类模板的别名 [英] alias of class template

问题描述

在下面的代码中考虑一个类似于 A 的别名模板。现在让 B 为 A 的别名模板。

Consider an alias template like the A in the code below. Now let B be an alias template of A.

在下面的代码中，这些类模板用作结构 C 的模板参数，该结构仅专用于一种类型名称（ A ）。 clang -std = c ++ 11 存在，错误为：未定义模板'C< B>'的隐式实例化指示

In the code below these class templates are used as template arguments for a struct C which is only specialized for one typename (A). clang -std=c++11 exists with error: implicit instantiation of undefined template 'C<B>' indicating that another specialization for B is needed.

template<int N>
using A = int;

template<int N>
using B = A<N>;

template<template<int> class I>
struct C;

template<>
struct C<A> {};

int main() {
  C<A> c;
  C<B> d; // clang error: implicit instantiation
}

为什么（即使是）-尽管不允许别名的特殊化- A 和 B 被视为不同的类模板？是否有解决方法可以让我重命名冗长的模板而不会引起此问题？

Why (if even) is it that - despite not allowing specializations of aliases - A and B are treated as different class templates? Is there a workaround allowing me to rename a lengthy template without incurring this problem?

推荐答案

这是 CWG问题＃1286 ，该示例涉及以下示例：

This is CWG issue #1286, which deals with this example:

template<template<class> class TT> struct X { };
template<class> struct Y { };
template<class T> using Z = Y<T>;
X<Y> y;
X<Z> z;

询问是否 y 和 z 具有相同的类型。

questioning whether or not y and z have the same type.

基本上，根据标准，c是正确的拒绝代码。 [temp.alias]告诉我们的是：

Basically, according to the Standard, clang is correct in rejecting the code. All [temp.alias] tells us is:

当 template-id 引用别名模板的特殊化，等效于通过将其 template-argument s替换为 template-parameter s而获得的关联类型
在别名
模板的 type-id 中。

When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the type-id of the alias template.

因此，当 A< X> 等效于 B< X> （对于所有 X ！），没有措辞说 A 等效于 B 。但是在某种程度上，这实际上没有任何意义，因为应该是 B 和 A >等价物。有一项拟议的决议将使它们如此，但尚未获得批准。

So while A<X> is equivalent to B<X> (for all X!), there is no wording that A is equivalent to B. But on some level that doesn't really make any sense since B and A should be equivalent. There is a proposed resolution that would make them so, but it has not yet been approved.

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