尾随返回类型不是类的完整类上下文是否有特定原因? [英] Is there a specific reason why a trailing-return-type is not a complete-class context of a class?

问题描述

请注意，

一个类的完整类上下文是

A complete-class context of a class is a

(6.1)函数主体，
(6.2)默认参数，
(6.3)noexcept-specifier([except.spec])，
(6.4)合同条件，或
(6.5)默认成员初始化器

(6.1) function body,
(6.2) default argument,
(6.3) noexcept-specifier ([except.spec]),
(6.4) contract condition, or
(6.5) default member initializer

在类的成员规范内. [注意:完整的课程 嵌套类的上下文也是任何内容的完整类上下文 封闭类(如果嵌套类是在 封闭类的成员规范. —尾注]

within the member-specification of the class. [ Note: A complete-class context of a nested class is also a complete-class context of any enclosing class, if the nested class is defined within the member-specification of the enclosing class. — end note ]

[expr.prim.this] p2 也有一个关于此的注意事项:

[expr.prim.this]p2 also has a note about this:

如果声明声明一个成员函数或成员函数 X类的模板，它是类型的prvalue的表达式 可选cv-qualifier-seq之间的指向cv-qualifier-seq X的指针" 以及函数定义，成员声明器或函数的末尾 声明者.它不应出现在可选的cv-qualifier-seq之前 并且它不应出现在静态成员的声明中 函数(尽管其类型和值类别是在 静态成员函数，因为它们在非静态成员中 功能). [注意:这是因为声明匹配没有发生 直到知道完整的声明符为止. —尾注] [注:在 尾随返回类型，不需要定义所定义的类 完成以供类成员访问.班级成员宣布 以后将不可见. [示例:...

If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type "pointer to cv-qualifier-seq X" between the optional cv-qualifier-seq and the end of the function-definition, member-declarator, or declarator. It shall not appear before the optional cv-qualifier-seq and it shall not appear within the declaration of a static member function (although its type and value category are defined within a static member function as they are within a non-static member function). [ Note: This is because declaration matching does not occur until the complete declarator is known. — end note ] [ Note: In a trailing-return-type, the class being defined is not required to be complete for purposes of class member access. Class members declared later are not visible. [ Example: ...

推荐答案

因为您不想要它.

 struct Test {
     auto foo() -> decltype(bar());
     auto bar() -> int;

     auto baz() -> decltype(qux());
     auto qux() -> decltype(baz());
 }; 

现在，您需要各种规则来说明上面的哪些是允许的，哪些是不允许的.

Now you need all sorts of rules explaining which of the above is allowed and which is not.

那为什么标准将 noexcept-specifier 放在完整类上下文中?它不会在代码中允许基本上相同的东西吗?

So why does the standard places noexcept-specifier in complete-class context? Wouldn't it allow essentially the same thing in code like this:

struct Test { 
    void foo() noexcept(noexcept(bar())); 
    void bar() noexcept(noexcept(foo())); 
};

?

似乎该标准不能很好地解决这个问题，并且编译器对此的处理方式也有所不同. Clang抱怨上面的代码，但是吃了这个:

It seems the standard doesn't address this well, and the compilers differ in treatment of this. Clang complains about the above code, but eats this:

struct Test { 
    void foo() noexcept(Test::b);
    static const bool b = true;
};

GCC也抱怨第二个代码，但是接受交换了成员声明的代码.似乎根本没有将noexcept指定符视为完整类上下文.

GCC complains about the second code too, but accepts code with the member declarations swapped. It seems it doesn't treat the noexcept specifier as complete-class context at all.

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