Fixes the incomplete type and ambiguous dependent name problem for CRTP: https://godbolt.org/z/3WGe1q5K5

CRTP-(de)incomplete-(de)ambiguous.cpp

template <typename U>
struct A_trait {
    using T = int;    
};

template <typename U, template <typename ...> class Trait = A_trait>
struct A {
    using T = typename Trait<U>::T;
};

template <typename U>
struct B_trait {
    using T = int;
};

template <typename U, template <typename ...> class Trait = B_trait>
struct B {
    using T = typename Trait<U>::T;
};

template <typename U>
struct C_trait {
};

template <typename U, template <typename ...> class Trait = C_trait>
struct C {
    using T = typename Trait<U>::T;
};

// using multiple inherits to implements multi-inherits without the ambiguous error
//
// asusme the ambiguous dependent types are actually the same type

template <typename U>
struct Trait0 {};

template <typename U>
struct Trait1: public Trait0<U>, public A_trait<U> {};

template <typename U>
struct Trait2: public Trait1<U>, public B_trait<U> {};

template <typename U>
struct Trait: public Trait2<U>, public C_trait<U> {};

// pass the final trait as the type parameter
struct K: public A<K, Trait>, public B<K, Trait>, public C<K, Trait> {
};

int main() {
    K k{};
    K::T x{};
}