C99， "Despite the name, a non-directive is a preprocessing directive."

Question

标题中引用的脚注是什么意思？这是 6.10.3p11 的脚注

If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives,¹⁴⁷⁾ the behavior is undefined.

我查了一下发现

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that ...

而且我没有找到与该语法匹配的非终结符 non-directive。它可以但不必以 # 预处理标记开头。那么我们不是必须说以下吗？

"Despite the name, a preprocessing directive is a non-directive."

此外，该脚注的目的是什么？

Answer 1

参见 http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_250.htm是为了说明

#define nothing(x) // Nothing    /* Case 1 */
nothing (
#nonstandard
)

是 UB。