Which will print the following code:

#include <iostream>

using namespace std;

namespace NA
{
  struct A {
    A() : a(1) {}
    int a;
  };

  void f(A &b)
  {
    cout << b.a << endl;
  }
}

NA::A a;

void f(NA::A &b) 
{
  cout << b.a*2 << endl;
}

int main()
{
  f(a);
  return 0;
}

Explanation

According to the König search rule (Koenig lookup), if the function parameter is a class object, the function search is performed in the namespace of this class along with the standard scope. Thus, there are two identical signatures, which leads to ambiguity and compilation error, as a consequence.