C++和CRTP模式的实现与编译器困境

我正在尝试编译以下代码，但似乎有一个问题我无法解决：

template <int x>
struct count_x
{
enum { x_size = x };
};
template <typename y>
struct crtp_base
{
typedef typename y::count_t count_t;
crtp_base(const count_t&){}
};
template <int x>
struct derived : public crtp_base<derived<x> >
{
typedef typename count_x<x> count_t;
typedef crtp_base<derived<x> > base_t;
derived(const count_t& c) : base_t(c){}
};

int main()
{
derived<2> d((count_x<2>()));
return 0;
}

当使用clang 3.1编译时，以下是错误：

c:clangllvmcodeexample.cc:18:21: error: expected a qualified name after 'typename'
typedef typename count_x<x> count_t;
^
c:clangllvmcodeexample.cc:18:21: error: typedef name must be an identifier
typedef typename count_x<x> count_t;
^~~~~~~~~~
c:clangllvmcodeexample.cc:18:28: error: expected ';' at end of declaration list
typedef typename count_x<x> count_t;
^
;
c:clangllvmcodeexample.cc:20:18: error: no template named 'count_t'; did you mean 'count_x'?
derived(const count_t& c)
^~~~~~~
count_x
c:clangllvmcodeexample.cc:2:8: note: 'count_x' declared here
struct count_x
^
c:clangllvmcodeexample.cc:20:18: error: use of class template count_x requires template arguments
derived(const count_t& c)
^
c:clangllvmcodeexample.cc:2:8: note: template is declared here
struct count_x
^
5 errors generated.

我相信这与模板在编译时的确定方式有关，也与它们是否在正确的时间被确定为类型有关。我还尝试添加"using base_t:：count_t；"，但没有成功。除此之外，编译器产生的诊断让我真的很失落。如果您能就这一错误提供答案或建议，我们将不胜感激。