嵌套类友元函数访问在 gcc 和 clang 中有效，但在 msvc 中无效

问：

我有这个程序，在类中定义了一个朋友函数，它使用 gcc 和 clang 进行编译，但被 msvc 拒绝。

#include <iostream>
template<typename T>
class Outer
{
     struct Inner;
     friend void foo(); 
};

template<typename T>
struct Outer<T>::Inner
{
  friend void bar(Inner) 
  { 
    //gcc and clang accepts but msvc rejects this 
    typename Outer<int>::Inner k;    
    std::cout << "bar called";
  }
};
void foo()
{
    std::cout << "foo called";
    Outer<int>::Inner i;
    bar(i);    
}
int main()
{
    Outer<int> outer;
    foo();
}

工作演示

根据标准，哪个编译器在这里？

MSVC 说道：

<source>(14): error C2248: 'Outer<int>::Inner': cannot access private struct declared in class 'Outer<int>'
<source>(5): note: see declaration of 'Outer<int>::Inner'
<source>(14): note: see declaration of 'Outer<int>'
<source>(14): note: the template instantiation context (the oldest one first) is
<source>(17): note: see reference to class template instantiation 'Outer<T>::Inner' being compiled
<source>(14): note: see reference to class template instantiation 'Outer<int>::Inner' being compiled
<source>(12): note: while compiling class template member function 'void bar(Outer<int>::Inner)'
<source>(22): note: see the first reference to 'bar' in 'foo'
Compiler returned: 2