class A {}
    
    // foo
    
    func foo<T>(_ v: T, handler: () -> Void) -> T {
        print("regular")
        return v
    }
    
    func foo<T>(_ v: T?, handler: () throws -> Void) rethrows -> T? {
        print("optional")
        return v
    }
    
    // bar
    
    func bar<T>(_ v: T, handler: () -> Void) -> T {
        print("regular")
        return v
    }
    
    func bar<T>(_ v: T?, handler: () -> Void) -> T? {
        print("optional")
        return v
    }
    
    let result1: A? = foo(A(), handler: {}) // error: ambiguous use of 'foo(_:handler:)'
    let result2: A? = bar(A(), handler: {}) // ok, "optional"