GCC 编译器是否应该对涉及 [[fallthrough]] 属性的这种格式错误的 C++ 代码进行诊断？

Question

我在 GCC 编译器版本 7.1.0 上测试 C++17 功能。这与 fallthrough 属性和下面的示例 (live example) is adapted from online CPP reference here

#include "iostream"
using namespace std;

int f(int n) {

  switch (n) {
    case 1:
    case 2:
      n = n + 20;
     [[fallthrough]];
    case 3: // no warning on fallthrough      
      n = n + 30;
    case 4: // compiler may warn on fallthrough      
      [[fallthrough]]; // illformed, not before a case label
      //n = n + 40;  //commented out to test if compiler will warn.
  }
  return n;
}    

int main()
{
    cout << f(1) << endl;
    cout << f(2) << endl;
    cout << f(3) << endl;
    cout << f(4) << endl;
    return 0;
}

最后一个 [[fallthrough]]（对于 case 4:）格式错误。

关于 "What is the C++ compiler required to do with ill-formed programs according to the Standard?" here has the top answer 的问题指出：

So to sum it up: if an ill-formed program contains a diagnosable violation for which the Standard does not explicitly specify "no diagnostic required", conforming implementations should emit a diagnostic.

因此，我查阅了标准 (N4713)，看它是否说明此问题不需要诊断。我找不到任何这样的声明。

有趣的是，在这一切之后，当我在最后一个 [[fallthrough]]

之后添加以下语句时

n = n + 40;

编译器警告（live example）：

warning: attribute 'fallthrough' not preceding a case label or default label

所以，这里有两个问题：

编译器是否错过了发出诊断信息，或者我是这里缺少什么？
如果是编译器问题，严重到需要报告吗？

Answer 1

If it is a compiler issue, is it serious enough to be reported?

是的，一致性错误是重要的错误，开发人员依赖于符合标准的编译器（编译器可能具有不需要严格一致性的模式，但即 gcc 要求 -pedantic to obtain all diagnostics required by the standard）错误获得的优先级是不同的故事，但仅仅记录错误并让编译器团队将其确认为错误可能对运行进入错误的未来开发人员有很大帮助。

Has the compiler missed out on emitting a diagnostic, or am I missing something here?

是的，根据 [dcl.attr.fallthrough#]p1:

格式不正确

... The next statement that would be executed after a fallthrough statement shall be a labeled statement whose label is a case label or default label for the same switch statement. The program is ill-formed if there is no such statement.

并且编译器需要至少根据 [intro.compliance]p2.2:

发出诊断

If a program contains a violation of any diagnosable rule or an occurrence of a construct described in this document as “conditionally-supported” when the implementation does not support that construct, a conforming implementation shall issue at least one diagnostic message.

Answer 2

在我发布这个问题后的某个时间，有人根据这个问题报告了 bug。
我等着看它是否会被接受。它已被 接受并分配 ，并且基于错误报告中的 comment， 将生成补丁 。所以这有效地回答了我的两个问题。

但是，我会接受@ShafikYaghmour 给出的答案，因为它包含解决我问题的要点。

GCC 编译器是否应该对涉及 [[fallthrough]] 属性的这种格式错误的 C++ 代码进行诊断？

Should there be a diagnostic from GCC compiler for this ill-formed C++ code involving [[fallthrough]] attribute?

c++

gcc

gcc-warning

fall-through

c++17