double(?) definition of template member operators

if I look into < complex> header from msvc 2008, I find that each operator definition is doubled with _Other type template operation, like for example

_Myt& operator+=(const _Myt& _Right)
{   // add other complex
   this->_Add(_Right);
   return (*this);
}
template<class _Other> inline
_Myt& operator+=(const complex<_Other>& _Right)
{   // add other complex
   this->_Add(_Right);
   return (*this);
}

The question is why would not the second definition alone suffice?

PS: It seems in gcc there is only second definition present, now I do not worry any more. :)开发者_Python百科

The first case also catches a right-hand-side that's convertible to _Myt.

class MyComplex {
   // ...
   public:
     operator std::complex<double>() const;
     operator std::complex<float>() const;
};
std::complex<double> i;
i += MyComplex(1,1); // Unambiguously uses the first form.