text: //===----------------------------------------------------------------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is dual licensed under the MIT and the University of Illinois Open
  // Source Licenses. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  
  // <mutex>
  
  // template <class Mutex> class lock_guard;
  
  // explicit lock_guard(mutex_type& m);
  
  #include <mutex>
  #include <thread>
  #include <cstdlib>
  #include <cassert>
  
  std::mutex m;
  
  typedef std::chrono::system_clock Clock;
  typedef Clock::time_point time_point;
  typedef Clock::duration duration;
  typedef std::chrono::milliseconds ms;
  typedef std::chrono::nanoseconds ns;
  
  void f()
  {
      time_point t0 = Clock::now();
      time_point t1;
      {
      std::lock_guard<std::mutex> lg(m);
      t1 = Clock::now();
      }
      ns d = t1 - t0 - ms(250);
      assert(d < ms(200));  // within 200ms
  }
  
  int main()
  {
      m.lock();
      std::thread t(f);
      std::this_thread::sleep_for(ms(250));
      m.unlock();
      t.join();
  }
  