"Non-void function should return a value" when capturing Self in Swift 4.2 guard statement in closure in function

Question

How can we use the Swift 4.2 style of capturing self in closures in functions that returns a type?

For example:

func checkEmailExists(_ email: String) -> Observable<Bool> {
    return Observable.create { [weak self] observer in
        guard let self = self else { return }
        self.callAPI()
    }
}

This generates an error Non-void function should return a value but also it should not return false as this will influence the outcome of the function call.

EDIT: Another example, throws Non-void function should return a value

func loginWithCredentials() -> Observable<User> {
    return Observable.create { [weak self] observer in
        guard let self = self else { return }

        let decoder = JSONDecoder()
        let json = ["id": 1, "name": "John Doe", "email": "[email protected]"] as [String: Any]

        do {
            let user = try decoder.decode(User.self, from: json.jsonData()!)
            observer.onNext(user) // Simulation of successful user authentication.
        } catch {
            print("error")
        }

        return Disposables.create()
    }
}

Answer 1

You have to return a disposable in any case.

func checkEmailExists(_ email: String) -> Observable<Bool> {
    return Observable.create { [weak self] observer in
        guard let self = self else { return Disposables.create() }
        // and so on...
        return Disposables.create { /* cancelation code */ }
    }
}

This feels very wrong though. Your function callAPI() should itself be returning an Observable of some sort which would make this function pointless.