c – 由多重继承隐藏的嵌套类

struct Base {
    template <typename>
    struct nested;
};
struct Derived1 : Base { };
struct Derived2 : Base { };
struct Derived3 : Derived1,Derived2 { };

typedef Derived3::nested<int> xxx;

我知道的

上面的代码无法编译：

> Apple LLVM 5.0(clang-500.2.75)
> Clang 3.4

但它成功编译：

> gcc 4.9.0 20131110(实验性)
> gcc 4.8

另外,如果我将嵌套类型更改为非模板类​​型,即

struct Base {
    struct nested;
};
...
typedef Derived3::nested xxx;

然后它适用于上述编译器.

[编辑]
将嵌套模板结构更改为模板别名也不会改变任何内容;

template <typename> struct dependent { struct type; };
struct Base {
    template <typename T>
    using nested = typename dependent<T>::type;
};

与上述编译器产生相同的结果.
[结束编辑]

来自N3242§10.1[class.mi]

A class can be an indirect base class more than once and can be a direct and an indirect base class. There are limited things that can be done with such a class. The non-static data members and member functions of the direct base class cannot be referred to in the scope of the derived class. However,the static members,enumerations and types can be unambiguously referred to.

我认为这意味着代码应该是有效的,但我不确定.