let f: () -> Void = { print("hi") }
f()
//f(()) // ERROR

// let f2: (()) -> Void = { print("hi") } // ERROR
let f2: (()) -> Void = { _ in print("hi2") }
//f2() // ERROR
f2(())
f2((()))
f2(((())))

let f3: ((())) -> Void = { _ in print("hi3") }
//f3() // ERROR
f3(())
f3((()))
f3(((())))

//let g: (Void) -> Void = { print("void") } // ERROR
let g: (Void) -> Void = { _ in print("void") } // WARNING: When calling this function in Swift 4 or later, you must pass a '()' tuple; did you mean for the input type to be '()'?
//g() // ERROR
g(())
g((()))
g(((())))