static_casting a constexpr void* 的结果是常量表达式吗？

Question

clang 正在拒绝 gcc 允许的代码：

int main() {
    static constexpr const void *vp = nullptr;
    static constexpr const char *cp = static_cast<const char*>(vp);
}

具有以下内容：

error: constexpr variable 'cp' must be initialized by a constant expression
  static constexpr const char *cp = static_cast<const char*>(vp);

阅读 N3797 5.9/2 中的最终清单后，我没有看到任何禁止 static_cast 在常量表达式中使用的内容。我是在看错地方还是误读了什么？或者我应该针对 clang 打开一个错误？

Answer 1

好吧，C++14 标准（以及您的（非最终）草案！）要求

A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions:

— a conversion from type cv void * to a pointer-to-object type;

static_casting a constexpr void* 的结果是常量表达式吗？

Is the result of static_casting a constexpr void* a constant expression?

c++

clang

language-lawyer

c++14

constexpr