Bug Report

🔎 Search Terms

Parameters, Overload

🕗 Version & Regression Information

5.0.4

⏯ Playground Link

Playground link with relevant code

💻 Code

interface X {
    a(x: string): void;
    a(x: number): void;
}

type T = Parameters<X["a"]>[0];

let n: T = 1; // works
let s: T = "a"; // ERROR: Type 'string' is not assignable to type 'number'

🙁 Actual behavior

When using the Parameters<T> helper type on a function with overloads, the resulting type considers only the very last overload.

I noticed this while trying to extract tpe type of the second argument of String.replace which can be both a string and a replacer function; only the replacer function is seen.

🙂 Expected behavior

I would expect the previous code to consider valid all types which can be used for the function's parameters.
In the given example T should be resolved as string | number.

Possibly related to #53541