import MachO

enum Lock {
    @available(macOS 10.12, *)
    case unfairLock(os_unfair_lock_t)

    func lock() {
        switch self {
        case let .unfairLock(lock):
            // Unreachable in macOS version <10.12, but
            // `#available` is still required here for a green light
            // from the compiler.
            os_unfair_lock_lock(lock)
        }
    }
}