Why is "using namespace X;" not allowed at class/struct level?

This is probably disallowed because of openness vs closedness.

• Classes and structs in C++ are always closed entities. They are defined in exactly one place (although you can split declaration and implementation).
• namespaces can be opened, re-opened and extended arbitrarily often.

Importing namespaces into classes would lead to funny cases like this:

namespace Foo {}

struct Bar { using namespace Foo; };

namespace Foo {
using Baz = int; // I've just extended `Bar` with a type alias!
void baz(); // I've just extended `Bar` with what looks like a static function!
// etc.
}

I think it's a defect of the language. You may use workaround below. Keeping in mind this workaround, it is easy to suggest rules of names conflicts resolution for the case when the language will be changed.

namespace Hello
{
    typedef int World;
}
// surround the class (where we want to use namespace Hello)
// by auxiliary namespace (but don't use anonymous namespaces in h-files)
namespace Blah_namesp {
using namespace Hello;

class Blah
{
public:
    World DoSomething1();
    World DoSomething2();
    World DoSomething3();
};

World Blah::DoSomething1()
{
}

} // namespace Blah_namesp

// "extract" class from auxiliary namespace
using Blah_namesp::Blah;

Hello::World Blah::DoSomething2()
{
}
auto Blah::DoSomething3() -> World
{
}

You can't use using namespace inside of a class, but what you can do is simply use #define and then #undef inside of the structure. It will act the exact same way as namespace a = b;

struct foo
{
#define new_namespace old_namespace
     
    void foo2()
    {
        new_namespace::do_something();
    }

#undef new_namespace 
};