No type named 'type' in CTRP derived class

Question

I\'ve been experimenting with the Curiously Recurring Template Pattern for a generic single-argument functor and have two implementations: one using a template template para

Answer 1

Note: This question has already been answered by @r-sahu, but I'd like to elaborate on this and address clang's output specifically.

The problem can be demonstrated on a much smaller code sample: (@vtt suggested something similar)

template 
struct A {
    using _C = typename _CRTP::C;
};

struct B : public A {
    using C = int;
};

Compiling that with clang will result in completely misleading error message: (godbolt)

:3:32: error: no type named 'C' in 'B'
    using _C = typename _CRTP::C;
               ~~~~~~~~~~~~~~~~^
:6:19: note: in instantiation of template class 'A' requested here
struct B : public A {
                  ^
1 error generated.

GCC's error message is a little more helpful: (godbolt)

: In instantiation of 'struct A':
:6:19:   required from here
:3:33: error: invalid use of incomplete type 'struct B'
    3 |     using _C = typename _CRTP::C;
      |                                 ^
:6:8: note: forward declaration of 'struct B'
    6 | struct B : public A {
      |        ^

As suggested in the accepted answer, implementing a trait type fixes the issue:

// this declaration must appear before the definition of A
template 
struct a_traits;

template 
struct A {
    // `a_traits<_CRTP>::type` is an incomplete type at this point,
    // but that doesn't matter since `A` is also incomplete
    using _C = typename a_traits<_CRTP>::type;
};

// this specialization must appear before the definition of B
template <>
struct a_traits { // adding the type specifier `struct` will declare B
    using type = int;
};

// specifying the template parameter will complete the type `A`, which works since
// `a_traits` is already complete at this point
struct B : public A {
    using C = int;
};