程序与“noexcept”构造函数由gcc接受，被clang拒绝 [英] Program with "noexcept" constructor accepted by gcc, rejected by clang

问题描述

代码：

  struct T {T（）{}}; 
 
 struct S 
 {
 T t; 
 
 S（）noexcept = default; 
}; 
 
 int main（）
 {
 // S s; 
} 
  

g ++ 4.9.2接受这个没有错误或警告， 3.7报告第7行：

 错误：显式默认构造函数的异常规范与计算的不匹配
  

但是，如果 S s; 行未注释掉， g ++ 4.9.2现在报告：

  noex.cc：在函数'int main（）'中：
 noex。 cc：12：7：错误：使用已删除的函数'S :: S（）'
 S s; 
 ^ 
 noex.cc:7:5：注意：'S :: S（）noexcept'被隐式删除，因为它的异常规范不匹配隐式异常规范
 S（）noexcept = default; 
 ^ 
  

哪个编译器适合原始代码？

---

背景：

g ++甚至允许添加到主：

  std :: cout< std :: is_constructible< S> :: value<< '\\\
';输出 0 的
  

我遇到这个问题，当使用clang编译一些复杂的代码，大量使用模板，SFINAE和noexcept。在该代码 S 和 T 是模板类;因此行为取决于 S 被实例化的类型。 Clang拒绝它与一些类型的这个错误，而g ++允许它和SFINAE基于 is_constructible 和类似的traits工作。

N3337 [dcl.fct.def.default] /

p2：

显式默认的函数[...]可能有一个明确的

这是由 CWG问题1778 （N4296 [dcl.fct.def.default] / p3）：

如果一个明确默认的函数使用与隐式声明的异常规范不兼容（15.4）的
异常规范 p>

• 如果函数在其第一个声明中显式默认，则定义为删除;

这意味着构造函数现在只被定义为已删除。 （上述措词包含 N4285 所做的更改， C ++ 14纸张做一些清理更改纯粹是编辑的N3936版本是基本一样的）

可能GCC实现CWG1778的分辨率，而Clang不' t。

The code:

struct T { T() {} };

struct S
{
    T t;

    S() noexcept = default;
};

int main()
{
//    S s;
}

g++ 4.9.2 accepts this with no errors or warnings, however clang 3.6 and 3.7 report for line 7:

error: exception specification of explicitly defaulted default constructor does not match the calculated one

However, if the line S s; is not commented out, g++ 4.9.2 now reports:

noex.cc: In function 'int main()':
noex.cc:12:7: error: use of deleted function 'S::S()'
     S s;
       ^
noex.cc:7:5: note: 'S::S() noexcept' is implicitly deleted because its  exception-specification does not match the implicit exception-specification ''
     S() noexcept = default;
     ^

Which compiler is right for the original code?

---

Background:

g++ even allows the following to be added to main:

std::cout << std::is_constructible<S>::value << '\n';

which outputs 0. I encountered this problem when using clang to compile some complicated code that made heavy use of templates, SFINAE and noexcept. In that code S and T are template classes; so the behaviour depends on which types S was instantiated with. Clang rejects it with this error for some types, whereas g++ permits it and the SFINAE works based on is_constructible and similar traits.

解决方案

Depends on the version of the standard you are consulting.

N3337 [dcl.fct.def.default]/p2:

An explicitly-defaulted function [...] may have an explicit exception-specification only if it is compatible (15.4) with the exception-specification on the implicit declaration.

which renders your original code ill-formed.

This was changed by CWG issue 1778 to read (N4296 [dcl.fct.def.default]/p3):

If a function that is explicitly defaulted is declared with an exception-specification that is not compatible (15.4) with the exception specification on the implicit declaration, then

• if the function is explicitly defaulted on its first declaration, it is defined as deleted;
• otherwise, the program is ill-formed.

which means that the constructor is now merely defined as deleted. (The above wording incorporated changes made by N4285, a post-C++14 paper making some cleanup changes intended to be purely editorial. The N3936 version is substantively the same.)

Presumably GCC implements CWG1778's resolution, while Clang doesn't.

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