Is it possible to access child types in c++ using CRTP?

Question

I have this toy example,

template <typename TChild>
struct Base {
    template <typename T>
    using Foo = typename TChild::template B<T>;
};

struct Child : Base<Child> {
    template <typename T>
    using B = T;
};


using Bar = Child::Foo<int>;

which fails to compile. The intention is that I have a parent class that provides type computations based on members of the child class. The child class is provided via CRTP. However the line

using Foo = typename TChild::template B<T>;

fails to compile:

<source>: In instantiation of 'struct Base<Child>':
<source>:16:16:   required from here
<source>:13:11: error: invalid use of incomplete type 'struct Child'
   13 |     using Foo = typename TChild::template B<T>;
      |           ^~~
<source>:16:8: note: forward declaration of 'struct Child'
   16 | struct Child : Base<Child> {
      |        ^~~~~

Am I being naive in expecting such a construct to work?

Failing code at https://godbolt.org/z/5Prb84

Answer 1

Let me post another way to do it:

template<typename TChild, class T>
struct GetB {
    using Type = typename TChild::template B<T>;
};

template<typename TChild>
struct Base {
    template<typename T>
    using Foo = typename GetB<TChild, T>::Type;
};

struct Child : Base<Child> {
    template<typename T>
    using B = T;
};

I don't have a language-lawyer-type explanation why this works, but it should be related to having an additional level of indirection. When a compiler sees

using Foo = typename TChild::template B<T>;

it can (and will) check and complain right at this point that an incomplete type is used. However, when we wrap access to B<T> into a function or a struct,

using Foo = typename GetB<TChild, T>::Type;

then we're not accessing internals of TChild at this point, we're just using the name of it, which is fine.