Hidden overloaded operator in multiple inheritance

All,

I found in multiple inheritance only the first base class's overloaded
operators are visible. My overloaded operators has different signatures in
the different base classes. They are not ambiguous. 'using operator..'
statements can bring them back. Please see my minimal example below. I'd
like to know if it is a bug or a feature.

Thanks for your help,
Tamas

#include <iostream>
#include <string>

template <class Parent>
class basei
{
public:
    int bi;
    int operator + (int i)
    { bi += i; std::cout << "basei::operator+ : " << bi << std::endl; return
bi; }
    Parent& operator << (int i)
    { bi += i; std::cout << "basei::operator<< : " << bi << std::endl;
return (Parent&)*this; }
};
template <class Parent>
class bases
{
public:
    std::string bs;
    std::string operator + (std::string s)
    { bs += s; std::cout << "bases::operator+ : " << bs << std::endl; return
bs; }
    Parent& operator << (std::string s)
    { bs += s; std::cout << "bases::operator<< : " << bs << std::endl;
return (Parent&)*this; }
};
class comp1 : public basei<comp1>, public bases<comp1>
{
};
class comp2 : public bases<comp1>, public basei<comp1>
{
};
class comp3 : public basei<comp1>, public bases<comp1>
{
public:
    using basei::operator+;
    using basei::operator<<;
    using bases::operator+;
    using bases::operator<<;
};
int main(int argc, char* argv[])
{
std::string s;
int i = 0;
comp1 c1;
c1.bi = 0;
c1 << 1;
// c1 << std::string ("hello"); // C2679
i = c1 + 1;
// s = c1 + std::string (" world!"); // C2679
comp2 c2;
c2.bi = 0;
// c2 << 1; // C2679
c2 << std::string ("hello");
// i = c2 + 1; // C2679
s = c2 + std::string (" world!");
comp3 c3; // all good
c3.bi = 0;
c3 << 1;
c3 << std::string ("hello");
i = c3 + 1;
s = c3 + std::string (" world!");
return 0;
}