From my understanding, appending :: to the front of a namespace refers to the global namespace, regardless of any using statements or parent namespaces. If that is the case, and I haven't mis-understood anything, then why does such code compile (at least in Visual Studio):
namespace Foo { namespace Bar { class X; } } using namespace Foo; int main(void) { ::Bar::X x; } using namespace Foo; brings all the contents of namespace Foo into the context of the current namespace.
Since namespace Bar is among the contents of namespace Foo and the current namespace on the line with the using statement is the global namespace, namespace Bar is brought into the context of the global namespace.
:: overrides using statements, but I guess we don't always get what we want :/
:: at the start of a statement is going to refer to scope resolution on the global namespace regardless of what using statements you've introduced. What do you mean by wanting it to "[override] using statements"?
namespace K { using namespace Foo; } puts the content of the namespace Foo under K::, that's all is happening here, except for the fact that the global namespace :: is implied.using namespace Foo::Bar instead. Then, if someone referred to ::X in their code, then there is no easy way of knowing whether that was a reference to Foo::Bar::X or if there is a class X defined elsewhere outside of a namespace. I'm really just nit-picking at this point.::Foo::Bar::X - even if you have a using namespace Foo::Bar; statement.