String(number) does not return `${number}` type

Bug Report

🔎 Search Terms

string constructor number type template literal

🕗 Version & Regression Information

4.2.3

⏯ Playground Link

Playground link with relevant code

💻 Code

const stringNumber: `${number}` = String(123)

🙁 Actual behavior

Error: Type 'string' is not assignable to type '${number}'.(2322)

🙂 Expected behavior

No error.

Why

String, according to ES (String(value), and ToString(argument)), uses toString method on passed in values, so ideally the fix should include only changes on Number interface, but currently it's String's def does not care about toString, so these two signature would have to be generic:

// lib.es5.d.ts
interface StringConstructor {
    new(value?: any): String;
    (value?: any): string;
}

And this one would need to be generic as well as the type of returned string would depend on the radix.

// lib.es5.d.ts
interface Number {
    toString(radix?: number): string;
}

This approach would make String easily extensible and compatible with template literal types, but I'm not sure how drastic of a change it is.

Most likely would be related to https://github.com/microsoft/TypeScript/issues/42996

Start by reproducing the assignment in the linked TypeScript Playground, then inspect the StringConstructor and Number definitions in lib.es5.d.ts and read related issue #42996. Done means reaching an agreed type-safe behavior for String(123) and Number.toString with template literal types, supported by appropriate compiler tests.