using x::y; - why is this not allowed?

[LB]

namespace x
{
    namespace y
    {
        void z(){}
    }
}
 
int main()
{
    using x::y; //error
    y::z();
}

Why is this not allowed? It seems so useful and so simple...

[ne555]

using namespace x::y;
Or namespace y = x::y;

[LB]

using namespace doesn't work in my case, thanks for the namespace y = x::y; solution!

[ne555]

using namespace x; then you can call y::z();

[LB]

It causes ambiguities because there are things in namespace x with the same names as things in the current scope, so I cannot use using namespace x;.