Union types do not match narrowed overloads

[Haringat]

Bug Report

I am not sure if this is a bug or a design limitation, but when you have a function with narrowed overloads (see example) you cannot call the function with a broader union type although the union type does not go out of bounds of the overloads.

🔎 Search Terms

union types overloads

🕗 Version & Regression Information

• I was unable to test this on prior versions because of time restraints

⏯ Playground Link

Simplified example

💻 Code

function isOneOrTwo(it: unknown): it is (1 | 2) {
    return it === 1 || it === 2;
}

function doSomething(it: unknown) {
    if (isOneOrTwo(it)) {
        return doSomethingElse(it);
    } else {
        throw new Error("out of range");
    }
}

function doSomethingElse(it: 1): "one";
function doSomethingElse(it: 2): "two";
function doSomethingElse(it: number) {
    switch (it) {
        case 1:
            return "one";
        case 2:
            return "two";
        default:
            throw new Error("out of range");
    }
}

🙁 Actual behavior

The compiler reports an error on the call to doSomethingElse because the union type is not assignable to either of the overloads.

🙂 Expected behavior

The function is callable and returns a union type of the relevant overload's return values.

[MartinJohns]

Duplicate of #17471.

[typescript-bot]

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