Testing existence of template type

Class TypeTraits tries to verify, whether its templated type contains
required types.
It works for non-template types, but I don't know, how to check for the
template one.
So is there any way to check for the existence of TestType::B?

#include <boost/utility/enable_if.hpp>
#include <boost/concept_check.hpp>

#include <iostream>
using namespace std;

template <typename T> struct TypeTautology { enum { value = true};};

template <class GP, typename Enable = void>
   struct TypeTraitsImpl
   { enum { value = false }; };

template <class GP>
   struct TypeTraitsImpl<
     GP
     ,typename boost::enable_if<
       TypeTautology<
         typename GP::A//works ok, checks existence of A
         //typename GP::B//compiles, but returns "test: 0"
         //typename GP::template B//doesn't compile
         //typename GP::template <class> B//doesn't compile
       >
     >::type
   >
   { enum { value = true }; };

template <class GP> struct TypeTraits: public TypeTraitsImpl<GP, void>
{};

struct TestType
{
   struct A {};

   template <class PG> struct B {};
};

int main(int const, char const* [])
{
   try
   {
     clog << "test: " << TypeTraits<TestType>::value << endl;
     //Expected output:
     //test: 1
     return EXIT_SUCCESS;
   }
   catch (...){
     cerr << "error" << endl;
   }
   exit(EXIT_FAILURE);
}

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