Results from the command line:

bash-2.05a$ gcc -v -save-temps Test-2.cpp
Reading specs from /usr/libexec/gcc/darwin/ppc/3.3/specs
Thread model: posix
gcc version 3.3 20030304 (Apple Computer, Inc. build 1435)
 /usr/libexec/gcc/darwin/ppc/3.3/cc1plus -E -D__GNUG__=3 -quiet -v -D__GNUC__=3 -
D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=0 -D__APPLE_CC__=1435 -D__DYNAMIC__ 
Test-2.cpp -fPIC -D__private_extern__=extern Test-2.ii
ignoring nonexistent directory "/usr/ppc-darwin/include"
ignoring nonexistent directory "/Local/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/gcc/darwin/3.3/c++
 /usr/include/gcc/darwin/3.3/c++/ppc-darwin
 /usr/include/gcc/darwin/3.3/c++/backward
 /usr/local/include
 /usr/include/gcc/darwin/3.3
 /usr/include
End of search list.
Framework search starts here:
 /System/Library/Frameworks
 /Library/Frameworks
End of framework search list.
 /usr/libexec/gcc/darwin/ppc/3.3/cc1plus -fpreprocessed Test-2.ii -fPIC -quiet -dumpbase 
Test-2.cpp -auxbase Test-2 -version -D__private_extern__=extern -o Test-2.s
GNU C++ version 3.3 20030304 (Apple Computer, Inc. build 1435) (ppc-darwin)
        compiled by GNU C version 3.3 20030304 (Apple Computer, Inc. build 1435).
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=131072
Test-2.cpp: In constructor `A<T>::A(const A<U>&) [with U = int, T = float]':
Test-2.cpp:26:   instantiated from here
Test-2.cpp:7: error: `int A<int>::_data' is protected
Test-2.cpp:10: error: within this context

The intermediate file:

# 1 "Test-2.cpp"
#pragma GCC set_debug_pwd "/Users/ktulu/Projects/Ktk/MacOs 10/Pb/Test"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "Test-2.cpp"
template <typename T>
class A
{
        friend class A<int>;
        friend class A<float>;
protected:
        T _data;
        inline A() : _data(0) {}
        template <typename U>
        inline A(const A<U>& r) : _data(r._data) {}


};

class B : public A<int>
{
public:
        inline B() {}
        inline B(const B& r) : A<int>(r) {}
};

class C : public A<float>
{
public:
        inline C() {}
        inline C(const B& r) : A<float>(r) {}
};


int main(int, char*[])
{
        B b1, b2(b1);
        C c(b1);
        return 0;
}

The original code snippet:

template <typename T>
class A
{
	friend class A<int>;
	friend class A<float>;
protected:
	T _data;
	inline A() : _data(0) {}
	template <typename U>
	inline A(const A<U>& r) : _data(r._data) {}
};

class B : public A<int>
{
public:
	inline B() {}
	inline B(const B& r) : A<int>(r) {}
};

class C : public A<float>
{
public:
	inline C() {}
	inline C(const B& r) : A<float>(r) {}
};

int main(int, char*[])
{
	B b1, b2(b1);
	C c(b1);
	return 0;
}