打开模板别名中的参数包

我遇到了将可变参数模板解包为模板别名的问题。打开模板别名中的参数包

以下代码可与锵3.4和GCC 4.8但失败GCC 4.9：

template <typename T, typename...> 
using front_type = T; 

template <typename... Ts> 
struct foo 
{ 
    using front = front_type<Ts...>; 
};

GCC 4.9抱怨：

test.cc:7:37: error: pack expansion argument for non-pack parameter 'T' of alias template 'template<class T, class ...> using front_type = T' 
     using front = front_type<Ts...>; 
            ^
test.cc:1:15: note: declared here 
    template <typename T, typename...> 
      ^

存在一个日提交GCC蝽（#59498），但这应该失败？下面是从C++ core language issue #1430, "pack expansion into fixed alias template parameter list"一些背景：

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 T s 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.

来源

2014-06-26 mavam

你可以在'foo'之外移动'front_type' - 它们似乎没有关系吗？ – PiotrNycz

是的，好点@PiotrNycz。（虽然它没有修复它。） – mavam

我用gcc4.8检查了这个修改过的代码 - 看起来很有效。 – PiotrNycz

据报道，gcc4.9的Bugzilla：

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

最少的代码来重现

template <typename T, typename ...> 
using alias = T; 

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

using Fail = variadic_alias<int>; 

int main() { }

从解释来自海外合作伙伴 - 它是否定的很明显这是一个真正的bug。 这仍然是在gcc bugzilla和DR 1430（http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1430）中进行的讨论 - 现在在上面的问题中进行总结。

来源

2014-06-26 15:07:17 PiotrNycz

它被报告为一个错误，但是看着这个评论，好像是设计。 –

是的，它看起来应该失败？我编辑了来自[＃1430]（http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1430） – mavam

@ T.C的上下文的问题。好点，我错过了。 – PiotrNycz

打开模板别名中的参数包

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