Two consecutive variadic elements in the middle part of tuple type can't be recognized

[olegdunkan]

🔎 Search Terms

tuple, variadic element

🕗 Version & Regression Information

Since 4.0.5

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABMAPAFUQUwB5U2AEwGdFwBrMOAdzAG0BdAGkQDUtd9jSwLq76AfAApsALloAjOHAA2mAIZhmAOlUsVqtE0Srl8gE4BzIqJYBKRAG8AUIjt39mKCH1JsAbmsBfRNesQEIihEeUQAXmQhWih9EExmACZmACIJZOYYuO0ARhT5ZMQzd0QAehKAUX19OH1mHAAHTGhMAkQEkKMQAFt8KCJmCRBgwzhggGYgA

💻 Code

function f<U extends unknown[], V extends unknown[]>(x:[boolean, ...V, ...U], ...args:V) {
     return x;
} 

const a = f([true, 2, "b", true], 1, "a" ); //Error, expected 2 arguments, but got 3

🙁 Actual behavior

Compiler sees error

🙂 Expected behavior

No error, because as #39094 states

If the middle part of T is exactly two variadic elements ...A and ...B, and an implied arity exists for A, infer from a tuple consisting of the initial middle part of S to A and from a tuple consisting of the remaining middle part of S to B, where the length of the initial middle part corresponds to the implied arity for A.

As I understand the middle part of S is 2, "b", true and in the middle part of T we have ...V, ...U then 2, "b" is ...V and true is ...U.

Additional information about the issue

https://stackoverflow.com/posts/79784883/edit

[RyanCavanaugh]

I don't think there's an implied arity of A here? Not sure. cc @ahejlsberg

[olegdunkan]

function f<T extends unknown[], V extends unknown[]>(x:[...V, ...T], ...args:V) {
     return x; 
} 

const a = f([2, "b", true], 1, "a" ); //ok

In the case above we have implied arity of A, but we don't have starting fixed part of T

[Andarist]

I can confirm the above - A has to have the implied arity in this example, otherwise my proposed patch wouldn't do anything as the changed line is essentially gated with if (typeof A.impliedArity === 'number')