How to handle multi-type variables with templates?

I was trying to make a function that assigns y to x regardless whether x, y are int or std::string. I wrote this code:

#include <iostream>
#include <string>
#include <typeinfo>

template <typename T>
T& assign(T& x, T& y){

    if ( typeid(x).name() == "Ss" && typeid(y).name() == "Ss" ){
        std::string k = static_cast<std::string>(y);
        x = k;
    }

else if ( typeid(x).name() == "i" && typeid(y).name() == "i" ){        
        int k = static_cast<int>(y);
        x = k;
}

else{    
        std::cout << "uncorrect assignment" << std::endl;    
    }

}

int main(){

    std::string a = "empty_string";
    std::string b = "Hi there";
    assign(a, b);
    std::cout << a << std::endl;

}

But it doesn’t work. It gives the error:

[Error] invalid static_cast from type ‘std::basic_string<char>’ to type 

at line 14:

 int k = static_cast<int>(y);

I can’t understand, what is the problem?

I know the objection: I might have just defined function assign as:

template <typename T>
T& assign(T& x, T& y){
    x = y;    
}

which works. However, I was working on an other more complex function on which I have to (or at least I haven’t found any way other than) use static_cast.

So, if you could, please, explain to me what is the mistake in this example, I may try to fix the function I am working on.

Thank you very much, Simone.