为什么我不能提前申报?

namespace O
{
    class A{};


    class A;   // ok
    typedef A K; // ok

    struct A;  // ok(C++11): A is a class but for references and pointer it have the same meaning
    class K;   // (1) error: K is a typedef (of a class...)
}

namespace U
{
    typedef O::A A;


    class A;      // (2) error: A is a typedef (of  a class...)
}
namespace O
{
    class A{};


    class A;   /