#include <iostream>
    #include <iterator>
    #include <sstream>

    using namespace std;

    namespace N
    {
      class C
      {
      public: enum class E { a, b, c };
      };
    }

    using U = N::C::E;

    istream& operator>>( istream& is, U& e )
    {
      //...
      return is;
    }

    int main()
    {
      istringstream ss{ "0 1 2" };

      U e;
      ss >> e; // fine

      istream_iterator<U> ii( ss ); // fails
    }