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

🔎 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

Start with the linked TypeScript playground and compare the conditional tuple inference behavior in 5.3.3 and 5.6.3, focusing on the generic Mask case versus the explicit [any, any] constraint. Done means TestType2 preserves the tuple element types in the same way as TestType1 without regressing the reported behavior.