Scope Senders & Asynchronous Objects

include/exec/elide.hpp

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+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+ *                         Copyright (c) 2025 Robert Leahy. All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ * Licensed under the Apache License, Version 2.0 with LLVM Exceptions (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <concepts>
+#include <functional>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+namespace exec {
+
+namespace detail::elide {
+
+template<typename T>
+concept result = !std::same_as<void, T>;
+
+template<typename T, typename... Args>
+concept c =
+  std::invocable<T, Args...> &&
+  result<std::invoke_result_t<T, Args...>> &&
+  std::is_reference_v<T> &&
+  (std::is_reference_v<Args> && ...);
+
+template<typename T, typename... Args>
+concept lvalue =
+  std::invocable<T&, Args&...> &&
+  result<std::invoke_result_t<T&, Args&...>>;
+
+struct tombstone {};
+
+template<typename T, typename... Args>
+struct lvalue_result : std::type_identity<tombstone> {};
+template<typename T, typename... Args>
+  requires lvalue<T, Args...>
+struct lvalue_result<T, Args...> : std::invoke_result<T&, Args&...> {};
+
+}
+
+template<typename T, typename... Args>
+  requires detail::elide::c<T, Args...>
+struct elide {
+  elide() = delete;
+  elide(const elide&) = delete;
+  elide& operator=(const elide&) = delete;
+  constexpr explicit elide(T t, Args... args) noexcept
+    : t_(std::forward<T>(t)),
+      args_(std::forward<Args>(args)...)
+  {}
+private:
+  static constexpr bool rvalue_noexcept_ =
+    std::is_nothrow_invocable_v<T, Args...>;
+  static constexpr bool lvalue_noexcept_ =
+    std::is_nothrow_invocable_v<T&, Args&...>;
+  using rvalue_result_type_ = std::invoke_result_t<T, Args...>;
+  using lvalue_result_type_ = typename detail::elide::lvalue_result<
+    T,
+    Args...>::type;
+public:
+  constexpr operator rvalue_result_type_() const&& noexcept(rvalue_noexcept_) {
+    return std::apply(
+      [&](auto&&... args) noexcept(rvalue_noexcept_) {
+        return std::invoke(
+          std::forward<T>(t_),
+          std::forward<Args>(args)...);
+      },
+      args_);
+  }
+  constexpr rvalue_result_type_ operator()() const&& noexcept(rvalue_noexcept_)
+  {
+    return rvalue_result_type_(std::move(*this));
+  }
+  constexpr operator lvalue_result_type_() const& noexcept(lvalue_noexcept_) {
+    if constexpr (detail::elide::lvalue<T, Args...>) {
+      return std::apply(
+        [&](auto&&... args) noexcept(lvalue_noexcept_) {
+          return std::invoke(t_, args...);
+        },
+        args_);
+    } else {
+      return {};
+    }
+  }
+  constexpr lvalue_result_type_ operator()() const& noexcept(lvalue_noexcept_)
+    requires detail::elide::lvalue<T, Args...>
+  {
+    return rvalue_result_type_(*this);
+  }
+private:
+  [[no_unique_address]]
+  T t_;
+  [[no_unique_address]]
+  std::tuple<Args...> args_;
+};
+
+template<typename T, typename... Args>
+explicit elide(T&&, Args&&...) -> elide<T&&, Args&&...>;
+
+} // namespace exec

include/exec/enter_scope_sender.hpp

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+/*
+ * SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+ *                         Copyright (c) 2025 Robert Leahy. All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ *
+ * Licensed under the Apache License, Version 2.0 with LLVM Exceptions (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <concepts>
+#include <type_traits>
+#include <utility>
+
+#include "exit_scope_sender.hpp"
+#include "../stdexec/execution.hpp"
+
+namespace exec {
+
+template<typename Sender>
+concept enter_scope_sender = ::STDEXEC::sender<Sender>;
+
+namespace detail::exit_scope_sender_of {
+
+template<typename Env, typename... Args>
+struct transform_set_value_impl;
+template<typename Env, ::exec::exit_scope_sender_in<Env> Sender>
+struct transform_set_value_impl<Env, Sender> {
+  using type = ::STDEXEC::completion_signatures<
+    ::STDEXEC::set_value_t(Sender)>;
+};
+
+template<typename Env>
+struct transform_set_value {
+  template<typename... Args>
+  using fn = transform_set_value_impl<Env, Args...>::type;
+};
+
+template<typename T>
+using transform_set_error = ::STDEXEC::completion_signatures<>;
+
+template<typename Signatures>
+struct impl;
+template<typename Sender>
+struct impl<
+  ::STDEXEC::completion_signatures<
+    ::STDEXEC::set_value_t(Sender)>>
+{
+  using type = Sender;
+};
+
+}
+
+template<enter_scope_sender Constructor, typename Env>
+using exit_scope_sender_of_t = detail::exit_scope_sender_of::impl<
+  ::STDEXEC::transform_completion_signatures<
+    ::STDEXEC::completion_signatures_of_t<Constructor, Env>,
+    ::STDEXEC::completion_signatures<>,
+    detail::exit_scope_sender_of::transform_set_value<Env>::template fn,
+    detail::exit_scope_sender_of::transform_set_error,
+    ::STDEXEC::completion_signatures<>>>::type;
+
+template<typename Sender, typename Env>
+concept enter_scope_sender_in =
+  enter_scope_sender<Sender> &&
+  ::STDEXEC::sender_in<Sender, Env> &&
+  requires {
+    typename exit_scope_sender_of_t<Sender, Env>;
+  };
+
+}  // namespace exec