TypeScript unable to infer types properly between interfaces and overloaded methods.

[benlesh]

Bug Report

In all versions of TS I can find, there's no way to mark certain call patterns of a method as deprecated while maintaining inference with certain interfaces. If an interface is split out into different overloads so certain overloads can be deprecated, then it breaks inference in some cases.

🔎 Search Terms

I'm not sure what to search for here. Interfaces?

🕗 Version & Regression Information

• All versions

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Observer<T> {
    next(value: T): void;
    error(error: any): void;
    complete(): void;
}

interface Unsubscribable {
    unsubscribe(): void;
}

interface Subscribable<T> {
    subscribe(observer?: Partial<Observer<T>>): Unsubscribable;
}

class Observable<T> implements Subscribable<T> {
    /////////////////////////////////////////
    // Valid call pattern
    /////////////////////////////////////////
    subscribe(observerOrNext?: Partial<Observer<T>> | ((value: T) => void)): Unsubscribable;


    //////////////////////////////////////////
    // Deprecated call patterns
    //////////////////////////////////////////
    /**
     * @deprecated This call pattern is going away
     */
    subscribe(next: null | undefined, error: null | undefined, complete: () => void): Unsubscribable;
    /**
     * @deprecated This call pattern is going away
     */
    subscribe(next: null | undefined, error: (error: any) => void, complete?: () => void): Unsubscribable;
    /**
     * @deprecated This call pattern is going away
     */
    subscribe(next: (value: T) => void, error: null | undefined, complete: () => void): Unsubscribable;
    /**
     * @deprecated This call pattern is going away
     */
    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Unsubscribable;


    ////////////////////////////////////////////
    // Impl
    ////////////////////////////////////////////
    subscribe(...args: any[]): Unsubscribable {
        return {
            unsubscribe() {}
        };
    }
}


function somethingThatTakesSubscribable<T>(arg: Subscribable<T>): T {
    return null!;
}

/////////////////////////////////////
// Our failed expectation here
// (it's unknown)
/////////////////////////////////////
const result = somethingThatTakesSubscribable(new Observable<number>()); // $ExpectType number

🙁 Actual behavior

result is unknown above.

🙂 Expected behavior

result would be number.

[benlesh]

@DanielRosenwasser I'm not sure if the Needs Proposal means I'm supposed to propose a solution, or there just needs to be some discussion on your team?

[DanielRosenwasser]

It's either - I'm going to try to discuss this a bit on our side, but I don't know if we have a solution.