使用void\u t和受保护嵌套类的基于SFINAE的检测

最近,我遇到了一些clang和gcc之间关于 void_t 属性检测和受保护/私有类信息。考虑一个类型特征定义如下:

#include <type_traits>

template<typename T, typename = void>
constexpr const bool has_nested_type_v = false;

template<typename T>
constexpr const bool has_nested_type_v
<T, std::void_t<typename T::type>> = true;

type 类和一个简单的程序

#include "has_nested_type.hpp"
#include <iostream>

struct Protected {
protected:
  struct type{};
};

struct Private {
private:
  struct type{};
};

int main() {
  std::cout << "Protected: " 
            << (has_nested_type_v<Protected> ? "detected" : "not detected")
            << std::endl;
  std::cout << "Private: " 
            << (has_nested_type_v<Private> ? "detected" : "not detected")
            << std::endl;
}

• clang成功编译,但检测失败(如预期)。程序、编译和输出都复制到wandbox上 here

• gcc编译失败,发出诊断。此错误可在魔杖盒上重现 here .

GCC为此程序发出以下错误。

prog.cc:16:21: error: 'struct Protected::type' is protected within this context
                 << (has_nested_type_v<Protected> ? "detected" : "not detected")
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cc:6:14: note: declared protected here
       struct type{};
              ^~~~
prog.cc:19:21: error: 'struct Private::type' is private within this context
                 << (has_nested_type_v<Private> ? "detected" : "not detected")
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
prog.cc:11:14: note: declared private here
       struct type{};
              ^~~~