Fix returning an empty task_handle from task_group task (#1867)

Author: kboyarinov

include/oneapi/tbb/task_group.h

@@ -137,7 +137,7 @@ namespace {
         task_handle th = std::forward<F>(f)();
         task_handle_task* task_ptr = task_handle_accessor::release(th);
         // If task has unresolved dependencies, it can't be bypassed
-        if (task_ptr->has_dependencies() && !task_ptr->release_dependency()) {
+        if (task_ptr && task_ptr->has_dependencies() && !task_ptr->release_dependency()) {
             task_ptr = nullptr;
         }
 

test/tbb/test_task_group.cpp

@@ -1172,6 +1172,16 @@ TEST_CASE("Task handle for scheduler bypass"){
 
     tg.wait();
     CHECK_MESSAGE(run == true, "task handle returned by user lambda (bypassed) should be run");
+
+    // Test returning an empty handle
+    run = false;
+    tg.run([&] {
+        run = true;
+        return tbb::task_handle{};
+    });
+
+    tg.wait();
+    CHECK(run == true);
 }
 
 //! The test for task_handle inside other task waiting with run_and_wait
@@ -1187,6 +1197,13 @@ TEST_CASE("Task handle for scheduler bypass via run_and_wait"){
     });
 
     CHECK_MESSAGE(run == true, "task handle returned by user lambda (bypassed) should be run");
+
+    // Test returning an empty handle
+    run = false;
+    tg.run_and_wait([&] {
+        run = true;
+    });
+    CHECK(run == true);
 }
 #endif //__TBB_PREVIEW_TASK_GROUP_EXTENSIONS