[browser] Marshalling of resolved Tasks/promises is not resolved in JS/C#

src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.Task.cs

@@ -275,7 +275,7 @@ public void ToJS(Task? value)
                 }
                 else
                 {
-                    slot.ElementType = slot.Type;
+                    slot.ElementType = MarshalerType.Void;
                     slot.Type = MarshalerType.TaskResolved;
                     return;
                 }

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs

@@ -436,5 +436,12 @@ public async Task InternalsVisibleToDoesntBreak()
         {
             Assert.Equal(JavaScriptLibrary.JavaScriptInterop.ValidationMethod(5, 6), await JavaScriptTestHelper.callJavaScriptLibrary(5, 6));
         }
+
+        [Fact]
+        public async void JSExportCompletedTaskReturnsResolvedPromise()
+        {
+            string result = await JavaScriptTestHelper.InvokeReturnCompletedTask();
+            Assert.Equal("resolved", result);
+        }
     }
 }

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSImportTest.cs

@@ -1148,6 +1148,17 @@ public async Task JsImportTaskAwait()
             await task;
         }
 
+        [Fact]
+        public async Task JsImportResolvedPromiseReturnsCompletedTask()
+        {
+            var promise = JavaScriptTestHelper.ReturnResolvedPromise();
+#if !FEATURE_WASM_MANAGED_THREADS
+            Assert.False(promise.IsCompleted);
+#endif
+            await promise;
+            Assert.True(promise.IsCompleted);
+        }
+
         #endregion
 
         #region Action

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs

@@ -441,6 +441,18 @@ public static Exception EchoException([JSMarshalAs<JSType.Error>] Exception arg1
         [JSImport("invoke1", "JavaScriptTestHelper")]
         [return: JSMarshalAs<JSType.Promise<JSType.Number>>]
         internal static partial Task<int> invoke1_TaskOfInt([JSMarshalAs<JSType.Promise<JSType.Number>>] Task<int> value, [JSMarshalAs<JSType.String>] string name);
+
+        [JSImport("returnResolvedPromise", "JavaScriptTestHelper")]
+        internal static partial Task ReturnResolvedPromise();
+
+        [JSImport("invokeReturnCompletedTask", "JavaScriptTestHelper")]
+        internal static partial Task<string> InvokeReturnCompletedTask();
+
+        [JSExport]
+        internal static Task ReturnCompletedTask()
+        {
+            return Task.CompletedTask;
+        }
 
         [JSExport]
         [return: JSMarshalAs<JSType.Promise<JSType.Any>>]

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.mjs

@@ -246,6 +246,15 @@ export function invoke2(arg1, name) {
     return res;
 }
 
+export function returnResolvedPromise() {
+    return Promise.resolve();
+}
+
+export async function invokeReturnCompletedTask() {
+    await dllExports.System.Runtime.InteropServices.JavaScript.Tests.JavaScriptTestHelper.ReturnCompletedTask();
+    return "resolved";
+}
+
 export function invokeStructClassRecords(arg1) {
     return [
         dllExports.JavaScriptTestHelperNamespace.JavaScriptTestHelper.EchoString(arg1),