别名模板的打包扩展

Question

pack 参数似乎只能展开以代替别名模板的 pack 参数。这对于 class 或函数模板是不正确的：

template <class T, class... Args> struct x { using type = T; };

template <class T, class... Args> using x_t     = typename x<T, Args...>::type;
template <class... Args>          using x_fix_t = typename x<Args...>::type;

template <class... Args> auto f(Args...) -> void {
  typename x<Args...>::type v1; // OK
  x_t<Args...> v2; // Error
  x_fix_t<Args...> v3; // OK
}

更简单的案例：

template <class T, class U> using y_t = T;

template <class... Args> auto f(Args...) -> void {
  y_t<Args...> v4; // Error
}

上面的代码在 g++ 4.9、g++ 5.1 和 clang 3.5.

为什么不允许这样做？一般规则是什么？我认为没有理由限制这一点。好像是个很奇怪的禁令。

至于为什么不用第一个变体写成 x_fix_t 更清楚的是 x_t 有一个强制性的第一个参数。（例如，这就是不允许 f() 的原因）。但这并不重要，修复很容易。问题仍然存在：为什么？

gcc 错误：

error: pack expansion argument for non-pack parameter ‘T’ of
alias template ‘template<class T, class ... Args> using x_t = typename x::type’

clang 错误：

error: pack expansion used as argument for non-pack parameter of
alias template   x_t<Args...> v2;

Answer 1

这在 GCC 4.8 中编译但在 GCC 4.9 中失败，这证明它与 CWG 1430 和 bug report #59498 相关。 Roy Chrihfield 提出的修复与您的完全相同：

Rewriting the code to use a struct succeeds:

template <typename T, typename ...>
struct alias { using type = T; };

template <typename ...T>
using variadic_alias = typename alias<T...>::type;

此外，Jason Merrill 详细说明了失败的原因：

Actually, no, this is very much a Core 1430 issue; there's no way to mangle variadic_alias<T...> without mentioning the name of an alias template in the mangling, and they're supposed to be entirely transparent. This only works in 4.8 by accident because checking is disabled for the release.

错误报告中没有进一步的讨论，因此我们可以求助于 CWG 1430：

Originally, a pack expansion could not expand into a fixed-length template parameter list, but this was changed in N2555. This works fine for most templates, but causes issues with alias templates.

In most cases, an alias template is transparent; when it's used in a template we can just substitute in the dependent template arguments. But this doesn't work if the template-id uses a pack expansion for non-variadic parameters. For example:
template<class T, class U, class V>
struct S {};

template<class T, class V>
using A = S<T, int, V>;

template<class... Ts>
void foo(A<Ts...>);
There is no way to express A<Ts...> in terms of S, so we need to hold onto the A until we have the Ts to substitute in, and therefore it needs to be handled in mangling.

Currently, EDG and Clang reject this testcase, complaining about too few template arguments for A. G++ did as well, but I thought that was a bug. However, on the ABI list John Spicer argued that it should be rejected.

(See also issue 1558.)

Notes from the October, 2012 meeting:

The consensus of CWG was that this usage should be prohibited, disallowing use of an alias template when a dependent argument can't simply be substituted directly into the type-id.

Additional note, April, 2013:

For another example, consider:
  template<class... x> class list{};
  template<class a, class... b> using tail=list<b...>;
  template <class...T> void f(tail<T...>);

  int main() {
    f<int,int>({});
  }
There is implementation variance in the handling of this example.

换句话说，这是一个持续存在的问题，目前还没有任何解决方案 (AFAIC)。

别名模板的打包扩展

Pack expansion for alias template

c++

c++11

language-lawyer

variadic-templates

template-aliases