The following shouldn't compile, but does. The commented line will fail
as expected if it's includeed.

  namespace n1
  {
    struct x {};
  }

  namespace n2
  {
    void y() { using namespace n1; }
    //x a;
  }

  n2::x b;

This also causes the following code to fail with an 'ambiguous reference
to symbol' error:

  namespace n1
  {
    struct x {};
  }

  namespace n2
  {
    void y() { using namespace n1; }
    struct x {};
  }

  n2::x a;