C++ template function specialization as an argument of template function

Consider a c++ template function specialization :

namespace test {

   template < const int L, typename InputIt >
   typename std::iterator_traits<InputIt>::value_type
   Sum_L( InputIt beg, InputIt end)
   {
      typedef typename std::iterator_traits<InputIt>::value_type real_t;

      for( int i=0 ; i<L; ++i)
        call_rearrange_sum( beg, end);

      real_t sum( 0 );
      for( ; beg != end; ++beg)
        sum += *beg;

      return sum;
    }

   template < const int L, typename Real >
   Real
   Sum_L( const std::size_t len, Real * x)
   {          
      for( int i=0 ; i<L; ++i)
        call_rearrange_sum( x, x+len);

      Real sum( 0 );
      for( std::size_t i=0; i< len; ++i)
        sum += x[i];

      return sum;
    }

    template < typename Real, typename Func >
    Real
    special_sum( std::size_t len, const Real * const x, const Real * y, Func f)
    {
      std::vector<Real> res( 2*len );

      for( std::size_t i=0; i<len; ++i) {
        Real tmp;
        res.push_back( call_operator( x[i], y[i], &tmp);
        res.push_back( tmp );
      }

      return f( res.begin(), res.end(), f);
    }
}

Now I want to use the above functions as :

double my_test( const std::size_t len, double * x, double * y)
{
  return test::special_sum( len, x,y, 
                            test::Sum_L< 4, typename std::vector<double>::iterator> );
}

The gcc 4.9.2 it is not able to find the correct template specialization function. The error is "no matching function for call to 'special_sum(std::size_t&, const double*&, const double*&, < unresolved overloaded function type > )'.

Ι know that it is difficult to resolve by the compiler. From whatever I tried, it is one of the two template function 'Sum_L' to get an extra dummy-template argument. Is there any other way?

Thanks.