C++17: Template template parameter can be declared typename

Before C++17 , a template template parameter had to be declared class. Here is an example:

#include <complex>
#include <iostream>

//                +----- Template Parameter
//                |
//                V                          
template<typename T>
class A
{
 public:
  T mT = T();
}; 
//                                  +----- Template Template Parameter
//                                  |
//                                  V                          
template<  template<typename> class U  >
class B
{
 public:
  U<int> mU;
};
//                                  +----- Template Template Parameter
//                                  |
//                                  V                          
template<  template<typename> class U  >
class C
{
 public:
  U<std::complex<double>> mU;
};

int main()
{
  B<A> b;

  std::cout << b.mU.mT << " ";

  C<A> c;

  std::cout << c.mU.mT << std::endl;

  return 0;
}
// Output: 0 (0,0)

Reference: https://en.wikipedia.org/wiki/C%2B%2B17