Problem with expression templates of boost::mp

expression.cpp

#include <boost/multiprecision/cpp_int.hpp>

typedef boost::multiprecision::cpp_rational Rat;

struct E {
  Rat rat;
  E(const Rat& rat)
    : rat(rat)
  {}
};


int main()
{
  Rat r(1,3);

  E e = 2 * r;

  return 0;
}