Help with explicit instantiation

[E. Robert Tisdale]

       > cat X.h
       #ifndef GUARD_X_H
       #define GUARD_X_H 1
       #include <iostream>

       template<class T>
       class X {
       private:
         // representation
         T     D;
       public:
         // functions
         T data(void) const { return D; }
         // constructors
         X(T d = 0): D(d) { }
         X(const X& x);
         template<class S>
         X(const X<S>& x);
         friend
         std::ostream& operator<<(
         std::ostream& os, const X<T>& x) {
           return os << x.data();
           }
         };

       #endif//GUARD_X_H 1

       > cat X.cc
       #include "X.h"

       template<class T>
       X<T>::X(const X& x): D(x.D) {
         std::cerr << "copy constructor" << std::endl;
         }

       template<class T>
       template<class S>
       X<T>::X(const X<S>& x): D(x.data()) {
         std::cerr << "conversion constructor" << std::endl;
         }

       template X<float>::X(const X<int>& x);
       template X<float>::X(const X<float>& x);

       > g++ -Wall -ansi -pedantic -c X.cc
       X.cc:15: error: ambiguous template specialization `X<>' \
       for `X<float>::X(const X<float>&)'
       X.cc:4: error: candidates are: \
       X<T>::X(const X<T>&) [with T = float]
       X.h:17: error:                 \
       template<class S> X::X(const X<S>&) [with S = S, T = float]

What did I do wrong?