[C++]Template Argument Deduction: automatic type conversation is not allowed

#include <string>
using namespace std;
template <typename T>
inline T Add(const T& a,const T& b){
      return a+b;
}

string Add2(const string& a, const string& b)
{
      return a+b;
}
int _tmain(int argc, _TCHAR* argv[])
{
      string a("test");
      //Add(a,"b"); //error: template parameter 'T' is ambiguous
      Add2(a,"b"); //OK for none template parameter

      //how to fix it.
      Add(a,string("b"));
      Add(a,static_cast<string>("b"));
      Add<string>(a,"b");

      return 0;
}