From 96c9f7af992553550f21c73366996c7901a9953f Mon Sep 17 00:00:00 2001
From: Denis Blank <denis.blank@outlook.com>
Date: Tue, 25 Dec 2018 11:12:11 +0100
Subject: [PATCH] More

---
 include/continuable/continuable-base.hpp      |   2 +-
 include/continuable/continuable-coroutine.hpp |   2 +-
 .../continuable/continuable-primitives.hpp    |   8 ++
 .../continuable/continuable-promise-base.hpp  |   2 +-
 include/continuable/continuable-testing.hpp   |   2 +-
 include/continuable/continuable-trait.hpp     |  11 +-
 include/continuable/continuable-types.hpp     |  65 ++++--------
 include/continuable/continuable.hpp           |   1 -
 .../connection/connection-aggregated.hpp      |  26 ++---
 .../detail/connection/connection-any.hpp      |  18 ++--
 .../continuable/detail/core/annotation.hpp    |   6 +-
 include/continuable/detail/core/base.hpp      | 100 +++++++++---------
 include/continuable/detail/core/types.hpp     |   1 +
 .../continuable/detail/other/coroutines.hpp   |   4 +-
 include/continuable/detail/other/erasure.hpp  |  97 +++++++++++++++++
 include/continuable/detail/other/testing.hpp  |   6 +-
 .../continuable/detail/other/transforms.hpp   |   4 +-
 .../detail/utility/flat-variant.hpp           |   4 +-
 .../continuable/detail/utility/identity.hpp   |  54 ++++++++++
 include/continuable/detail/utility/traits.hpp |  14 +--
 .../multi/test-continuable-base-errors.cpp    |   2 +-
 test/unit-test/test-continuable.hpp           |  16 +--
 22 files changed, 282 insertions(+), 163 deletions(-)
 create mode 100644 include/continuable/detail/other/erasure.hpp
 create mode 100644 include/continuable/detail/utility/identity.hpp

diff --git a/include/continuable/continuable-base.hpp b/include/continuable/continuable-base.hpp
index 9a7039e..ae7889c 100644
--- a/include/continuable/continuable-base.hpp
+++ b/include/continuable/continuable-base.hpp
@@ -835,7 +835,7 @@ constexpr auto make_continuable(Continuation&& continuation) {
 template <typename... Args>
 auto make_ready_continuable(Args&&... args) {
   using detail::base::ready_continuation;
-  using detail::traits::identity;
+  using detail::identity;
   using detail::traits::unrefcv_t;
   return detail::base::attorney::create_from_raw(
       ready_continuation<unrefcv_t<Args>...>{std::forward<Args>(args)...},
diff --git a/include/continuable/continuable-coroutine.hpp b/include/continuable/continuable-coroutine.hpp
index fdd21fd..ee1680d 100644
--- a/include/continuable/continuable-coroutine.hpp
+++ b/include/continuable/continuable-coroutine.hpp
@@ -51,7 +51,7 @@ namespace std {
 namespace experimental {
 template <typename Data, typename... Args, typename... FunctionArgs>
 struct coroutine_traits<
-    cti::continuable_base<Data, cti::detail::traits::identity<Args...>>,
+    cti::continuable_base<Data, cti::detail::identity<Args...>>,
     FunctionArgs...> {
 
   using promise_type =
diff --git a/include/continuable/continuable-primitives.hpp b/include/continuable/continuable-primitives.hpp
index 054eb11..de26dda 100644
--- a/include/continuable/continuable-primitives.hpp
+++ b/include/continuable/continuable-primitives.hpp
@@ -32,6 +32,7 @@
 #define CONTINUABLE_PRIMITIVES_HPP_INCLUDED
 
 #include <continuable/detail/core/types.hpp>
+#include <continuable/detail/utility/identity.hpp>
 
 namespace cti {
 /// \defgroup Primitives Primitives
@@ -56,6 +57,13 @@ namespace cti {
 /// ```
 /// \{
 
+/// Represents the tag type that is used to specify the signature hint
+/// of a continuable_base or promise_base.
+///
+/// \since 4.0.0
+template <typename... Args>
+using signature_arg_t = detail::identity<Args...>;
+
 /// Represents the tag type that is used to query the continuation
 /// for whether it resolves the callback instantly with its arguments
 /// without having side effects.
diff --git a/include/continuable/continuable-promise-base.hpp b/include/continuable/continuable-promise-base.hpp
index fa4780f..de72523 100644
--- a/include/continuable/continuable-promise-base.hpp
+++ b/include/continuable/continuable-promise-base.hpp
@@ -62,7 +62,7 @@ class promise_base
   /// \cond false
   ;
 template <typename Data, typename... Args>
-class promise_base<Data, detail::traits::identity<Args...>>
+class promise_base<Data, detail::identity<Args...>>
     : detail::util::non_copyable
   /// \endcond
 { // clang-format on
diff --git a/include/continuable/continuable-testing.hpp b/include/continuable/continuable-testing.hpp
index df9847a..34e8710 100644
--- a/include/continuable/continuable-testing.hpp
+++ b/include/continuable/continuable-testing.hpp
@@ -158,7 +158,7 @@
 /// \since 1.0.0
 #define ASSERT_ASYNC_TYPES(CONTINUABLE, ...)                                   \
   cti::detail::testing::assert_async_types(                                    \
-      CONTINUABLE, cti::detail::traits::identity<__VA_ARGS__>{})
+      CONTINUABLE, cti::detail::identity<__VA_ARGS__>{})
 
 /// Asserts that the continuable is finished with the given exception
 ///
diff --git a/include/continuable/continuable-trait.hpp b/include/continuable/continuable-trait.hpp
index 7d42080..1918002 100644
--- a/include/continuable/continuable-trait.hpp
+++ b/include/continuable/continuable-trait.hpp
@@ -59,17 +59,12 @@ namespace cti {
 template <template <std::size_t, typename...> class CallbackWrapper,
           template <std::size_t, typename...> class ContinuationWrapper,
           typename... Args>
-class continuable_trait {
-
+struct continuable_trait {
   using callback = CallbackWrapper<0U, void(Args...)&&,
                                    void(exception_arg_t, exception_t) &&>;
 
-public:
-  /// The promise type which is used to resolve continuations
-  using promise = promise_base<callback, detail::traits::identity<Args...>>;
-
   /// The continuable type for the given parameters.
-  using continuable = continuable_base<
+  using continuation = continuable_base<
       ContinuationWrapper<
           // Size the buffer for the small functor optimization so the
           // result itself fits in and guarantee that a pointer can
@@ -81,7 +76,7 @@ public:
           bool(is_ready_arg_t) const,      //
           std::tuple<Args...>(query_arg_t) //
           >,
-      detail::traits::identity<Args...>>;
+      detail::identity<Args...>>;
 };
 /// \}
 } // namespace cti
diff --git a/include/continuable/continuable-types.hpp b/include/continuable/continuable-types.hpp
index ab0f9a2..0e6e8ed 100644
--- a/include/continuable/continuable-types.hpp
+++ b/include/continuable/continuable-types.hpp
@@ -33,7 +33,10 @@
 
 #include <cstddef>
 #include <function2/function2.hpp>
-#include <continuable/continuable-trait.hpp>
+#include <continuable/continuable-base.hpp>
+#include <continuable/continuable-primitives.hpp>
+#include <continuable/continuable-promise-base.hpp>
+#include <continuable/detail/other/erasure.hpp>
 
 namespace cti {
 /// \defgroup Types Types
@@ -41,66 +44,40 @@ namespace cti {
 /// cti::promise promise\endlink facility for type erasure.
 /// \{
 
-namespace detail {
-/// A type erasure which isn't size adjusted and move only
-template <std::size_t, typename... Args>
-class type_erasure : public fu2::unique_function<Args...> {
-public:
-  using fu2::unique_function<Args...>::unique_function;
-  using fu2::unique_function<Args...>::operator=;
-  using fu2::unique_function<Args...>::operator();
-};
-
-/// A function which is size adjusted and move only
-template <std::size_t Size, typename... Args>
-class sized_type_erasure
-    : public fu2::function_base<true, false, Size, true, false, Args...> {
-
-public:
-  using fu2::function_base<true, false, Size, //
-                           true, false, Args...>::function_base;
-  using fu2::function_base<true, false, Size, //
-                           true, false, Args...>::operator=;
-  using fu2::function_base<true, false, Size, //
-                           true, false, Args...>::operator();
-};
-
-/// We adjust the internal capacity of the outer function wrapper so
-/// we don't have to allocate twice when using `continuable<...>`.
-template <typename... Args>
-using unique_trait_of = continuable_trait< //
-    type_erasure, sized_type_erasure,
-    Args... //
-    >;
-
-/// A type erasure for work objects
-class work_type_erasure : public fu2::unique_function<void()> {
-public:
-  using fu2::unique_function<void()>::unique_function;
-  using fu2::unique_function<void()>::operator=;
-  using fu2::unique_function<void()>::operator();
-};
-} // namespace detail
+/// Deduces to the preferred continuation capacity for a possible
+/// small functor optimization. The given capacity size is always enough to
+/// to avoid any allocation when storing ready a continuable_base.
+///
+/// \since 4.0.0
+// template <typename... Args>
+// using continuation_capacity = detail::erasure::continuation_capacity<Args...>;
 
 /// Defines a non-copyable continuation type which uses the
 /// function2 backend for type erasure.
 ///
 /// Usable like: `continuable<int, float>`
 template <typename... Args>
-using continuable = typename detail::unique_trait_of<Args...>::continuable;
+using continuable = continuable_base<detail::erasure::continuation<Args...>, //
+                                     signature_arg_t<Args...>>;
 
 /// Defines a non-copyable promise type which is using the
 /// function2 backend for type erasure.
 ///
 /// Usable like: `promise<int, float>`
 template <typename... Args>
-using promise = typename detail::unique_trait_of<Args...>::promise;
+using promise = promise_base<detail::erasure::callback<Args...>, //
+                             signature_arg_t<Args...>>;
 
 /// Defines a non-copyable type erasure which is capable of carrying
 /// callable objects passed to executors.
 ///
 /// \since 4.0.0
-using work = detail::work_type_erasure;
+class work : public fu2::unique_function<void()> {
+public:
+  using fu2::unique_function<void()>::unique_function;
+  using fu2::unique_function<void()>::operator=;
+  using fu2::unique_function<void()>::operator();
+};
 
 // TODO channel
 // TODO sink
diff --git a/include/continuable/continuable.hpp b/include/continuable/continuable.hpp
index ef325ea..ca016a4 100644
--- a/include/continuable/continuable.hpp
+++ b/include/continuable/continuable.hpp
@@ -52,7 +52,6 @@ namespace cti {}
 #include <continuable/continuable-promise-base.hpp>
 #include <continuable/continuable-promisify.hpp>
 #include <continuable/continuable-result.hpp>
-#include <continuable/continuable-trait.hpp>
 #include <continuable/continuable-transforms.hpp>
 #include <continuable/continuable-traverse-async.hpp>
 #include <continuable/continuable-traverse.hpp>
diff --git a/include/continuable/detail/connection/connection-aggregated.hpp b/include/continuable/detail/connection/connection-aggregated.hpp
index aee8382..42a3044 100644
--- a/include/continuable/detail/connection/connection-aggregated.hpp
+++ b/include/continuable/detail/connection/connection-aggregated.hpp
@@ -75,13 +75,13 @@ T&& unpack_lazy(container::flat_variant<T>&& value) {
 template <typename Continuable>
 class continuable_box;
 template <typename Data>
-class continuable_box<continuable_base<Data, traits::identity<>>> {
+class continuable_box<continuable_base<Data, identity<>>> {
 
-  continuable_base<Data, traits::identity<>> continuable_;
+  continuable_base<Data, identity<>> continuable_;
 
 public:
   explicit continuable_box(
-      continuable_base<Data, traits::identity<>>&& continuable)
+      continuable_base<Data, identity<>>&& continuable)
       : continuable_(std::move(continuable)) {
   }
 
@@ -101,14 +101,14 @@ public:
   }
 };
 template <typename Data, typename First>
-class continuable_box<continuable_base<Data, traits::identity<First>>> {
+class continuable_box<continuable_base<Data, identity<First>>> {
 
-  continuable_base<Data, traits::identity<First>> continuable_;
+  continuable_base<Data, identity<First>> continuable_;
   lazy_value_t<First> first_;
 
 public:
   explicit continuable_box(
-      continuable_base<Data, traits::identity<First>>&& continuable)
+      continuable_base<Data, identity<First>>&& continuable)
       : continuable_(std::move(continuable)) {
   }
 
@@ -130,14 +130,14 @@ public:
 };
 template <typename Data, typename First, typename Second, typename... Rest>
 class continuable_box<
-    continuable_base<Data, traits::identity<First, Second, Rest...>>> {
+    continuable_base<Data, identity<First, Second, Rest...>>> {
 
-  continuable_base<Data, traits::identity<First, Second, Rest...>> continuable_;
+  continuable_base<Data, identity<First, Second, Rest...>> continuable_;
   lazy_value_t<std::tuple<First, Second, Rest...>> args_;
 
 public:
   explicit continuable_box(
-      continuable_base<Data, traits::identity<First, Second, Rest...>>&&
+      continuable_base<Data, identity<First, Second, Rest...>>&&
           continuable)
       : continuable_(std::move(continuable)) {
   }
@@ -209,12 +209,12 @@ constexpr auto unbox_continuables(Args&&... args) {
 
 namespace detail {
 template <typename Callback, typename Data>
-constexpr auto finalize_impl(traits::identity<void>, Callback&& callback,
+constexpr auto finalize_impl(identity<void>, Callback&& callback,
                              Data&&) {
   return std::forward<Callback>(callback)();
 }
 template <typename... Args, typename Callback, typename Data>
-constexpr auto finalize_impl(traits::identity<std::tuple<Args...>>,
+constexpr auto finalize_impl(identity<std::tuple<Args...>>,
                              Callback&& callback, Data&& data) {
   // Call the final callback with the cleaned result
   return traits::unpack(std::forward<Callback>(callback),
@@ -223,7 +223,7 @@ constexpr auto finalize_impl(traits::identity<std::tuple<Args...>>,
 
 struct hint_mapper {
   template <typename... T>
-  constexpr auto operator()(T...) -> traits::identity<T...> {
+  constexpr auto operator()(T...) -> identity<T...> {
     return {};
   }
 };
@@ -233,7 +233,7 @@ template <typename Callback, typename Data>
 constexpr auto finalize_data(Callback&& callback, Data&& data) {
   using result_t = decltype(unbox_continuables(std::forward<Data>(data)));
   // Guard the final result against void
-  return detail::finalize_impl(traits::identity<std::decay_t<result_t>>{},
+  return detail::finalize_impl(identity<std::decay_t<result_t>>{},
                                std::forward<Callback>(callback),
                                std::forward<Data>(data));
 }
diff --git a/include/continuable/detail/connection/connection-any.hpp b/include/continuable/detail/connection/connection-any.hpp
index cc9c1fa..fbdde2b 100644
--- a/include/continuable/detail/connection/connection-any.hpp
+++ b/include/continuable/detail/connection/connection-any.hpp
@@ -88,30 +88,30 @@ private:
 
 struct result_deducer {
   template <typename T>
-  static auto deduce_one(std::false_type, traits::identity<T>) {
+  static auto deduce_one(std::false_type, identity<T>) {
     static_assert(traits::fail<T>::value,
                   "Non continuable types except tuple like and homogeneous "
                   "containers aren't allowed inside an any expression!");
   }
   template <typename T>
-  static auto deduce_one(std::true_type, traits::identity<T> id) {
+  static auto deduce_one(std::true_type, identity<T> id) {
     return base::annotation_of(id);
   }
   template <typename T>
   static auto deduce(traversal::container_category_tag<false, false>,
-                     traits::identity<T> id) {
+                     identity<T> id) {
     return deduce_one<T>(base::is_continuable<T>{}, id);
   }
 
   /// Deduce a homogeneous container
   template <bool IsTupleLike, typename T>
   static auto deduce(traversal::container_category_tag<true, IsTupleLike>,
-                     traits::identity<T>) {
+                     identity<T>) {
 
     // Deduce the containing type
     using element_t = std::decay_t<decltype(*std::declval<T>().begin())>;
     return deduce(traversal::container_category_of_t<element_t>{},
-                  traits::identity<element_t>{});
+                  identity<element_t>{});
   }
 
   template <typename First, typename... T>
@@ -125,19 +125,19 @@ struct result_deducer {
 
   template <std::size_t... I, typename T>
   static auto deduce_tuple_like(std::integer_sequence<std::size_t, I...>,
-                                traits::identity<T>) {
+                                identity<T>) {
 
     return deduce_same_hints(deduce(
         traversal::container_category_of_t<
             std::decay_t<decltype(std::get<I>(std::declval<T>()))>>{},
-        traits::identity<
+        identity<
             std::decay_t<decltype(std::get<I>(std::declval<T>()))>>{})...);
   }
 
   /// Traverse tuple like container
   template <typename T>
   static auto deduce(traversal::container_category_tag<false, true>,
-                     traits::identity<T> id) {
+                     identity<T> id) {
 
     constexpr auto const size = std::tuple_size<T>::value;
     return deduce_tuple_like(std::make_index_sequence<size>{}, id);
@@ -172,7 +172,7 @@ struct connection_finalizer<connection_strategy_any_tag> {
   static auto finalize(Connection&& connection, util::ownership ownership) {
     constexpr auto const signature = decltype(any::result_deducer::deduce(
         traversal::container_category_of_t<std::decay_t<Connection>>{},
-        traits::identity<std::decay_t<Connection>>{})){};
+        identity<std::decay_t<Connection>>{})){};
 
     return base::attorney::create_from(
         [connection =
diff --git a/include/continuable/detail/core/annotation.hpp b/include/continuable/detail/core/annotation.hpp
index d78b191..6017a83 100644
--- a/include/continuable/detail/core/annotation.hpp
+++ b/include/continuable/detail/core/annotation.hpp
@@ -42,7 +42,7 @@ struct annotation_trait;
 
 /// Specialization for a present signature hint
 template <typename... Args>
-struct annotation_trait<traits::identity<Args...>> {
+struct annotation_trait<identity<Args...>> {
   template <typename Continuable>
   static Continuable&& finish(Continuable&& continuable) {
     return std::forward<Continuable>(continuable);
@@ -55,9 +55,9 @@ namespace hints {
 ///
 /// This is the overload taking an arbitrary amount of args
 template <typename... HintArgs>
-struct from_args : std::common_type<traits::identity<HintArgs...>> {};
+struct from_args : std::common_type<identity<HintArgs...>> {};
 template <>
-struct from_args<void> : std::common_type<traits::identity<>> {};
+struct from_args<void> : std::common_type<identity<>> {};
 } // namespace hints
 } // namespace detail
 } // namespace cti
diff --git a/include/continuable/detail/core/base.hpp b/include/continuable/detail/core/base.hpp
index 6d8d9aa..ff2b21e 100644
--- a/include/continuable/detail/core/base.hpp
+++ b/include/continuable/detail/core/base.hpp
@@ -122,7 +122,7 @@ struct ready_continuation<> {
 template <typename Hint, typename Continuation>
 struct proxy_continuable;
 template <typename... Args, typename Continuation>
-struct proxy_continuable<traits::identity<Args...>, Continuation>
+struct proxy_continuable<identity<Args...>, Continuation>
     : Continuation {
 
   explicit proxy_continuable(Continuation continuation)
@@ -192,9 +192,9 @@ struct attorney {
 
 /// Returns the signature hint of the given continuable
 template <typename Data, typename... Args>
-constexpr traits::identity<Args...>
-annotation_of(traits::identity<continuable_base<Data, //
-                                                traits::identity<Args...>>>) {
+constexpr identity<Args...>
+annotation_of(identity<continuable_base<Data, //
+                                                identity<Args...>>>) {
   return {};
 }
 
@@ -275,30 +275,30 @@ constexpr void invoke_no_except(T&& callable, Args&&... args) noexcept {
 }
 
 template <typename... Args, typename T>
-void invoke_void_no_except(traits::identity<exception_arg_t, Args...>,
+void invoke_void_no_except(identity<exception_arg_t, Args...>,
                            T&& /*callable*/) noexcept {
   // Don't invoke the next failure handler when being in an exception handler
 }
 template <typename... Args, typename T>
-void invoke_void_no_except(traits::identity<Args...>, T&& callable) noexcept {
+void invoke_void_no_except(identity<Args...>, T&& callable) noexcept {
   std::forward<T>(callable)();
 }
 
 template <typename T, typename... Args>
-constexpr auto make_invoker(T&& invoke, traits::identity<Args...>) {
-  return invoker<std::decay_t<T>, traits::identity<Args...>>(
+constexpr auto make_invoker(T&& invoke, identity<Args...>) {
+  return invoker<std::decay_t<T>, identity<Args...>>(
       std::forward<T>(invoke));
 }
 
 /// - continuable<?...> -> result(next_callback);
 template <typename Data, typename Annotation>
 constexpr auto
-invoker_of(traits::identity<continuable_base<Data, Annotation>>) {
+invoker_of(identity<continuable_base<Data, Annotation>>) {
   /// Get the hint of the unwrapped returned continuable
   using Type =
       decltype(std::declval<continuable_base<Data, Annotation>>().finish());
 
-  auto constexpr const hint = base::annotation_of(traits::identify<Type>{});
+  auto constexpr const hint = base::annotation_of(identify<Type>{});
 
   return make_invoker(
       [](auto&& callback, auto&& next_callback, auto&&... args) {
@@ -317,7 +317,7 @@ invoker_of(traits::identity<continuable_base<Data, Annotation>>) {
 
 /// - ? -> next_callback(?)
 template <typename T>
-constexpr auto invoker_of(traits::identity<T>) {
+constexpr auto invoker_of(identity<T>) {
   return make_invoker(
       [](auto&& callback, auto&& next_callback, auto&&... args) {
         CONTINUABLE_BLOCK_TRY_BEGIN
@@ -329,26 +329,26 @@ constexpr auto invoker_of(traits::identity<T>) {
                            std::move(result));
         CONTINUABLE_BLOCK_TRY_END
       },
-      traits::identify<T>{});
+      identify<T>{});
 }
 
 /// - void -> next_callback()
-inline auto invoker_of(traits::identity<void>) {
+inline auto invoker_of(identity<void>) {
   return make_invoker(
       [](auto&& callback, auto&& next_callback, auto&&... args) {
         CONTINUABLE_BLOCK_TRY_BEGIN
           invoke_callback(std::forward<decltype(callback)>(callback),
                           std::forward<decltype(args)>(args)...);
           invoke_void_no_except(
-              traits::identity<traits::unrefcv_t<decltype(args)>...>{},
+              identity<traits::unrefcv_t<decltype(args)>...>{},
               std::forward<decltype(next_callback)>(next_callback));
         CONTINUABLE_BLOCK_TRY_END
       },
-      traits::identity<>{});
+      identity<>{});
 }
 
 /// - empty_result -> <cancel>
-inline auto invoker_of(traits::identity<empty_result>) {
+inline auto invoker_of(identity<empty_result>) {
   return make_invoker(
       [](auto&& callback, auto&& next_callback, auto&&... args) {
         (void)next_callback;
@@ -362,11 +362,11 @@ inline auto invoker_of(traits::identity<empty_result>) {
           (void)result;
         CONTINUABLE_BLOCK_TRY_END
       },
-      traits::identity<>{});
+      identity<>{});
 }
 
 /// - exceptional_result -> <throw>
-inline auto invoker_of(traits::identity<exceptional_result>) {
+inline auto invoker_of(identity<exceptional_result>) {
   return make_invoker(
       [](auto&& callback, auto&& next_callback, auto&&... args) {
         util::unused(callback, next_callback, args...);
@@ -381,12 +381,12 @@ inline auto invoker_of(traits::identity<exceptional_result>) {
                            std::move(result).get_exception());
         CONTINUABLE_BLOCK_TRY_END
       },
-      traits::identity<>{});
+      identity<>{});
 }
 
 /// - result<?...> -> next_callback(?...)
 template <typename... Args>
-auto invoker_of(traits::identity<result<Args...>>) {
+auto invoker_of(identity<result<Args...>>) {
   return make_invoker(
       [](auto&& callback, auto&& next_callback, auto&&... args) {
         CONTINUABLE_BLOCK_TRY_BEGIN
@@ -416,7 +416,7 @@ auto invoker_of(traits::identity<result<Args...>>) {
         // asynchronous chain.
         CONTINUABLE_BLOCK_TRY_END
       },
-      traits::identity<Args...>{});
+      identity<Args...>{});
 }
 
 /// Returns a sequenced invoker which is able to invoke
@@ -443,15 +443,15 @@ inline auto sequenced_unpack_invoker() {
 
 // - std::pair<?, ?> -> next_callback(?, ?)
 template <typename First, typename Second>
-constexpr auto invoker_of(traits::identity<std::pair<First, Second>>) {
+constexpr auto invoker_of(identity<std::pair<First, Second>>) {
   return make_invoker(sequenced_unpack_invoker(),
-                      traits::identity<First, Second>{});
+                      identity<First, Second>{});
 }
 
 // - std::tuple<?...>  -> next_callback(?...)
 template <typename... Args>
-constexpr auto invoker_of(traits::identity<std::tuple<Args...>>) {
-  return make_invoker(sequenced_unpack_invoker(), traits::identity<Args...>{});
+constexpr auto invoker_of(identity<std::tuple<Args...>>) {
+  return make_invoker(sequenced_unpack_invoker(), identity<Args...>{});
 }
 
 #undef CONTINUABLE_BLOCK_TRY_BEGIN
@@ -510,7 +510,7 @@ template <handle_results HandleResults, typename Base, typename Hint>
 struct result_handler_base;
 template <typename Base, typename... Args>
 struct result_handler_base<handle_results::no, Base,
-                           traits::identity<Args...>> {
+                           identity<Args...>> {
   void operator()(Args... args) && {
     // Forward the arguments to the next callback
     std::move(static_cast<Base*>(this)->next_callback_)(std::move(args)...);
@@ -518,13 +518,13 @@ struct result_handler_base<handle_results::no, Base,
 };
 template <typename Base, typename... Args>
 struct result_handler_base<handle_results::yes, Base,
-                           traits::identity<Args...>> {
+                           identity<Args...>> {
   /// The operator which is called when the result was provided
   void operator()(Args... args) && {
     // In order to retrieve the correct decorator we must know what the
     // result type is.
     constexpr auto result =
-        traits::identify<decltype(decoration::invoke_callback(
+        identify<decltype(decoration::invoke_callback(
             std::move(static_cast<Base*>(this)->callback_),
             std::move(args)...))>{};
 
@@ -556,7 +556,7 @@ struct error_handler_base<handle_errors::forward, Base> {
   /// The operator which is called when an error occurred
   void operator()(exception_arg_t, exception_t exception) && {
     constexpr auto result =
-        traits::identify<decltype(decoration::invoke_callback(
+        identify<decltype(decoration::invoke_callback(
             std::move(static_cast<Base*>(this)->callback_), exception_arg_t{},
             std::move(exception)))>{};
 
@@ -580,16 +580,16 @@ struct callback_base;
 template <typename... Args, handle_results HandleResults,
           handle_errors HandleErrors, typename Callback, typename Executor,
           typename NextCallback>
-struct callback_base<traits::identity<Args...>, HandleResults, HandleErrors,
+struct callback_base<identity<Args...>, HandleResults, HandleErrors,
                      Callback, Executor, NextCallback>
     : proto::result_handler_base<
           HandleResults,
-          callback_base<traits::identity<Args...>, HandleResults, HandleErrors,
+          callback_base<identity<Args...>, HandleResults, HandleErrors,
                         Callback, Executor, NextCallback>,
-          traits::identity<Args...>>,
+          identity<Args...>>,
       proto::error_handler_base<
           HandleErrors,
-          callback_base<traits::identity<Args...>, HandleResults, HandleErrors,
+          callback_base<identity<Args...>, HandleResults, HandleErrors,
                         Callback, Executor, NextCallback>>,
       util::non_copyable {
 
@@ -606,14 +606,14 @@ struct callback_base<traits::identity<Args...>, HandleResults, HandleErrors,
   /// Pull the result handling operator() in
   using proto::result_handler_base<
       HandleResults,
-      callback_base<traits::identity<Args...>, HandleResults, HandleErrors,
+      callback_base<identity<Args...>, HandleResults, HandleErrors,
                     Callback, Executor, NextCallback>,
-      traits::identity<Args...>>::operator();
+      identity<Args...>>::operator();
 
   /// Pull the error handling operator() in
   using proto::error_handler_base<
       HandleErrors,
-      callback_base<traits::identity<Args...>, HandleResults, HandleErrors,
+      callback_base<identity<Args...>, HandleResults, HandleErrors,
                     Callback, Executor, NextCallback>>::operator();
 
   /// Resolves the continuation with the given values
@@ -684,21 +684,21 @@ struct final_callback : util::non_copyable {
 template <typename T, typename... Args>
 constexpr auto
 next_hint_of(std::integral_constant<handle_results, handle_results::yes>,
-             traits::identity<T> /*callback*/,
-             traits::identity<Args...> /*current*/) {
+             identity<T> /*callback*/,
+             identity<Args...> /*current*/) {
   // Partial Invoke the given callback
   using Result = decltype(
       decoration::invoke_callback(std::declval<T>(), std::declval<Args>()...));
 
   // Return the hint of thr given invoker
-  return decltype(decoration::invoker_of(traits::identify<Result>{}).hint()){};
+  return decltype(decoration::invoker_of(identify<Result>{}).hint()){};
 }
 /// Don't progress the hint when we don't continue
 template <typename T, typename... Args>
 constexpr auto
 next_hint_of(std::integral_constant<handle_results, handle_results::no>,
-             traits::identity<T> /*callback*/,
-             traits::identity<Args...> current) {
+             identity<T> /*callback*/,
+             identity<Args...> current) {
   return current;
 }
 
@@ -730,8 +730,8 @@ struct chained_continuation;
 template <typename... Args, typename... NextArgs, handle_results HandleResults,
           handle_errors HandleErrors, typename Continuation, typename Callback,
           typename Executor>
-struct chained_continuation<traits::identity<Args...>,
-                            traits::identity<NextArgs...>, HandleResults,
+struct chained_continuation<identity<Args...>,
+                            identity<NextArgs...>, HandleResults,
                             HandleErrors, Continuation, Callback, Executor> {
 
   Continuation continuation_;
@@ -761,7 +761,7 @@ struct chained_continuation<traits::identity<Args...>,
     // - Continuation: continuation<[](auto&& callback) { callback("hi"); }>
     // - Callback: [](std::string) { }
     // - NextCallback: []() { }
-    auto proxy = callbacks::make_callback<traits::identity<Args...>,
+    auto proxy = callbacks::make_callback<identity<Args...>,
                                           HandleResults, HandleErrors>(
         std::move(callback_), std::move(executor_),
         std::forward<decltype(next_callback)>(next_callback));
@@ -793,7 +793,7 @@ struct chained_continuation<traits::identity<Args...>,
 template <typename... Args, typename... NextArgs, handle_results HandleResults,
           handle_errors HandleErrors, typename Callback, typename Executor>
 struct chained_continuation<
-    traits::identity<Args...>, traits::identity<NextArgs...>, HandleResults,
+    identity<Args...>, identity<NextArgs...>, HandleResults,
     HandleErrors, ready_continuation<Args...>, Callback, Executor> {
 
   ready_continuation<Args...> continuation_;
@@ -815,7 +815,7 @@ struct chained_continuation<
 
   template <typename NextCallback>
   void operator()(NextCallback&& next_callback) {
-    auto proxy = callbacks::make_callback<traits::identity<Args...>,
+    auto proxy = callbacks::make_callback<identity<Args...>,
                                           HandleResults, HandleErrors>(
         std::move(callback_), std::move(executor_),
         std::forward<decltype(next_callback)>(next_callback));
@@ -851,10 +851,10 @@ auto chain_continuation(Continuation&& continuation, Callback&& callback,
   static_assert(is_continuable<std::decay_t<Continuation>>{},
                 "Expected a continuation!");
 
-  using Hint = decltype(base::annotation_of(traits::identify<Continuation>()));
+  using Hint = decltype(base::annotation_of(identify<Continuation>()));
   constexpr auto next_hint =
       next_hint_of(std::integral_constant<handle_results, HandleResults>{},
-                   traits::identify<decltype(callback)>{}, Hint{});
+                   identify<decltype(callback)>{}, Hint{});
 
   auto ownership = attorney::ownership_of(continuation);
   auto data =
@@ -885,11 +885,11 @@ void finalize_continuation(Continuation&& continuation) {
 template <typename Data, typename Annotation, typename Continuation>
 struct can_accept_continuation : std::false_type {};
 template <typename Data, typename... Args, typename Continuation>
-struct can_accept_continuation<Data, traits::identity<Args...>, Continuation>
+struct can_accept_continuation<Data, identity<Args...>, Continuation>
     : traits::conjunction<
           traits::is_invocable<Continuation, callbacks::final_callback>,
           std::is_convertible<
-              proxy_continuable<traits::identity<Args...>, Continuation>,
+              proxy_continuable<identity<Args...>, Continuation>,
               Data>> {};
 
 /// Workaround for GCC bug:
diff --git a/include/continuable/detail/core/types.hpp b/include/continuable/detail/core/types.hpp
index 5774c4a..5d5ae6f 100644
--- a/include/continuable/detail/core/types.hpp
+++ b/include/continuable/detail/core/types.hpp
@@ -33,6 +33,7 @@
 
 #include <utility>
 #include <continuable/detail/features.hpp>
+#include <continuable/detail/utility/identity.hpp>
 
 #ifndef CONTINUABLE_WITH_CUSTOM_ERROR_TYPE
 #ifndef CONTINUABLE_WITH_NO_EXCEPTIONS
diff --git a/include/continuable/detail/other/coroutines.hpp b/include/continuable/detail/other/coroutines.hpp
index 2ec98ba..4589f97 100644
--- a/include/continuable/detail/other/coroutines.hpp
+++ b/include/continuable/detail/other/coroutines.hpp
@@ -57,7 +57,7 @@ using std::experimental::coroutine_handle;
 template <typename T>
 struct result_from_identity;
 template <typename... T>
-struct result_from_identity<traits::identity<T...>> {
+struct result_from_identity<identity<T...>> {
   using result_t = result<T...>;
 };
 
@@ -65,7 +65,7 @@ struct result_from_identity<traits::identity<T...>> {
 /// for waiting on a continuable in a stackless coroutine.
 template <typename Continuable>
 class awaitable {
-  using hint_t = decltype(base::annotation_of(traits::identify<Continuable>{}));
+  using hint_t = decltype(base::annotation_of(identify<Continuable>{}));
   using result_t = typename result_from_identity<hint_t>::result_t;
 
   /// The continuable which is invoked upon suspension
diff --git a/include/continuable/detail/other/erasure.hpp b/include/continuable/detail/other/erasure.hpp
new file mode 100644
index 0000000..1371038
--- /dev/null
+++ b/include/continuable/detail/other/erasure.hpp
@@ -0,0 +1,97 @@
+
+/*
+
+                        /~` _  _ _|_. _     _ |_ | _
+                        \_,(_)| | | || ||_|(_||_)|(/_
+
+                    https://github.com/Naios/continuable
+                                   v4.0.0
+
+  Copyright(c) 2015 - 2018 Denis Blank <denis.blank at outlook dot com>
+
+  Permission is hereby granted, free of charge, to any person obtaining a copy
+  of this software and associated documentation files(the "Software"), to deal
+  in the Software without restriction, including without limitation the rights
+  to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+  copies of the Software, and to permit persons to whom the Software is
+  furnished to do so, subject to the following conditions :
+
+  The above copyright notice and this permission notice shall be included in
+  all copies or substantial portions of the Software.
+
+  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+  SOFTWARE.
+**/
+
+#ifndef CONTINUABLE_DETAIL_ERASURE_HPP_INCLUDED
+#define CONTINUABLE_DETAIL_ERASURE_HPP_INCLUDED
+
+#include <type_traits>
+#include <function2/function2.hpp>
+#include <continuable/detail/core/base.hpp>
+#include <continuable/detail/features.hpp>
+#include <continuable/detail/utility/traits.hpp>
+#include <continuable/detail/utility/util.hpp>
+
+namespace cti {
+namespace detail {
+namespace erasure {
+/*template <typename... Args>
+using continuation_capacity = std::integral_constant<
+    std::size_t, (sizeof(base::ready_continuation<Args...>) < sizeof(void*)
+                      ? sizeof(void*)
+                      : sizeof(base::ready_continuation<Args...>))>;*/
+
+template <std::size_t Size>
+struct sized {
+  template <typename... Args>
+  using erasure_base_t =                    //
+      fu2::function_base<true, false, Size, //
+                         true, false, Args...>;
+};
+
+template <template <typename...> class Wrapper, typename... Args>
+using callback_of_t = Wrapper<void(Args...)&&, //
+                              void(exception_arg_t, exception_t) &&>;
+
+template <typename... Args>
+using callback_base_t = callback_of_t<sized<0>::erasure_base_t, Args...>;
+
+template <typename... Args>
+class callback : public callback_base_t<Args...> {
+public:
+  using callback_base_t<Args...>::callback_base_t;
+  using callback_base_t<Args...>::operator=;
+  using callback_base_t<Args...>::operator();
+};
+
+template <template <typename...> class Wrapper, typename Promise,
+          typename... Args>
+using continuation_of_t = Wrapper<void(Promise),              //
+                                  bool(is_ready_arg_t) const, //
+                                  std::tuple<Args...>(query_arg_t)>;
+
+template <typename... Args>
+using continuation_base_t =
+    continuation_of_t<sized<0 /*
+          continuation_capacity<Args...>::value*/>::erasure_base_t,
+                      promise_base<callback<Args...>, signature_arg_t<Args>...>,
+                      Args...>;
+
+template <typename... Args>
+class continuation : public continuation_base_t<Args...> {
+public:
+  using continuation_base_t<Args...>::callback_base_t;
+  using continuation_base_t<Args...>::operator=;
+  using continuation_base_t<Args...>::operator();
+};
+} // namespace erasure
+} // namespace detail
+} // namespace cti
+
+#endif // CONTINUABLE_DETAIL_ERASURE_HPP_INCLUDED
diff --git a/include/continuable/detail/other/testing.hpp b/include/continuable/detail/other/testing.hpp
index e1aa735..cb6bc31 100644
--- a/include/continuable/detail/other/testing.hpp
+++ b/include/continuable/detail/other/testing.hpp
@@ -179,15 +179,15 @@ template <typename... Expected>
 struct assert_async_types_validator {
   template <typename... Actual>
   void operator()(Actual...) {
-    static_assert(std::is_same<traits::identity<Actual...>,
-                               traits::identity<Expected...>>::value,
+    static_assert(std::is_same<identity<Actual...>,
+                               identity<Expected...>>::value,
                   "The called arguments don't match with the expected ones!");
   }
 };
 
 template <typename C, typename... Args>
 void assert_async_types(C&& continuable,
-                        traits::identity<Args...> /*expected*/) {
+                        identity<Args...> /*expected*/) {
   assert_async_validation(std::forward<C>(continuable),
                           assert_async_types_validator<Args...>{});
 }
diff --git a/include/continuable/detail/other/transforms.hpp b/include/continuable/detail/other/transforms.hpp
index e3e4a84..7484a9d 100644
--- a/include/continuable/detail/other/transforms.hpp
+++ b/include/continuable/detail/other/transforms.hpp
@@ -77,7 +77,7 @@ template <typename Hint>
 class promise_callback;
 
 template <typename... Args>
-class promise_callback<traits::identity<Args...>>
+class promise_callback<identity<Args...>>
     : public future_trait<Args...> {
 
   typename future_trait<Args...>::promise_t promise_;
@@ -119,7 +119,7 @@ template <typename Data, typename Annotation>
 auto as_future(continuable_base<Data, Annotation>&& continuable) {
   // Create the promise which is able to supply the current arguments
   constexpr auto const hint =
-      base::annotation_of(traits::identify<decltype(continuable)>{});
+      base::annotation_of(identify<decltype(continuable)>{});
 
   promise_callback<std::decay_t<decltype(hint)>> callback;
   (void)hint;
diff --git a/include/continuable/detail/utility/flat-variant.hpp b/include/continuable/detail/utility/flat-variant.hpp
index 44cedce..0ea311c 100644
--- a/include/continuable/detail/utility/flat-variant.hpp
+++ b/include/continuable/detail/utility/flat-variant.hpp
@@ -62,7 +62,7 @@ template <typename T>
 using align_of_helper = std::integral_constant<std::size_t, alignof(T)>;
 
 template <typename... T>
-constexpr auto storage_of_impl(traits::identity<T...>) {
+constexpr auto storage_of_impl(identity<T...>) {
   constexpr auto size = max_element_of({(size_of_helper<T>::value)...});
   constexpr auto align = max_element_of({(align_of_helper<T>::value)...});
   return std::aligned_storage_t<size, align>{};
@@ -70,7 +70,7 @@ constexpr auto storage_of_impl(traits::identity<T...>) {
 
 /// Declares the aligned storage union for the given types
 template <typename... T>
-using storage_of_t = decltype(storage_of_impl(traits::identity<T...>{}));
+using storage_of_t = decltype(storage_of_impl(identity<T...>{}));
 
 /// The value fpr the empty slot
 using slot_t = std::uint8_t;
diff --git a/include/continuable/detail/utility/identity.hpp b/include/continuable/detail/utility/identity.hpp
new file mode 100644
index 0000000..064ee6d
--- /dev/null
+++ b/include/continuable/detail/utility/identity.hpp
@@ -0,0 +1,54 @@
+
+/*
+
+                        /~` _  _ _|_. _     _ |_ | _
+                        \_,(_)| | | || ||_|(_||_)|(/_
+
+                    https://github.com/Naios/continuable
+                                   v4.0.0
+
+  Copyright(c) 2015 - 2018 Denis Blank <denis.blank at outlook dot com>
+
+  Permission is hereby granted, free of charge, to any person obtaining a copy
+  of this software and associated documentation files(the "Software"), to deal
+  in the Software without restriction, including without limitation the rights
+  to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+  copies of the Software, and to permit persons to whom the Software is
+  furnished to do so, subject to the following conditions :
+
+  The above copyright notice and this permission notice shall be included in
+  all copies or substantial portions of the Software.
+
+  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+  SOFTWARE.
+**/
+
+#ifndef CONTINUABLE_DETAIL_IDENTITY_HPP_INCLUDED
+#define CONTINUABLE_DETAIL_IDENTITY_HPP_INCLUDED
+
+#include <type_traits>
+#include <continuable/detail/features.hpp>
+
+namespace cti {
+namespace detail {
+/// A tagging type for wrapping other types
+template <typename... T>
+struct identity {};
+
+template <typename>
+struct is_identity : std::false_type {};
+template <typename... Args>
+struct is_identity<identity<Args...>> : std::true_type {};
+
+template <typename T>
+using identify = std::conditional_t<is_identity<std::decay_t<T>>::value, T,
+                                    identity<std::decay_t<T>>>;
+} // namespace detail
+} // namespace cti
+
+#endif // CONTINUABLE_DETAIL_IDENTITY_HPP_INCLUDED
diff --git a/include/continuable/detail/utility/traits.hpp b/include/continuable/detail/utility/traits.hpp
index 6d27a0c..c3034f3 100644
--- a/include/continuable/detail/utility/traits.hpp
+++ b/include/continuable/detail/utility/traits.hpp
@@ -36,6 +36,7 @@
 #include <type_traits>
 #include <utility>
 #include <continuable/detail/features.hpp>
+#include <continuable/detail/utility/identity.hpp>
 
 namespace cti {
 namespace detail {
@@ -69,19 +70,6 @@ auto make_flat_tuple(T&&... args) {
   return std::tuple<T...>{std::forward<T>(args)...};
 }
 
-/// A tagging type for wrapping other types
-template <typename... T>
-struct identity {};
-
-template <typename>
-struct is_identity : std::false_type {};
-template <typename... Args>
-struct is_identity<identity<Args...>> : std::true_type {};
-
-template <typename T>
-using identify = std::conditional_t<is_identity<std::decay_t<T>>::value, T,
-                                    identity<std::decay_t<T>>>;
-
 #if defined(CONTINUABLE_HAS_CXX17_VOID_T)
 using std::void_t;
 #else
diff --git a/test/unit-test/multi/test-continuable-base-errors.cpp b/test/unit-test/multi/test-continuable-base-errors.cpp
index 48444ee..62d3d1b 100644
--- a/test/unit-test/multi/test-continuable-base-errors.cpp
+++ b/test/unit-test/multi/test-continuable-base-errors.cpp
@@ -134,7 +134,7 @@ TYPED_TEST(single_dimension_tests, are_exceptions_partial_applyable) {
   handled = false;
   ASSERT_ASYNC_INCOMPLETION(
       this->supply_exception(supply_test_exception(),
-                             detail::traits::identity<int, int>{})
+                             detail::identity<int, int>{})
           .fail([&]() -> result<int, int> {
             EXPECT_FALSE(handled);
             handled = true;
diff --git a/test/unit-test/test-continuable.hpp b/test/unit-test/test-continuable.hpp
index 2a0ca4c..0f14f22 100644
--- a/test/unit-test/test-continuable.hpp
+++ b/test/unit-test/test-continuable.hpp
@@ -34,7 +34,7 @@
 #include <continuable/continuable-testing.hpp>
 #include <continuable/continuable.hpp>
 
-using cti::detail::traits::identity;
+using cti::detail::identity;
 using cti::detail::util::unused;
 
 inline auto to_hint(identity<> /*hint*/) {
@@ -48,7 +48,7 @@ auto to_hint(identity<Args...> hint) {
 template <typename... Args>
 auto supplier_of(Args&&... args) {
   return [values = std::make_tuple(std::forward<Args>(args)...)](
-             auto&& promise) mutable {
+      auto&& promise) mutable {
     cti::detail::traits::unpack(
         [&](auto&&... passed) {
           promise.set_value(std::forward<decltype(passed)>(passed)...);
@@ -113,12 +113,12 @@ struct provide_copyable {
 struct provide_unique {
   template <typename... Args, typename... Hint, typename T>
   auto make(identity<Args...>, identity<Hint...>, T&& callback) {
-    return cti::make_continuable<Hint...>(
-        [callback = std::forward<T>(callback),
-         guard = std::make_unique<int>(0)](auto&&... args) mutable {
-          (void)(*guard);
-          return std::move(callback)(std::forward<decltype(args)>(args)...);
-        });
+    return cti::make_continuable<Hint...>([
+      callback = std::forward<T>(callback), guard = std::make_unique<int>(0)
+    ](auto&&... args) mutable {
+      (void)(*guard);
+      return std::move(callback)(std::forward<decltype(args)>(args)...);
+    });
   }
 };