C++ partial specialization with multiple optional parameters

Question

Try a partial specialization with multiple parameters. Sounds like

template <class T, int E=0>
class Info
{
public:
  Info& operator=(double rhs);
  T data;
};


template<int E> Info<std::string, E> &Info<std::string, E>::operator=(double rhs) {
    data = rhs;
    return *this;
}

But cannot compile and get

error: invalid use of incomplete type 'class
info<std::__cxx11::basic_string<char>, E>'  template<int E>
Info<std::string, E> &Info<std::string, E>::operator=(double rhs)...

Update
It seems that it is not possible to do partial spec. with multiple parameters (optional or not...) Am I wrong ?