Typelist of nested types - c++

Question

I've got a typelist providing the following interface :
template <typename... Ts>
struct type_list
{
static constexpr size_t length = sizeof...(Ts);
template <typename T>
using push_front = type_list<T, Ts...>;
template <typename T>
using push_back = type_list<Ts..., T>;
// hidden implementation of complex "methods"
template <uint64_t index>
using at;
struct pop_front;
template <typename U>
using concat;
template <uint64_t index>
struct split;
template <uint64_t index, typename T>
using insert;
template <uint64_t index>
using remove;
};
In another piece of code, I have such a typelist TL of types statically inheriting a base class providing such an interface :
template<typename Derived>
struct Expression {
using type1 = typename Derived::_type1;
using type2 = typename Derived::_type2;
};
struct Exp1 : Expression<Exp1> {
template<typename> friend struct Expression;
private:
using _type1 = float;
using _type2 = int;
};
struct Exp2 : Expression<Exp2> {
template<typename> friend struct Expression;
private:
using _type1 = double;
using _type2 = short;
};
I want to make the typelist of nested types from TL, something like :
using TL = type_list<Exp1, Exp2>;
using TL2 = type_list<TL::type1...>; // type_list<float, double>
but I can't expand TL as it's not an unexpanded parameter pack.
I've thought about index_sequence but can't manage to make it work.

Answer 1

The question is seemingly looking for map, also called transform in C++. TL is one list of types, and the desire is to apply some type-level function (extract ::type1) and have another list of types. Writing transform is straightforward:
template <template <typename> typename fn, typename TL>
struct type_list_transform_impl;
template <template <typename> typename fn, typename... Ts>
struct type_list_transform_impl<fn, type_list<Ts...>>
{
using type = type_list<fn<Ts>...>;
};
template <template <typename> typename fn, typename TL>
using type_list_transform = type_list_transform_impl<fn, TL>::type;
And then the type-level function:
template <typename T>
using type1_of = typename T::type1;
And combine the pieces to get TL2:
using TL2 = type_list_transform<type1_of, TL>; // type_list<float, double>
Example: https://godbolt.org/z/b7TMoac5c