包含命名空间的类模板的转发声明会导致编译错误

Question

我有一个example1和一个example2，它使用前缀为相应类模板命名空间的类模板的前向声明。第一个例子用visual-studio编译好，而第二个例子没有。我检查了两个例子与其他编译器（http://rextester.com）。现在我有两个问题：

在这里使用前向声明中的命名空间似乎是非法的。为什么呢？
Visual Studio（2015和2017）似乎允许在第一个示例中使用附加命名空间进行前向声明，但在第二个示例中不允许。这是一个错误吗？

例1：

#include <vector>

namespace N1
{
    template <typename T> struct MySystem {};
    template <typename T> class Other {};

    struct MyClass
    {
        MyClass() { typename Dependencies::TYPE_A oTYPE_A; }

        struct Dependencies
        {
            template <typename>
            class N1::Other;

            struct TypeX_Dependencies;

            using TYPE_A = N1::MySystem<N1::Other<TypeX_Dependencies>>;

            struct TypeX_Dependencies
            {
                using TYPE_A = typename Dependencies::TYPE_A;
            };

            using TYPE_X = N1::Other<TypeX_Dependencies>;
        };
    };
}

int main(){ return 0; }

c ++（gcc 5.4.0） source_file.cpp:15:23: error: invalid use of template-name ‘N1::Other’ without an argument list class N1::Other;

c ++（clang 3.8.0） source_file.cpp:15:23: error: non-friend class member 'Other' cannot have a qualified name class N1::Other; source_file.cpp:15:19: error: forward declaration of class cannot have a nested name specifier class N1::Other;

c ++（vc ++ 19.00.23506 for x64 / also vs community 2017 15.4.4） compiles fine

例2：

#include <vector>

namespace N1
{
    template <typename T> struct MySystem {};
    template <typename T> class Other {};

    template <typename T>
    struct MyClass
    {
        MyClass() { typename Dependencies::TYPE_A oTYPE_A; }

        struct Dependencies
        {
            template <typename>
            class N1::Other;

            struct TypeX_Dependencies;

            using TYPE_A = N1::MySystem<N1::Other<TypeX_Dependencies>>;

            struct TypeX_Dependencies
            {
                using TYPE_A = typename Dependencies::TYPE_A;
            };

            using TYPE_X = N1::Other<TypeX_Dependencies>;
        };
    };
}

int main(){ return 0; }

c ++（gcc 5.4.0） source_file.cpp:16:23: error: invalid use of template-name ‘N1::Other’ without an argument list class N1::Other;

c ++（clang 3.8.0） source_file.cpp:16:23: error: non-friend class member 'Other' cannot have a qualified name class N1::Other; source_file.cpp:16:19: error: forward declaration of class cannot have a nested name specifier class N1::Other;

c ++（vc ++ 19.00.23506 for x64 / also vs community 2017 15.4.4） source_file.cpp(16): error C3855: 'N1::Other': template parameter 'T' is incompatible with the declaration source_file.cpp(24): note: see reference to class template instantiation 'N1::MyClass<T>::Dependencies' being compiled source_file.cpp(29): note: see reference to class template instantiation 'N1::MyClass<T>' being compiled source_file.cpp(20): error C3203: 'Other': unspecialized class template can't be used as a template argument for template parameter 'T', expected a real type