cannot specialize a member of an unspecialized template

gistfile1.cpp

#include <iostream>

template<typename T, typename U>
class Foo
{
public:
  template<typename V>
  void operator()(V const &)
  {
    std::cout << "default" << std::endl;
  }
};


template<typename T, typename U>
template<>
void
Foo<T, U>::
operator()<int>(int const &)
{
  std::cout << "specialized" << std::endl;
}

int main()
{
  Foo<int, double> foo;
  foo(42);
}

// cout@ubuntu:~/tmp $ clang++ test.cpp -std=c++11
// test.cpp:19:1: error: cannot specialize (with 'template<>') a member of an unspecialized template
// operator()<int>(int const &)
// ^