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