Type inference issue with overload signatures and string literal types

[marcomura]

TypeScript Version: 2.2.1 / nightly (2.2.0-dev.201xxxxx)

Code

interface Base {
    key: string;
}
interface A extends Base {
}
interface B extends Base {
}

export function test(key: "aa"): A;
export function test(key: "bb1"): B;
export function test(key: "bb2"): B;

export function test(key: string): Base {
    return { key: key };
}

export function bbTest(key: "bb1" | "bb2"): B {
    return test(key);
}

playground url:
here

Expected behavior:
No compiler errors; type inference to understand that "return test(key)" returns a B type.

Actual behavior:
"return test(key)" doesn't compile because of an argument type error

[aluanhaddad]

This is working correctly since the overloads only differ by the type of one argument but have the same return type.

You should rather write

export function test(key: "aa"): A;
export function test(key: "bb2" | "bb1"): B;
export function test(key: string): Base {
    return { key: key };
}

See here https://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html for more information (end of page)

[marcomura]

I'm sorry @aluanhaddad, but no, it's not working correctly.
It doesn't make sense to define the test() overloads as you described, because I could have way more definitions and exponentials permutations.

E.g.

export function test(key: "bb1"): B;
export function test(key: "bb2"): B;
export function test(key: "bb3"): B;
export function test(key: "bb4"): B;

export function testB1(key: "aa1" | "aa2"): B { return test(key); }
export function testB2(key: "aa1" | "aa3"): B { return test(key); }
export function testB3(key: "aa1" | "aa2" | "aa3"): B { return test(key); }
export function testB4(key: "aa2" | "aa3" | "aa4"): B { return test(key); }
export function testB5(key: "aa1" | "aa2" | "aa3" | "aa4"): B { return test(key); }
// etc ...

Typescript has all the information at compile time to understand that calling test() with an argument of type "bb1" | "bb2" will surely produce a return of type B, so it's definitely an issue that it's not working right now.

Thanks

[mhegazy]

Duplicate of #14107

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.