friend 注入会是病态的吗？

Question

曾经在standard中有一段说

the names of a namespace-scope friend functions of a class template specialization are not visible during an ordinary lookup unless explicitly declared at namespace scope. Such names may be found under for associated classes.

template <typename T> struct number {
    number(int);
    friend number gcd(number x, number y) { return 0; }
};

void g() {
    number<double> a(3), b(4);
    a = gcd(a, b);    // finds gcd becuase numer<double> is an associated class,
                      // making gcd visible in its namespace (global scope)
    b = gcd(3, 4);    // error: gcd is not visible
}

此功能已用于在编译时捕获和检索元编程状态。

template <int X>
struct flag {
    friend consteval int f(flag<X>);
};

template <int X, int N>
struct injecter {
    friend consteval int f(flag<X>) { return N; }
};

template <int N = 0, auto = []{}>
consteval auto inject() {
    if constexpr (requires { f(flag<X>{}); }) {
        return inject<N+1>();
    } else {
        void(injecter<X>{});
        return f(flag<X>{});
    }
}

static_assert(inject() == 0);
static_assert(inject() == 1);
static_assert(inject() == 2);
static_assert(inject() == 3);
// ...

据我所知，CWG 有一个 issue 旨在使此类行为成为错误的行为，尽管禁止它们的机制尚未确定。但是，由于最新草案中似乎没有该段落，我想知道友元注入在 C++23 中是否格式错误。

Answer 1

来自P1787R6：

Merged [temp.inject] into [temp.friend]

（2021 年 3 月 N4885 通过）

当前草案 (N4901) 为 ([temp.friend]p2):

Friend classes, class templates, functions, or function templates can be declared within a class template. When a template is instantiated, its friend declarations are found by name lookup as if the specialization had been explicitly declared at its point of instantiation

里面好像覆盖了旧的[temp.inject]p2

friend 注入会是病态的吗？

Will friend injections be ill-formed?

c++

templates

language-lawyer

friend

c++23