func f(x: Int?, y: Int?) {
    switch (x, y) {
    case (let x?, _):
        _ = x // use x
    case (_, let y?):
        _ = y // use y
    case (nil, nil): //  switch must be exhaustive, consider adding a default clause
        break
    }
}

func f(x: Int?, y: Int?) {
    switch (x, y) {
    case (let x?, _):
        _ = x
    case (_, let y?):
        _ = y
    case (let x?, let y?): // case will never be executed
        _ = x
        _ = y
    case (nil, nil):
        break
    }
}