using declaration引用C，它是D的baseclass（别名），但不被识别为有效

Question

这看起来像是 clang 中的一个问题（我已经打开了 bug to clang），但我想确定我没有做错。

考虑以下代码：

struct B { };

template<typename...>
struct D;

template<typename T, typename U, typename... A>
struct D<T, U, A...>: D<U, A...> {
    using C = D<U, A...>; // (1)
    // using D<U, A...>::D; // (2)
    // using C::C; // (3)
    using C::D; // (4)
};

template<typename T>
struct D<T>: T { using T::T; };

int main() {
    D<int, double, B> d{};
}

第 (2) 行（如果注释掉 (1) 和 (4)）和 (3)（如果注释掉 (4)）按预期工作，而 (1)（上面的示例原样）给出以下错误：

11 : error: dependent using declaration resolved to type without 'typename'
using C::D;

[...]

11 : error: using declaration refers into 'C::', which is not a base class of 'D'
using C::D;

无论如何，C 是 D<U, A...> 的别名，它是 D<T, U, A...> 的基础 class。
据我所知，该片段应该可以编译。我错了吗？

请注意 GCC 至少从 v4.8.1 to v6.1.

开始编译它

Answer 1

如 clang 的票中所述，它不能被视为错误（可能）：

Clang is following the direction of core issue 2070 (http://wg21.link/cwg2070), which matches the intent of the original proposal (see the end of "Outline of the Solution" in http://wg21.link/n2540). As such, this is working as intended. To inherit a constructor, you need to use the same identifier before and after the ::.

using declaration引用C，它是D的baseclass（别名），但不被识别为有效

Using declaration refers into C, which is a base class (alias) of D, but it is not recognized as valid

gcc

templates

clang

using-declaration

c++11