问为什么在ADL中重载更倾向于显式专门化？

EN

#include <iostream>
#include <algorithm> // std::swap C++98
#include <utility> // std::swap C++11

namespace A
{
template<typename T>
struct Foo {};

template<typename T>
void swap(Foo<T> &lhs, Foo<T> &rhs)
{
    std::cout << "A::swap<T>" << std::endl;
}

} /* end namespace A */

namespace std // we explicitly specialize std::swap here
{

template<> // explicit template specialization for std::swap<int>
void swap(A::Foo<int> &lhs, A::Foo<int> &rhs) 
noexcept 
(is_nothrow_move_constructible<A::Foo<int>>::value && is_nothrow_move_assignable<A::Foo<int>>::value)
{
    std::cout << "std::swap<int>" << std::endl;
} 

} /* end namespace std */

int main()
{
    using std::swap;
    A::Foo<int> a, b; 
    A::Foo<double> x, y;

    swap(a, b); // ADL, expected to call std::swap<Foo<int>>, but NO
    swap(x, y); // ADL, expected to call A::swap<T>, YES
}