c – 模板中的朋友功能(为什么这在Visual Studio中失败但在GCC和

#include <utility>

namespace Generic
{
    class Whatever
    {
    public:
        // ISSUE1 (member "swap()" has same name as non-member template)
        void swap()
        {
        }
    };

    // Forward declaration of class template "Test"
    template <class>
    class Test;

    // Prototype of function template "swap()" for class template "Test"
    template<class T>
    void swap(Test<T> &,Test<T> &);

    /////////////////////////////////////////////////////////////////
    // Class template "Test"
    /////////////////////////////////////////////////////////////////
    template <class T>
    class Test : public T
    {
    public:
        // Default constructor
        Test() = default;

        // Move constructor
        Test(Test &&test)
            : BaseClass(std::move(test))
        {
        }

        // Assignment operator
        Test& operator=(Test test)
        {
            // Note that the "Generic:" prefix here has no impact on the results
            Generic::swap(*this,test);

            return (*this);
        }

    private:
        using BaseClass = T;

        // ISSUE2 (qualified call - prefixing with "Generic:")
        friend void Generic::swap<T>(Test &,Test &);
    };

    /////////////////////////////////////////////////////////////////
    // Canonical swap function for class "Test" just above
    /////////////////////////////////////////////////////////////////
    template<class T>
    void swap(Test<T> &,Test<T> &)
    {
    }

} // namespace Generic

int main()
{
    using namespace Generic;
    Test<Whatever> test1;
    Test<Whatever> test2;
    test2 = std::move(test1);

    return 0;
}

任何人都可以看到它不应该编译的任何理由.请注意注释中的ISSUE1和ISSUE2.这些会影响结果. ISSUE1和ISSUE2有4种组合可以让事情发挥作用(见下表),虽然我知道另一种纠正方法,但这4种组合是我想要关注的.请注意,所有4种组合都在GCC和Clang中完全编译,但只有第4项在Visual Studio 2017(最新版本)中干净地编译.其他组合(1到3)失败,并在下面的RESULT列中看到错误(编译器引用了所有3种情况下ISSUE2行的问题)：

ISSUE1                               ISSUE2                          RESULT
   ------                               ------                          ------
1) Leave unchanged                      Leave unchanged                 'Generic::Test': use of class template requires template argument list
                                                                        'Generic::swap': not a function

2) Leave unchanged                      Remove "Generic:" qualifier     syntax error: missing ';' before '<'
                                                                        'swap': illegal use of type 'void'
                                                                        'Generic::swap': 'friend' not permitted on data declarations
                                                                        'Generic::swap': redefinition; previous definition was 'function'
                                                                        unexpected token(s) preceding ';'

3) Comment out member "swap()"          Leave unchanged                 'Generic::Test': use of class template requires template argument list
                                                                        'Generic::swap': not a function

4) Comment out member "swap()"          Remove "Generic:" qualifier     Works!!!

是Visual Studio只是错误或GCC和Clang错了(前者似乎是基于我对合格/非限定名称查找规则,ADL等的理解).谢谢.