Handle ((IAsyncResult)task).AsyncWaitHandle.Dispose better in task completion

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs

@@ -260,7 +260,21 @@ internal sealed class ContingentProperties
             internal void SetCompleted()
             {
                 ManualResetEventSlim? mres = m_completionEvent;
-                if (mres != null) mres.Set();
+                if (mres != null) SetEvent(mres);
+            }
+
+            internal static void SetEvent(ManualResetEventSlim mres)
+            {
+                try
+                {
+                    mres.Set();
+                }
+                catch (ObjectDisposedException)
+                {
+                    // The event may have been exposed to external code, in which case it could have been disposed
+                    // prematurely / erroneously.  To avoid that causing problems for the unrelated stack trying to
+                    // set the event, eat any disposed exceptions.
+                }
             }
 
             /// <summary>
@@ -1483,7 +1497,7 @@ internal ManualResetEventSlim CompletedEvent
                     {
                         // We published the event as unset, but the task has subsequently completed.
                         // Set the event's state properly so that callers don't deadlock.
-                        newMre.Set();
+                        ContingentProperties.SetEvent(newMre);
                     }
                 }
 
@@ -1619,7 +1633,7 @@ protected virtual void Dispose(bool disposing)
                         // will deadlock; an ensuing Set() will not wake them up.  In the event of an AppDomainUnload,
                         // there is no guarantee that anyone else is going to signal the event, and it does no harm to
                         // call Set() twice on m_completionEvent.
-                        if (!ev.IsSet) ev.Set();
+                        ContingentProperties.SetEvent(ev);
 
                         // Finally, dispose of the event
                         ev.Dispose();

src/libraries/System.Threading.Tasks/tests/Task/TaskAPMTest.cs

@@ -110,6 +110,14 @@ public void WaitOnAsyncWaitHandleTechnique(bool hasReturnType)
             Assert.False(asyncResult.CompletedSynchronously, "Should not have completed synchronously.");
         }
 
+        [Fact]
+        public void AsyncWaitHandle_DisposeHandleThenCompleteTask_Succeeds()
+        {
+            var tcs = new TaskCompletionSource();
+            ((IAsyncResult)tcs.Task).AsyncWaitHandle.Dispose();
+            tcs.SetResult();
+        }
+
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         [OuterLoop]
         [InlineData(true)]