为什么模板别名专门化取决于引用它的上下文? [英] Why a template alias specialization depends on the context in which it is referred?

问题描述

请考虑以下示例代码:

template <class T>
using pt_type = typename T::type;

template <class T>
class V {
  using type = int;
  public:
  using pt = pt_type<V>;
};

void g() {
  V<int>::pt a; // Does compile
  pt_type<V<int>> b; // Does not compile
}

V< int> :: pt 是 pt_type< V< int>> 的别名.尽管如此，定义它的事实取决于引用它的上下文.

V<int>::pt is an alias for pt_type<V<int>>. Nevertheless the fact it is defined depends on the context where it is referred.

C ++标准在何处说明，在引用别名专门化的上下文中执行了用template参数替换template参数?

Where is it explained in the C++ standard that the substitution of the template parameter by the template argument is performed in the context where is refered the alias specialization?

推荐答案

无处.这是核心问题1554 .

从当前的14.5.7 [temp.alias]措词来看，别名模板和访问控制之间的交互作用尚不清楚.例如:

The interaction of alias templates and access control is not clear from the current wording of 14.5.7 [temp.alias]. For example:

template <class T> using foo = typename T::foo;

class B {
  typedef int foo;
  friend struct C;
};

struct C {
  foo<B> f;    // Well-formed?
};

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