C++: nested class of a template class

Question

Consider the following code:

template < typename T >
struct A
{
    struct B { };
};

template < typename T >
void f( typename A::B ) {          


        

Answer 1

typename A::B

Here, T is in a nondeduced context, which means that T cannot be deduced from the function argument.

The problem is that in the general case, there is a potentially infinite number of possible types T that could match. Consider, for example, if instead of struct B { }; you had typedef int B;.