There is no need to call PostQueuedCompletionStatus to stop the io_service if
the outstanding work count is initially zero. A run/poll call can only be
calling GetQueuedCompletionStatus if it the count was non-zero at some point.
However, if it was previously non-zero, then the io_service will have already
been stop()-ed when the count was decremented to zero.

asio/include/asio/detail/impl/win_iocp_io_service.ipp

diff --git a/asio/include/asio/detail/impl/win_iocp_io_service.ipp b/asio/include/asio/detail/impl/win_iocp_io_service.ipp
index 030f5a8..2580ee2 100644
--- a/asio/include/asio/detail/impl/win_iocp_io_service.ipp
+++ b/asio/include/asio/detail/impl/win_iocp_io_service.ipp
@@ -147,7 +147,7 @@
 {
   if (::InterlockedExchangeAdd(&outstanding_work_, 0) == 0)
   {
-    stop();
+    InterlockedExchange(&stopped_, 1);
     ec = asio::error_code();
     return 0;
   }
@@ -165,7 +165,7 @@
 {
   if (::InterlockedExchangeAdd(&outstanding_work_, 0) == 0)
   {
-    stop();
+    InterlockedExchange(&stopped_, 1);
     ec = asio::error_code();
     return 0;
   }
@@ -179,7 +179,7 @@
 {
   if (::InterlockedExchangeAdd(&outstanding_work_, 0) == 0)
   {
-    stop();
+    InterlockedExchange(&stopped_, 1);
     ec = asio::error_code();
     return 0;
   }
@@ -197,7 +197,7 @@
 {
   if (::InterlockedExchangeAdd(&outstanding_work_, 0) == 0)
   {
-    stop();
+    InterlockedExchange(&stopped_, 1);
     ec = asio::error_code();
     return 0;
   }