struct B
{
    struct M
    {
        void f()
        {
        }
    };
};

namespace N
{
    struct M
    {
        void f()
        {
        }
    };
}

template<typename>
struct A : N::M,B::M
{
    typedef B N;
};

struct D : A<int>
{
    A<int> m;
    void f()
    {
        m.N::M::f(); // found class-name 'A<int>::N' (unambiguous)
    }
};

template<typename T>
struct C : A<T>
{
    A<T> m;
    void f()
    {
        m.N::M::f(); // found namespace-name 'N' (ambiguous?)
    }
};

template struct C<int>;

在咨询标准之后,对于C< T> :: f()中的表达式,我不清楚哪种行为是正确的.

因为N在对象表达式n(它是依赖的)的类的范围内并且在整个后缀表达式的上下文中查找(即,函数C< T> :: f()的范围),所以它有必要延迟查找直到实例化点.

在实例化的时刻,如果找到命名空间N和typedef A< T> :: N,则查找将是不明确的.只有当A< T> :: N的声明不被隐藏时,N的声明才可见.

问题是,在整个后缀表达式的上下文中“N”和“在模板的定义点”时,命名空间N是否应被视为被查找N的typedef A< T> :: N隐藏”.

引用C工作草案标准N3242 = 11-0012(2011年2月)：

3.4.5 Class member access [basic.lookup.classref]

If the id-expression in a class member access is a qualified-id of the form

class-name-or-namespace-name::...

the class-name-or-namespace-name following the . or -> operator is looked up both in the context of the
entire postfix-expression and in the scope of the class of the object expression. If the name is found only in
the scope of the class of the object expression,the name shall refer to a class-name. If the name is found
only in the context of the entire postfix-expression,the name shall refer to a class-name or namespace-name. If the name is found in both contexts,the class-name-or-namespace-name shall refer to the same
entity.

14.6.4 Dependent name resolution [temp.dep.res]

In resolving dependent names,names from the following sources are considered:

— Declarations that are visible at the point of definition of the template.

— Declarations from namespaces associated with the types of the function arguments both from the
instantiation context (14.6.4.1) and from the definition context.