Inferred subtuple types in extends expression not narrowed if variadic (i.e. has a rest element)

[dregre]

Bug Report

Given the following behavior, which correctly narrows a subtuple:

One would expect similar narrowing when a tuple involves a rest element, but no narrowing occurs:

Fixing this bug would be useful as a non-recursive method of grabbing a subtuple of a predefined length, while also maintaining the subtuple's labels—which I think cannot be done today.

🔎 Search Terms

narrowing, tuple, extend, variadic

🕗 Version & Regression Information

Typescript 4.7, 4.8, and 4.9.0-dev.20221004

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about tuples (and labeled tuples), conditional types, type inference.

⏯ Playground Link

Playground link with relevant code

💻 Code

// Narrows as expected
type SubTup2FixedLength<T extends unknown[]> =
T extends [...(infer B extends [any, any]), any]
? B
: never;


type SubTup2FixedLengthTest = SubTup2FixedLength<[a: 0, b: 1, c: number]>;


// Narrowing not occurring as expected 👇
type SubTup2Variadic<T extends unknown[]> =
T extends [...(infer B extends [any, any]), ...any]
? B
: never;

type SubTup2VariadicTest = SubTup2Variadic<[a: 0, b: 1, ...c: number[]]>;

🙁 Actual behavior

🙂 Expected behavior

[a: 0, b: 1]