与类和命名空间一起使用 差异/歧义

我只是好奇为什么用using指令来设计它。对于1）结构被视为命名空间，而对于2）它不是：

struct foo
{
  using type0 = int;
};
namespace bar
{
  using type1 = int;
}
using bar::type1; 
using type0 = foo::type0; // 1)
using foo::type0;         // 2)

clang version 3.3 (branches/release_33 186829)
clang -std=c++11 test.cpp
test.cpp:13:12: error: using declaration can not refer to class member
using foo::type0;

~~~~~^

gcc version 4.8.1
c++ -std=c++11 test.cpp
test.cpp:13:12: error: ‘foo’ is not a namespace
using foo::type0;