Can extends [any, any] but not generic which is exactly [any, any] (woks till 5.3.3 breaks from 5.4.5)

[zedryas]

🔎 Search Terms

generic, tuple extends

🕗 Version & Regression Information

This code works in the playground with typescript version 5.3.3 but breaks since version 5.4.5 (version between are not available in the playground)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.3#code/C4TwDgpgBAIghsOUC8UDacBcUCMAaKAI2wCYCA6SgY2wGY0BdBgWACg3RIoAVCAZ2DdwEHAB5uUCAA9gEAHYATPlDhyQjAHwoobKHp6SZ8pekrkAlnIBmEAE5QASodmLlGNQVUgGFSl8YMUAD8jlDYchAAbnYcwlCyAjjavAJCkGLwiBpsAPQ5+gB6QWyxXCmCwiTizsbK-j5QALJwfADWNa4qaoza7iCeagxaqLr6EtIuJmhmljb2ThO1TS2tvuT1gSFO4VExrJzQCcAkyfwVkFWZcNmseYVBQA

💻 Code

type Data = [a: 1, b: 2, ...c: 3[]]

type TestType1<T extends any[]> = 
 T extends [...infer R extends [any, any], ...any[]] ? R : never
type test1 = TestType1<Data>
// ^?

type TestType2<T extends any[], Mask extends any[] = [any, any]> = 
 T extends [...infer R extends Mask, ...any[]] ? R : never
type test2 = TestType2<Data>
// ^?

🙁 Actual behavior

When passing the Mask as a generic and trying to extends the tuple with it to extract the relevant part, the resulting tuple is well formed, but item types are replaced with any

🙂 Expected behavior

Expect that constraint on infer to works wether as an explicit form or a generic holding a type with the exact same form, which used to be the case priori to typescript 5.4 bu breaks since

Additional information about the issue

No response

[jcalz]

looks like this was caused by #57362

[Andarist]

I'll do some digging into it then 😛

[zedryas]

Mateusz Burzyński (@Andarist) a nice use case (using version 5.3.3)

[Andarist]

Extra test cases:

type Interpolable = string | number | bigint | boolean | null | undefined;

type Test1<
 T extends string,
 TOutput extends Interpolable = number,
> = T extends `${infer R extends TOutput}` ? R : never;
type result1 = Test1<`100`>;
// ^? actual: number, expected: 100 (like in 5.3)

type Test2<
 T extends string,
 TOutput extends Interpolable,
> = T extends `${infer R extends TOutput}` ? R : never;
type result2 = Test2<`100`, number>;
// ^? actual: number, expected: 100 (like in 5.3)

I think I roughly know how to fix this issue. Stay tuned 😉