[Regression 3.9 -> 4.X] Variadic tuple type is not correctly inferred

Bug Report

🔎 Search Terms

variadic tuple, regression, inference

🕗 Version & Regression Information

• This changed between versions 3.9.X and 4.X.X

⏯ Playground Link

Typescript 3.9 (works as expected)

Typescript 4.0 (does not work any more)

💻 Code

type Sink<T> = (t: number, payload: T) => void;

type Producer<T> = (t: number, sink: Sink<T>) => void;

type ExtractContent<T extends [Producer<unknown>, ...Producer<unknown>[]]> = {
    [k in keyof T]: T[k] extends Producer<infer U> ? U : never;
}

function combineWith<T extends [Producer<unknown>, ...Producer<unknown>[]], U>(
    fn: (...args: ExtractContent<T>) => U,
    ...sources: T
): Producer<U> {
    return undefined as any;
}

function of<T>(n: T): Producer<T> {
    return undefined as any;
}

const x = combineWith((x, y) => x + y, of(1), of(2));

With typescript 3.9, the arguments of the lambda passed to combineWith are correctly read as (number, number). With 4.0 everything is just unknown, even the Producer arguments later

🙁 Actual behavior

x and y (of the lambda function) are types unknown

🙂 Expected behavior

x and y (of the lambda function) are types number

Beginne mit den verlinkten Playground-Reproduktionen für TypeScript 3.9 und 4.0 sowie den Definitionen von combineWith, ExtractContent, Producer und of im Issue. Vergleiche das Inferenzverhalten zwischen den Versionen und bestätige den Abschluss, wenn die Callback-Parameter und die späteren Producer-Argumente als number statt als unknown inferiert werden.