struct M {
    friend void f() {}
    M() {
        f(); // error: 'f' was not declared in this scope
    }
};

int main() {
    M m;
}

Live example

g 4.8和clang3.4都无法编译,因为f在M中是不可见的,所以他们说.

然而,该标准给出了类似代码的示例

class M {
  friend void f() { } // definition of global f,a friend of M,// not the definition of a member function
};

并说

A friend function defined in a class is in the (lexical) scope of the
class in which it is defined.

(ISO / IEC 14882：2011 11.3朋友[class.friend] p6,p7)

从这里我无法理解编译器如何找不到f,它被定义在同一个类中.

两个编译器都有同样的错误.
那么,我错过了什么？