgWJlKtBk1btSXHvyGiJKGkAOlD4JFQMHCcScmp6RhEGUOCZWVkjE3BoAFkQcORAgsjrOwcYl2o+QWEGABYRTIBjAHtSCmAoRAQABgBGHTziwOk+hgAmBgBmDMMgA

💻 Code

type ArrayWithNElements<T, N extends number> = T[] & {length: N};

type ArrayWithNPlusElements<T, N extends number> = [...ArrayWithNElements<T, N>, ...T[]];

type MyArray = ArrayWithNPlusElements<number, 4>;

const arr01: MyArray = [1, 2, 3];

🙁 Actual behavior

const arr01: MyArray = [1, 2, 3]; in the code is not validated as error.

While in the following code, arr02 is validated as error.

type NumberX4 = [number, number, number, number];
type MyArray2 = [...NumberX4, ...number[]];

const arr02: MyArray2 = [1, 2, 3];

🙂 Expected behavior

const arr01: MyArray = [1, 2, 3]; in the code should be validated as error.

Validation of MyArray should be an attempt of splitting it into two arrays to validate against each array rest operation.

Additional information about the issue

No response

[MartinJohns]

T[] & {length: N} is not a valid tuple type, it's an array. So when you spread it, it's spread as an array, and your ArrayWithNPlusElements<T, N> type ends up being just T[]. You can see this when hovering over the type. When you use a proper tuple type you don't have this issue, that's why the suggested solution for the "N Tuple type" is #54740 (comment).

[KingMario]

Thanks MartinJohns

I'm just reporting a bug that

either

typescript should be able to report an error while declaring the type ArrayWithNPlusElements<T, N> as the error reported for type MixedArray<T, U> = [...T[], ...U[]], which is, A rest element cannot follow another rest element.,

or

typescript should be able to validate arr01 as error.

[typescript-bot]

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

[KingMario]

Hi @RyanCavanaugh

IMHO, this is not a question. It's the buggy behavior of typescript compiler/language service that

either

typescript should be able to report an error while declaring the type ArrayWithNPlusElements<T, N> as the error reported for type MixedArray<T, U> = [...T[], ...U[]], which is, A rest element cannot follow another rest element.,

or

typescript should be able to validate arr01 as error.

@MartinJohns I can't @ you because you blocked me. But I'm not talking about Tuple at all as a subset of the Array type. I don't use Typle much because I think it's immature, whereas you can call pop so that the tuple type of [string, string] becomes [string] or [] without realizing it.