使用来自元函数的typedef声明成员函数

问题描述

考虑以下代码:

template <class>
struct Foo_s {
    using type = void();
};

template <class>
using Foo_u = void();

template <class T>
struct Bar {
             Foo_u<void>       foo1; // OK
    typename Foo_s<void>::type foo2; // OK
             Foo_u<T>          foo3; // OK
    typename Foo_s<T>::type    foo4; // Boom.
};

template struct Bar<void>;

foo4的声明在GCC 7.2，Clang 5.0.0和MSVC 19.10.25017上失败.

The declaration of foo4 fails on GCC 7.2, Clang 5.0.0 and MSVC 19.10.25017.

海湾合作委员会:

<source>: In instantiation of 'struct Bar<void>':
18 : <source>:18:17:   required from here
15 : <source>:15:29: error: field 'Bar<void>::foo4' invalidly declared function type
     typename Foo_s<T>::type foo4;
                             ^~~~

C语:

15 : <source>:15:29: error: data member instantiated with function type 'typename Foo_s<void>::type' (aka 'void ()')
    typename Foo_s<T>::type foo4;
                            ^
18 : <source>:18:17: note: in instantiation of template class 'Bar<void>' requested here
template struct Bar<void>;
                ^

MSVC:

15 : <source>(15): error C2207: 'Bar<T>::foo4': a member of a class template cannot acquire a function type
18 : <source>(18): note: see reference to class template instantiation 'Bar<void>' being compiled

所有人似乎都认为我正在尝试声明具有函数类型的数据成员.如您所见，这仅在嵌套类型(不是using模板)且依赖于类的参数时发生.这看起来像个错误，但是这三个编译器都同意的事实令我感到怀疑.

All of them seem to think that I am trying to declare a data member with a function type. As you can see, this only happens when the type is nested (not a using template), and is dependent on the parameter of the class. This looks like a bug, but the fact that for once all of these three compilers agree has me doubting.

这是标准行为吗?如果是这样，是否有不允许这样做的理由，是否有办法声明其成员类型是通过元程序计算的成员函数?

Is this standard behaviour? If so, is there a rationale behind disallowing this, and is there a way to declare a member function whose type is computed with a metaprogram?

推荐答案

temp.spec]/8 :

这使您的最后一行绝对格式错误.这是有道理的，因为与其他依赖结构的情况一样，我们不希望短语的含义过分依赖模板参数.

This makes your last line decidedly ill-formed. Which makes sense, since, as in other cases of dependent constructs, we don't want the meaning of a phrase to depend too strongly on template arguments.

但是， [dcl.fct]/13 :

这使您的前三行格式正确-直接前两行，对于第三行，请注意

This makes your first three lines well-formed---the first two directly, and for the third one, note that

来自 [temp.alias] .

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