guarded_thread类：移动赋值运算符错误

Question

Stroustrup演示了一个guarded_thread类，它继承自线程并在其上使用RAII（“The C ++ Programming Language”，4th Ed，pg1215）：

struct guarded_thread : std::thread
{
   /// inherit the thread class' constructor
   /// [CPL], $20.3.5.1
   using std::thread::thread;

   ~guarded_thread()
   {
      if (joinable())
         join();
   }
};

我使用代码（也由Stroustrup提供）测试了这个：http://coliru.stacked-crooked.com/a/da67ae51157bcbe4

/// simple use of a guard
void run(int i, int n)
{
   guarded_thread t1 {f};
   guarded_thread t2;

   vector<T> v;

   /// ...

   if (i < n)
   {
      thread t3 {g};

      /// ...

      /// move t3 to outer scope
      t2 = move(t3);
   }

   /// might throw
   v[i] = T {};

   /// ...

   cout << "i="  << i
        << " n=" << n
        << "  OK" << endl;
}

但是，测试仪发生以下编译错误：

main.cpp:51:10: error: no viable overloaded '='

      t2 = move(t3);
      ~~ ^ ~~~~~~~~

./guarded_thread.h:12:8: note: candidate function (the implicit copy assignment operator) not viable: no known conversion from 'typename remove_reference<thread &>::type' (aka 'std::__1::thread') to 'const guarded_thread' for 1st argument

struct guarded_thread : std::thread
       ^
1 error generated.

guarded_thread类也不能继承线程类的赋值运算符，因为后者是私有的。

如何解决此错误？