If you define BREAK_IT, this doesn't compile. I'm not sure why.

broken_templates.cpp

#include <iostream>
using namespace std;

template <typename T>
class A;

template <typename T>
class B;

template <typename T>
class A {
public:
	T data;
	A() { data = 0; }
	A(T data_) { data = data_; }
	A(const A<T> &src) { data = src.data; }
	const A<T> &operator=(const A<T> &rhs) { data = rhs.data; return *this; }

	template <class U> A<U> *copy( ) { return new A<U>(data); }
	template <class U> B<U> *to_B( ); 
};

template <typename T>
class B {
public:
	T data;
	B() { data = 0; }
	B(T data_) { data = data_; }
	B(const B<T> &src) { data = src.data; }
	const B<T> &operator=(const B<T> &rhs) { data = rhs.data; return *this; }

	template <class U> A<U> *to_A( );
	template <class U> B<U> *copy( ) { return new B<U>(data); }
};

template <typename T> template <typename U>
B<U> *A<T>::to_B( ) {
	return new B<U>(data);
}

template <typename T> template <typename U>
A<U> *B<T>::to_A( ) {
	return new A<U>(data);
}

#ifdef BREAK_IT
template <typename T>
class C {
public:
	C(T data_) { data = data_; }

	template <typename U> void x(A<U> &a) {
		a.data = data;
	}

	template <typename U> void x(B<U> &b) {
		A<U> tmp;
		x(tmp);	/* tmp.data = data */
		B<U> *tmp2 = tmp.to_B<U>( ); /* tmp2->data = tmp.data */
		b = *tmp2; /* b.data = tmp2->data */
		delete tmp2;
	}

	T data;
};
#endif

template <typename U> B<U> *copy_b(B<U> &b) {
	return b.copy<U>( );
}

int main( ) {
	A<float> a(2.0f);
	B<float> b(2.0f);

	A<int> *a2 = b.to_A<int>( );
	B<int> *b2 = a.to_B<int>( );
	B<float> *b3 = copy_b(b);

	cout << "a.data=" << a.data << endl;
	cout << "b.data=" << b.data << endl;
	cout << "a2->data=" << a2->data << endl;
	cout << "b2->data=" << b2->data << endl;
	cout << "b3->data=" << b3->data << endl;
	
	delete a2;
	delete b2;
	delete b3;

#ifdef BREAK_IT
	C<int> c(1);
	c.x(a);
	c.x(b);

	cout << "a.data=" << a.data << endl;
	cout << "b.data=" << b.data << endl;
#endif
}