模板类中的变量模板-意外错误(可能的错误?) [英] Variable template in template class - unexpected error (possible bug?)

问题描述

具有:

struct Value
{
    template<class T>
    static constexpr T value{0};
};

---

(0) ideone

template<typename TValue>
struct Something
{
    void x()
    {
        static_assert(TValue::template value<int> == 0, "");
    }
};

int main() { Something<Value>{}.x(); return 0; } 

• 不使用clang ++ 3.6进行编译.

  • Does not compile with clang++ 3.6.

    错误:如果没有模板参数列表，就无法引用变量模板'value'

    error: cannot refer to variable template 'value' without a template argument list

  • 无法使用g ++ 5.2进行编译.

  • Does not compile with g++ 5.2.

    错误:"template constexpr const T Value :: value"不是函数模板

    error: ‘template constexpr const T Value::value’ is not a function template

  (1) ideone

  使用clang ++和g ++进行编译.

  Compiles with both clang++ and g++.

  struct Something
  {
      void x()
      {
          static_assert(Value::template value<int> == 0, "");
      }
  };
  
  int main() { Something{}.x(); return 0; } 
  

  ---

  为什么(0)无法编译?

  ---

  Why does (0) fail to compile?

  如果通过模板参数(在本例中为TValue)访问变量模板，似乎会出现此问题.为TValue定义类型别名或使用typename关键字不能解决该问题.

  It seems that the issue occurs if the variable template is accessed through a template parameter (in this case, TValue). Defining a type alias for TValue or using the typename keyword does not fix the issue.

  这是怎么回事?

  推荐答案

  在将变量模板视为从属名称时，这肯定是gcc和clang错误.我提交了 gcc 67248 和

  This is definitely a gcc and clang bug in their treatment of variable templates as dependent names. I submitted gcc 67248 and clang 24473.

  目前，这是一种解决方法，两个编译器都支持执行变量模板的旧方法，即是否添加了:

  As a workaround for now, both compilers support the old way of doing variable templates, namely if you added:

  struct Value
  {
      template<class T>
      static constexpr T value = 0;
  
      template <typename T>
      struct variable_template_ish {
          static constexpr T value = Value::value<T>;
      };
  };
  

  然后执行以下编译:

  template<typename TValue>
  struct Something
  {
      void foo() {
          static_assert(TValue::template variable_template_ish<int>::value == 0, "");
      }
  };
  
  int main() { 
      Something<Value>{}.foo();
  }
  

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