fix: ignore Task<T> result for activities declared as Task

src/Temporalio/Activities/ActivityDefinition.cs

@@ -37,8 +37,8 @@ private ActivityDefinition(
         public string? Name { get; private init; }
 
         /// <summary>
-        /// Gets the return type for the definition. This may be a Task for async activities. This
-        /// is currently unused (callers are expected to provide the return type as needed).
+        /// Gets the return type for the definition. This may be a Task for async activities. When
+        /// it is a non-generic Task, the activity completes with no result.
         /// </summary>
         public Type ReturnType { get; private init; }
 
@@ -84,7 +84,7 @@ public static ActivityDefinition Create(Delegate del)
         /// Create an activity definition manually from the given values.
         /// </summary>
         /// <param name="name">Name to use for the activity or null for dynamic.</param>
-        /// <param name="returnType">Return type of the activity. This is currently unused.</param>
+        /// <param name="returnType">Return type of the activity. A non-generic Task means no result.</param>
         /// <param name="parameterTypes">Parameter types for the invoker.</param>
         /// <param name="requiredParameterCount">Minimum number of parameters that must be provided
         /// to the invoker.</param>
@@ -212,9 +212,10 @@ public static IReadOnlyCollection<ActivityDefinition> CreateAll(Type type, objec
             if (result is Task resultTask)
             {
                 await resultTask.ConfigureAwait(false);
-                // We have to use reflection to extract value if it's a Task<>
+                // We have to use reflection to extract value if it's a Task<>, but not if the
+                // activity is declared as a non-generic Task
                 var resultTaskType = resultTask.GetType();
-                if (resultTaskType.IsGenericType)
+                if (ReturnType != typeof(Task) && resultTaskType.IsGenericType)
                 {
                     result = resultTaskType.GetProperty("Result")!.GetValue(resultTask);
                 }

tests/Temporalio.Tests/Activities/ActivityDefinitionTests.cs

@@ -123,6 +123,15 @@ public async Task InvokeAsync_AsyncDelegate_CanInvoke()
             await ActivityDefinition.Create(act).InvokeAsync(new object?[] { 123 }));
     }
 
+    [Fact]
+    public async Task InvokeAsync_TaskDelegateReturningGenericTask_HasNoResult()
+    {
+        var act = [Activity("MyActivity")] Task (int param) => Task.FromResult(param + 5);
+        Assert.Equal(
+            ValueTuple.Create(),
+            await ActivityDefinition.Create(act).InvokeAsync(new object?[] { 123 }));
+    }
+
     [Fact]
     public async Task InvokeAsync_ManualInvoker_IsCalled()
     {