Inner template class as template template parameter

Question

I have a class that takes a template template parameter:

template <template <typename> class F>
class A {};

And another templated class with an inner templated class:

template <typename T>
class B {
  public:
    template <typename U>
    class C {};
};

I want to be able to use C as the template template parameter for A in a templated context:

template <typename T>
using D = A<B<T>::C>;

However, this results in an error message:

"template argument for template template parameter must be a class template or type alias template"

I am assuming that I am missing some magic incantation of typename and template in the declaration of D, but I cannot figure it out for the life of me and the error message isn't particularly useful.

Answer 1

The error message complains that B<T>::C is not a template, thus it doesn't match the template template parameter of A.

You need to use template keyword to tell the compiler that the dependent name B<T>::C (which depends on the template parameter T) is a template.

template <typename T>
using D = A<B<T>::template C>;
//                ~~~~~~~~