ReturnType<SomeClass["someMethodWithOverloads"]> incorrect result

[david-fong]

Bug Report

🔎 Search Terms

• ReturnType
• overload

🕗 Version & Regression Information

• This is the behavior in every version I tried (on the playground, the lowest version is 3.3.3), and I reviewed the FAQ for entries about function and method typings.

⏯ Playground Link

Playground link with relevant code

💻 Code

class Class {
    public method(param: 0): boolean;
    public method(param: 1): string;
    public method(param: 2): this;
    public method(param: number): boolean | string | this {
        switch (param) {
            case 0: return true;
            case 1: return "string";
            case 2: return this;
            default: throw new Error();
        }
    }
}
type TypeAlias = ReturnType<Class["method"]>; // is `Class`. expected `boolean | string | Class`;

🙁 Actual behavior

If you shuffle the order of the overload definitions, the last one (the non-implementation one) becomes the type given by ReturnType<Class["method"]>.

🙂 Expected behavior

The return type indicated by the method signature that provides the function body should be given by ReturnType<Class["method"]>

[MartinJohns]

Conditional types do not work with overloads. This is mentioned in the documentation.

[david-fong]

Conditional types do not work with overloads. This is mentioned in the documentation.

I see. I wish the Conditional Types page on the typescript website made this clearer. Thank you, Martin for explaining and sorry I didn't catch this before I made this issue! Feel free to close it.

[david-fong]

For anyone that stumbles upon this issue I mistakenly made, here are a couple of related links:

• https://stackoverflow.com/a/52760599/11107541
• Can not get return type when util.promisify an function with overloading definations #26048
• ConstructorParameters<T> cannot get correct types of overloaded constructor #37079

I clearly was not looking hard enough. Again, sorry.

[MartinJohns]

I wish the Conditional Types page on the typescript website made this clearer.

Type inference in 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.

But I've noticed that the v2 handbook (which is marked as work in progress) does not mention this constraint at all.