Fix channels to support C++14 and (partially) C++11.

C++11 support is limited to channels with a single completion signature,
or channels with a void() signature (plus the error signature added by
the channel traits).

include/boost/asio/detail/config.hpp

@@ -1443,6 +1443,30 @@
 # endif // !defined(BOOST_ASIO_DISABLE_STD_ANY)
 #endif // !defined(BOOST_ASIO_HAS_STD_ANY)
 
+// Standard library support for std::variant.
+#if !defined(BOOST_ASIO_HAS_STD_VARIANT)
+# if !defined(BOOST_ASIO_DISABLE_STD_VARIANT)
+#  if defined(__clang__)
+#   if (__cplusplus >= 201703)
+#    if __has_include(<variant>)
+#     define BOOST_ASIO_HAS_STD_VARIANT 1
+#    endif // __has_include(<variant>)
+#   endif // (__cplusplus >= 201703)
+#  elif defined(__GNUC__)
+#   if (__GNUC__ >= 7)
+#    if (__cplusplus >= 201703)
+#     define BOOST_ASIO_HAS_STD_VARIANT 1
+#    endif // (__cplusplus >= 201703)
+#   endif // (__GNUC__ >= 7)
+#  endif // defined(__GNUC__)
+#  if defined(BOOST_ASIO_MSVC)
+#   if (_MSC_VER >= 1910) && (_MSVC_LANG >= 201703)
+#    define BOOST_ASIO_HAS_STD_VARIANT 1
+#   endif // (_MSC_VER >= 1910) && (_MSVC_LANG >= 201703)
+#  endif // defined(BOOST_ASIO_MSVC)
+# endif // !defined(BOOST_ASIO_DISABLE_STD_VARIANT)
+#endif // !defined(BOOST_ASIO_HAS_STD_VARIANT)
+
 // Standard library support for std::source_location.
 #if !defined(BOOST_ASIO_HAS_STD_SOURCE_LOCATION)
 # if !defined(BOOST_ASIO_DISABLE_STD_SOURCE_LOCATION)

include/boost/asio/experimental/detail/channel_message.hpp

@@ -18,6 +18,7 @@
 #include <boost/asio/detail/config.hpp>
 #include <tuple>
 #include <boost/asio/detail/type_traits.hpp>
+#include <boost/asio/detail/utility.hpp>
 
 #include <boost/asio/detail/push_options.hpp>
 
@@ -105,11 +106,17 @@ public:
   template <typename Handler>
   void receive(Handler& h)
   {
-    std::apply(BOOST_ASIO_MOVE_OR_LVALUE(Handler)(h),
-        BOOST_ASIO_MOVE_CAST(args_type)(args_));
+    this->do_receive(h, boost::asio::detail::index_sequence_for<Args...>());
   }
 
 private:
+  template <typename Handler, std::size_t... I>
+  void do_receive(Handler& h, boost::asio::detail::index_sequence<I...>)
+  {
+    BOOST_ASIO_MOVE_OR_LVALUE(Handler)(h)(
+        std::get<I>(BOOST_ASIO_MOVE_CAST(args_type)(args_))...);
+  }
+
   typedef std::tuple<typename decay<Args>::type...> args_type;
   args_type args_;
 };

include/boost/asio/experimental/detail/channel_payload.hpp

@@ -16,10 +16,14 @@
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
-#include <variant>
 #include <boost/asio/detail/type_traits.hpp>
+#include <boost/system/error_code.hpp>
 #include <boost/asio/experimental/detail/channel_message.hpp>
 
+#if defined(BOOST_ASIO_HAS_STD_VARIANT)
+# include <variant>
+#endif // defined(BOOST_ASIO_HAS_STD_VARIANT)
+
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
@@ -28,28 +32,7 @@ namespace experimental {
 namespace detail {
 
 template <typename... Signatures>
-class channel_payload
-{
-public:
-  template <typename Signature>
-  channel_payload(BOOST_ASIO_MOVE_ARG(channel_message<Signature>) m)
-    : message_(BOOST_ASIO_MOVE_CAST(channel_message<Signature>)(m))
-  {
-  }
-
-  template <typename Handler>
-  void receive(Handler& handler)
-  {
-    std::visit(
-        [&](auto& message)
-        {
-          message.receive(handler);
-        }, message_);
-  }
-
-private:
-  std::variant<channel_message<Signatures>...> message_;
-};
+class channel_payload;
 
 template <typename R>
 class channel_payload<R()>
@@ -85,6 +68,69 @@ private:
   channel_message<Signature> message_;
 };
 
+#if defined(BOOST_ASIO_HAS_STD_VARIANT)
+
+template <typename... Signatures>
+class channel_payload
+{
+public:
+  template <typename Signature>
+  channel_payload(BOOST_ASIO_MOVE_ARG(channel_message<Signature>) m)
+    : message_(BOOST_ASIO_MOVE_CAST(channel_message<Signature>)(m))
+  {
+  }
+
+  template <typename Handler>
+  void receive(Handler& handler)
+  {
+    std::visit(
+        [&](auto& message)
+        {
+          message.receive(handler);
+        }, message_);
+  }
+
+private:
+  std::variant<channel_message<Signatures>...> message_;
+};
+
+#else // defined(BOOST_ASIO_HAS_STD_VARIANT)
+
+template <typename R1, typename R2>
+class channel_payload<R1(), R2(boost::system::error_code)>
+{
+public:
+  typedef channel_message<R1()> void_message_type;
+  typedef channel_message<R2(boost::system::error_code)> error_message_type;
+
+  channel_payload(BOOST_ASIO_MOVE_ARG(void_message_type))
+    : message_(0, boost::system::error_code()),
+      empty_(true)
+  {
+  }
+
+  channel_payload(BOOST_ASIO_MOVE_ARG(error_message_type) m)
+    : message_(BOOST_ASIO_MOVE_CAST(error_message_type)(m)),
+      empty_(false)
+  {
+  }
+
+  template <typename Handler>
+  void receive(Handler& handler)
+  {
+    if (empty_)
+      channel_message<R1()>(0).receive(handler);
+    else
+      message_.receive(handler);
+  }
+
+private:
+  error_message_type message_;
+  bool empty_;
+};
+
+#endif // defined(BOOST_ASIO_HAS_STD_VARIANT)
+
 } // namespace detail
 } // namespace experimental
 } // namespace asio

include/boost/asio/experimental/detail/channel_send_functions.hpp

@@ -65,6 +65,12 @@ public:
   auto async_send(Args... args,
       BOOST_ASIO_MOVE_ARG(CompletionToken) token
         BOOST_ASIO_DEFAULT_COMPLETION_TOKEN(Executor))
+    -> decltype(
+        async_initiate<CompletionToken, void (boost::system::error_code)>(
+          declval<typename conditional<false, CompletionToken,
+            Derived>::type::initiate_async_send>(), token,
+          declval<typename conditional<false, CompletionToken,
+            Derived>::type::payload_type>()))
   {
     typedef typename Derived::payload_type payload_type;
     typedef typename detail::channel_message<R(Args...)> message_type;
@@ -114,6 +120,12 @@ public:
   auto async_send(Args... args,
       BOOST_ASIO_MOVE_ARG(CompletionToken) token
         BOOST_ASIO_DEFAULT_COMPLETION_TOKEN(Executor))
+    -> decltype(
+        async_initiate<CompletionToken, void (boost::system::error_code)>(
+          declval<typename conditional<false, CompletionToken,
+            Derived>::type::initiate_async_send>(), token,
+          declval<typename conditional<false, CompletionToken,
+            Derived>::type::payload_type>()))
   {
     typedef typename Derived::payload_type payload_type;
     typedef typename detail::channel_message<R(Args...)> message_type;

include/boost/asio/experimental/detail/channel_service.hpp

@@ -537,6 +537,7 @@ struct channel_service<Mutex>::implementation_type<
   implementation_type()
     : size_(0)
   {
+    first_.count_ = 0;
   }
 
   // Move from another buffer.
@@ -637,7 +638,7 @@ private:
   struct buffered_value
   {
     boost::system::error_code value_;
-    std::size_t count_ = 0;
+    std::size_t count_;
   };
 
   struct value_handler