求教个简单的 C++ 语法问题

代码如下

#include<iostream>

enum ExprValueType : uint8_t {
  MinInf = 1
};

int main(int argc, char* argv[]) {
  ExprValueType type = MinInf;
  std::cout <<"Hello world" << std::endl;
  std::cout << type << std::endl;
  return 0;
}

使用 g++ abc.cc -o a.out -std=c++20 报错，报错信息如下所示

abc.cc:3:20: error: found ‘:’ in nested-name-specifier, expected ‘::’
    3 | enum ExprValueType : uint8_t {
      |                    ^
      |                    ::
abc.cc:3:6: error: ‘ExprValueType’ has not been declared
    3 | enum ExprValueType : uint8_t {
      |      ^~~~~~~~~~~~~
abc.cc:3:30: error: expected unqualified-id before ‘{’ token
    3 | enum ExprValueType : uint8_t {
      |                              ^
abc.cc: In function ‘int main(int, char**)’:
abc.cc:8:3: error: ‘ExprValueType’ was not declared in this scope
    8 |   ExprValueType type = MinInf;
      |   ^~~~~~~~~~~~~
abc.cc:10:16: error: ‘type’ was not declared in this scope; did you mean ‘std::__cmp_cat::type’?
   10 |   std::cout << type << std::endl;
      |                ^~~~
      |                std::__cmp_cat::type
In file included from /usr/include/c++/13/bits/char_traits.h:56,
                 from /usr/include/c++/13/ios:42,
                 from /usr/include/c++/13/ostream:40,
                 from /usr/include/c++/13/iostream:41,
                 from abc.cc:1:
/usr/include/c++/13/compare:49:11: note: ‘std::__cmp_cat::type’ declared here
   49 |     using type = signed char;
      |           ^~~~

个人猜测是 gcc 版本的问题，于是在 https://gcc.godbolt.org/ 切换不同的编译器查看。

• 发现自 x86-64 gcc 13.1 开始会报错，之前版本（比如 x86-64 gcc 12.3）就不会报错。
• 同样 x86-64 clang 17.0.1 开始会报错，之前版本（比如 x86-64 clang 16.0.0 ）就不会报错。

于是产生了两个困惑

• 编译器最近是出了什么新的标准导致不支持 enum 的这种写法了吗？
• 如果想要在 x86-64 gcc 13.1 上跑起来，应该怎么改？