Curious Mixin with variadic constructor

Question

I have a code snipped including a variadic mixin crtp of some sorts and a few related questions. Do I understand correctly that in the following code, the second constructor is merely passing copies of instances of those classes which were used to instantiate X to the constructors of those very same classes?

template<class DerivedT>
struct CuriousBase{};

template<template<typename> typename... Features>
struct X : Features<X<Features...>> ... {
  X() = default;
  X(Features<X<Features...>> ...f) : Features<X<Features...>>(f)... {}
};

int main(){
  auto x = X<CuriousBase>{};
}

what are use cases for such a behaviour and what is the difference between this sample and the following snippet?

template<class DerivedT>
struct CuriousBase{};

template<template<typename> typename... Features>
struct X : Features<X<Features...>> ... {
  X() = default;
  X(Features<X> ...f) : Features<X>(f)... {}
};

int main(){
  auto x = X<CuriousBase>{};
}

Both do compile but I thought, that one has to specify explicitly that X is itself a variadic class template but that does not seem to be necessary...

help is greatly appreciated

Answer 1

X is a injected class name, so indeed X and X<Features...> are equivalent inside the class.