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

Inizia con il TypeScript playground collegato e confronta il comportamento dell’inferenza condizionale delle tuple in 5.3.3 e 5.6.3, concentrandoti sul caso generico Mask rispetto al vincolo esplicito [any, any]. Il lavoro è completato quando TestType2 preserva i tipi degli elementi della tupla nello stesso modo di TestType1, senza regressioni nel comportamento segnalato.