C++20哪里禁止sizeof...T（不带括号）

Question

g++ 和 clang++ 都拒绝在参数未加括号时使用 sizeof...。例如，以下代码被拒绝：

template<typename... T> auto packsize = sizeof...T;

$ g++  -std=c++20 -ggdb -O -Wall -Werror   -c -o z.o z.cc
z.cc:1:50: error: 'sizeof...' argument must be surrounded by parentheses [-fpermissive]
    1 | template<typename... T> auto packsize = sizeof...T;
      |                                                  ^
$ clang++  -std=c++20 -ggdb -O -Wall -Werror   -c -o z.o z.cc
z.cc:1:50: error: missing parentheses around the size of parameter pack 'T'
template<typename... T> auto packsize = sizeof...T;
                                                 ^
                                                 ()
1 error generated.

CPPreference.org also seems to require parentheses around T. However, such a restriction does not appear in any of the obvious places in the C++20 standard, e.g., the discussion of sizeof... or the discussion of pack expansions。但是，标准中的所有非规范示例都包含括号。

我的问题：在 C++20 中，哪种规范语言禁止将 sizeof... 与未加括号的标识符一起使用？

Answer 1

语法要求：

[expr.unary.general]/1

unary-expression:
      ...
      sizeof ... ( identifier )
      ...