c++ - VS2015 和 clang 编译此代码，但 g++ 拒绝它。哪一个是正确的？

Question

VS2015 和 clang 编译这段代码，但是 g++ rejects it .

namespace A {
   struct B {
      friend void f();
   };
}
void A::f() {}

int main(){
}

我认为 g++ 是正确的，因为 7.3.1.2/3 中的注释:

If a friend declaration in a non-local class first declares a class, function, class template or function template⁹⁷ the friend is a member of the innermost enclosing namespace. The friend declaration does not by itself make the name visible to unqualified lookup ([basic.lookup.unqual]) or qualified lookup ([basic.lookup.qual]). [ Note: The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship).  — end note ] If a friend function or function template is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments ([basic.lookup.argdep]). If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace. [ Note: The other forms of friend declarations cannot declare a new member of the innermost enclosing namespace and thus follow the usual lookup rules.  — end note ]

Answer 1

你引用的这一部分比你强调的注释更明确:

The friend declaration does not by itself make the name visible to unqualified lookup ([basic.lookup.unqual]) or qualified lookup ([basic.lookup.qual]).

您的定义依赖于限定查找来查找已在命名空间内声明的函数。但该名称对合格查找不可见。应该拒绝此代码。

这是在第 8.3 节 [dcl.meaning] 中找到的相关规则:

When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers (or, in the case of a namespace, of an element of the inline namespace set of that namespace) or to a specialization thereof; the member shall not merely have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier of the declarator-id .