std::pair of strings as custom key of unordered_map defined in std fails with template errors

I have a map defined and used like this

// def.h
struct X{};
struct Y{};
struct myStruct
{
   X x;
   Y y;
};

typedef std::unordered_map<std::pair<std::string, std::string>, myStruct> myMap;
namespace std
{
   template<> struct pair<std::string, std::string>
   {
      std::string s1,s2;
      pair(const std::string& a, const std::string& b):s1(a),s2(b){}
      bool operator < (const pair<std::string,std::string>& r)
      {
         return (0 < r.s1.compare(s1) && (0 < r.s2.compare(s2)));
      }
   };
} 

//use.cpp
class CUse
{
  myMap m;
public:
   CUse():m(0){}
};

Some errors emitted by the compiler are extracted as below

1. At the constructor CUse initialization,

note: see reference to function template instantiation 'std::unordered_map,myStruct,std::hash<_Kty>,std::equal_to<_Kty>,std::allocator>>::unordered_map(unsigned __int64)' being compiled

2. At the declaration of m in CUse

note: see reference to class template instantiation 'std::unordered_map,myStruct,std::hash<_Kty>,std::equal_to<_Kty>,std::allocator>>' being compiled