Add asio::lazy.

example/cpp17/lazy/lazy_1.cpp

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+//
+// lazy_1.cpp
+// ~~~~~~~~~~
+//
+// Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/ts/net.hpp>
+#include <boost/asio/lazy.hpp>
+#include <iostream>
+
+int main()
+{
+  boost::asio::io_context ctx;
+  boost::asio::steady_timer timer(ctx);
+  timer.expires_after(std::chrono::seconds(1));
+  auto x = timer.async_wait(boost::asio::lazy);
+  x([](auto){ std::cout << "timer done\n"; });
+  ctx.run();
+}

example/cpp17/lazy/lazy_2.cpp

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+//
+// lazy_2.cpp
+// ~~~~~~~~~~
+//
+// Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/ts/net.hpp>
+#include <boost/asio/lazy.hpp>
+#include <iostream>
+
+template <boost::asio::completion_token_for<void(int)> CompletionToken>
+auto async_foo(int i, CompletionToken&& token)
+{
+  return boost::asio::async_initiate<CompletionToken, void(int)>(
+      [](auto&& handler, int i)
+      {
+        auto ex = boost::asio::get_associated_executor(handler);
+        auto alloc = boost::asio::get_associated_allocator(handler);
+        int result = i * 2;
+        ex.post(
+            [handler = std::move(handler), result]() mutable
+            {
+              handler(result);
+            }, alloc);
+      }, token, i);
+}
+
+int main()
+{
+  auto x = async_foo(21, boost::asio::lazy);
+  x([](auto result){ std::cout << "result is " << result << "\n"; });
+  boost::asio::system_executor().context().join();
+}

include/boost/asio/lazy.hpp

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+//
+// lazy.hpp
+// ~~~~~~~~
+//
+// Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BOOST_ASIO_LAZY_HPP
+#define BOOST_ASIO_LAZY_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include <boost/asio/detail/config.hpp>
+
+#include <tuple>
+#include <utility>
+#include <boost/asio/async_result.hpp>
+
+#include <boost/asio/detail/push_options.hpp>
+
+namespace boost {
+namespace asio {
+
+struct lazy_t {};
+
+template <BOOST_ASIO_COMPLETION_SIGNATURE Signature>
+class async_result<lazy_t, Signature>
+{
+public:
+  template <typename Initiation, typename... InitArgs>
+  static auto initiate(Initiation initiation, lazy_t, InitArgs... init_args)
+  {
+    return [
+        initiation = std::move(initiation),
+        init_arg_pack = std::make_tuple(std::move(init_args)...)
+      ](auto&& token) mutable
+    {
+      return std::apply(
+          [&](auto&&... args)
+          {
+            return async_initiate<decltype(token), Signature>(
+                std::move(initiation), token,
+                std::forward<decltype(args)>(args)...);
+          },
+          std::move(init_arg_pack)
+        );
+    };
+  }
+};
+
+/// A completion token object that indicates that an Awaitable should be returned.
+#if defined(BOOST_ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
+constexpr lazy_t lazy;
+#elif defined(BOOST_ASIO_MSVC)
+__declspec(selectany) lazy_t lazy;
+#endif
+
+} // namespace asio
+} // namespace boost
+
+#include <boost/asio/detail/pop_options.hpp>
+
+#endif // BOOST_ASIO_LAZY_HPP