Forward declaration of std::hash template specialization

Question

Why forward declaration as follows :

template<typename T> struct std::hash;

fails to compile with gcc and clang, but compiles with Visual Studio 2015?

gcc 6.1.0 (using coliru):

main.cpp:11:34: error: invalid use of template-name 'std::hash' without an argument list
 template<typename T> struct std::hash;
                                  ^~~~

clang 3.8.0 (using coliru):

main.cpp:11:29: error: forward declaration of struct cannot have a nested name specifier
template<typename T> struct std::hash;
                            ^~~~~

it works under VS (http://webcompiler.cloudapp.net/). Which compiler is right?

btw. the same declaration is used in C++ Primer 5th edition. Well - nearly the same it uses class instead of struct: template <class T> class std::hash; which is wrong.

full code:

#include <unordered_map>

/*
// compiles with gcc,clang,VS
namespace std {
  template<typename T>
  struct hash;
}*/

// Compiles only with VS
template<typename T> struct std::hash;

struct MyData {
  MyData() {}
  MyData(int d1, int d2) : data1(d1), data2(d2) {}
  bool operator==(const MyData& rop) const {
    return rop.data1 == data1 && rop.data2 == data2;
  }

  friend struct std::hash<MyData>;
 private:
  int data1;
  int data2;
};

namespace std {
  template<>
  struct hash<MyData> {
    typedef MyData argument_type;
    typedef size_t result_type;
    size_t operator()(const argument_type& data) const noexcept;
  };

  size_t hash<MyData>::operator()(const argument_type& data) const noexcept {
    return hash<unsigned>()(data.data1) ^ hash<unsigned>()(data.data2);
  }
}

int main() {
  std::unordered_map<MyData, std::string> mm;
  mm[MyData(1,1)] = "test1";
  mm[MyData(2,2)] = "test1";
}

Answer 1

The reason, seems to be largely because a forward declaration has to function much like a regular declaration. i.e. encased in a namespace, not prefixed by one. I guess this would allow the same parser to be used for declarations and forward declarations which makes sense.