Parameters<> helper only shows the last declared overload

[m-gallesio]

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

[MartinJohns]

This is a design limitation and working as intended. Parameters<> is a conditional type using type inference, and these have trouble with overloads. See Inferring Within Conditional Types:

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types.

[m-gallesio]

Understood, thank you for your quick response.

[microsoft-github-policy-service]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.