Это нестандартная реализация std :: pair, ошибка компилятора, или нестандартный код?

The following code compiles on VS2005 and gcc-4.3.4.

#include 
#include 

template
struct X { };

typedef std::pair Pr;
X var;

int main()
{
    std::cout << "hello\n";
}

But it fails to compile on VS2010 with the error message:

1>d:\a\testvs10\testvs10.cpp(13): error C2440: 'specialization' : cannot convert from 'int std::_Pair_base<_Ty1,_Ty2>::* ' to 'int std::pair<_Ty1,_Ty2>::* '
1>          with
1>          [
1>              _Ty1=int,
1>              _Ty2=int
1>          ]
1>          Standard conversion from pointer-to-member of base to pointer-to-member of derived is not applied for template arguments

I understand that in microsoft's implementation of VS2010 Pr::first is actually a member of _Pair_base. However, AFAIK, it doesn't matter. &Pr::first still must be of a type int Pr::*. Note that the following code compiles fine:

int Pr::* x = &Pr::first;

So, is it a non-standard std::pair implementation, a compiler bug, or a non-standard code?