c++ - 为什么 noexcept 说明符在完整的类上下文中不起作用，但默认参数和函数体却起作用

Question

我正在使用列出的书籍学习 C++ here .特别是，我阅读了完整类上下文并了解到它包括函数体、默认参数、noexcept-specifier 等。现在，为了进一步明确我对该主题的理解，我编写了以下程序，其中 # 1 和 #2 有效，但 #3 失败。我不知道为什么 #3 失败，因为我读到所有三个(函数体、默认参数和 noexcept 说明符)都包含在完整类上下文中。

struct A {
   constexpr static bool func() 
   { 
       return true; 
   }
   //--------------vvvvvv------->works as expected   #1
   void f(bool V1 = func())
   { 
      bool V2 = func(); //works as expected          #2
   }
   //-----------------vvvvvv---->DOESN'T WORK?       #3
   void g()  noexcept(func()) 
   {
      ;
   }

};

A complete-class context of a class is a

• function body
• default argument
• noexcept specifier

如您所见，第三点是“noexcept specifier”，所以我希望 #3 也能正常工作，但事实并非如此。

所以我的问题是为什么#3 与#1 和#2 不同？ Demo

GCC 给出了 #3 的错误:

 error: 'static constexpr bool A::func()' called in a constant expression before its definition is complete
   20 |    void g()  noexcept(func())

Clang 给出:

error: noexcept specifier argument is not a constant expression
   void g()  noexcept(func()) 
                      ^~~~~~
<source>:20:23: note: undefined function 'func' cannot be used in a constant expression
<source>:10:26: note: declared here
   constexpr static bool func() 

MSVC 给出:

 error C2131: expression did not evaluate to a constant
<source>(20): note: failure was caused by call of undefined function or one not declared 'constexpr'
<source>(20): note: see usage of 'A::func'

Answer 1

该程序格式良好并且所有三个编译器在拒绝代码方面都是错误的，因为 noexcept-specifier 包含在 complete-问题中引用的类上下文。

以下是相应的错误报告:

GCC rejects use of static constexpr member function in noexcept complete-class context

Clang rejects use of static constexpr member function in noexcept complete-class context

MSVC rejects use of static constexpr member function in noexcept complete-class context