为什么GCC无法使用声明将其解析为正确的类型 [英] why can't GCC resolve this using declaration to the correct type
问题描述
在我正在研究的项目中进行编码时,我发现了一些奇怪的东西:
While coding around in a project I'm working on, I discovered something really odd:
namespace detail {
struct tuplelike_tag { };
struct arraylike_tag { };
template<typename>
struct call_with_traits;
template<typename... Ts>
struct call_with_traits<std::tuple<Ts...>> {
using tag = tuplelike_tag;
enum { size = sizeof...(Ts) };
};
template<typename T, std::size_t Sz>
struct call_with_traits<std::array<T, Sz>> {
using tag = arraylike_tag;
enum { size = Sz };
};
template<typename T, std::size_t Sz>
struct call_with_traits<T[Sz]> {
using tag = arraylike_tag;
enum { size = Sz };
};
template<typename F, typename T, int... Is>
auto call_with(F && f, T && tup, indices<Is...>, tuplelike_tag) -> ResultOf<Unqualified<F>> {
return (std::forward<F>(f))(std::get<Is>(std::forward<T>(tup))...);
}
template<typename F, typename A, int... Is>
auto call_with(F && f, A && arr, indices<Is...>, arraylike_tag) -> ResultOf<Unqualified<F>> {
return (std::forward<F>(f))(std::forward<A>(arr)[Is]...);
}
}
template<typename F, typename Cont>
inline auto call_with(F && f, Cont && cont) -> ResultOf<Unqualified<F>> {
using unqualified = Unqualified<Cont>;
using traits = typename detail::call_with_traits<unqualified>;
using tag = typename detail::call_with_traits<unqualified>::tag;
using no_tag = typename traits::tag; // this is what it's all about
return detail::call_with(std::forward<F>(f), std::forward<Cont>(cont), build_indices<traits::size>(), tag());
}
此代码的奇怪之处在于 tag 解析为 typename detail :: call_with_traits :: tag; ,但出现 no_tag 错误:
The odd thing about this code is that tag gets resolved to typename detail::call_with_traits::tag;, but no_tag errors out:
error: no type named ‘tag’ in ‘using traits = struct detail::call_with_traits<typename std::remove_cv<typename std::remove_reference<_To>::type>::type>’
即使它应该在同一结构中引用相同的using声明。
我缺少什么,还是GCC中的某些错误?
even though it should refer to the same using declaration in the same struct. Is there something I'm missing, or is this some bug in GCC?
A 在线示例可以在此处找到,包括来自GCC的相关错误消息。
A live example can be found here, including the related error message from GCC.
推荐答案
这似乎是g ++ 4.7.2中的错误。这是一个最小的示例:
This appears to be a bug in g++ 4.7.2. Here is a minimal example:
template<typename> struct A { using tag = int; };
template<typename T>
inline void f() {
using AT = A<T>;
typename A<T>::tag x; // no error
typename AT::tag y; // error
}
int main(int argc, char ** argv) {
f<int>();
return 0;
}
如果 typedef $没有错误。 c $ c>代替了
using
声明,或者如果您使用 A< int>
代替了 A< T>
。
There are no errors if a typedef
is used instead of a using
declaration, or if you use A<int>
instead of A<T>
.
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