[366] - solution using friend injection

tips/366.md

@@ -104,4 +104,46 @@ int main() {
 
 > https://godbolt.org/z/P6sTdPEG8
 
+```cpp
+template<class...> struct type_list {};
+template<auto> struct nth { auto friend get(nth); auto friend get(nth); };
+template<auto N, class T> struct set { auto friend get(nth<N>) { return T{}; } };
+
+template<class T, template<class...> class TList, class... Ts> auto append_impl(TList<Ts...>) -> TList<Ts..., T>;
+
+template<class T, auto N = 0, auto unique = []{}>
+consteval auto append() {
+    if constexpr (requires { get(nth<N>{}); }) {
+        append<T, N+1, unique>();
+    } else if constexpr (N == 0) {
+        void(set<N, type_list<T>>{});
+    } else {
+        void(set<N, decltype(append_impl<T>(get(nth<N-1>{})))>{});
+    }
+}
+
+template<auto unique = []{}, auto N = 0>
+consteval auto get_list() {
+    if constexpr (requires { get(nth<N>{}); }) {
+        return get_list<unique, N+1>();
+    } else if constexpr (N == 0) {
+        return type_list{};
+    } else {
+        return get(nth<N-1>{});
+    }
+}
+
+int main() {
+  static_assert(typeid(get_list()) == typeid(type_list<>));
+
+  append<int>();
+  static_assert(typeid(get_list()) == typeid(type_list<int>));
+
+  append<float>();
+  static_assert(typeid(get_list()) == typeid(type_list<int, float>));
+}
+```
+
+> https://godbolt.org/z/d7GrW1j76
+
 </p></details>