Try to prevent slicing in `co_return ex;`

commit_hash:348f2fb14a4a93cfa3c8504419d021c1f41e4b8f

2 files changed, 11 insertions, 1 deletions

diff --git a/library/cpp/threading/future/core/coroutine_traits.h b/library/cpp/threading/future/core/coroutine_traits.h
index b36362db28a..9ad791e58d0 100644
--- a/library/cpp/threading/future/core/coroutine_traits.h
+++ b/library/cpp/threading/future/core/coroutine_traits.h
@@ -87,6 +87,11 @@ struct std::coroutine_traits<NThreading::TFuture<T>, Args...> {
             Y_ASSERT(success && "value already set");
         }
 
+        // Allow only rvalues to be returned to prevent slicing
+        template <typename E>
+        requires std::derived_from<std::remove_cvref_t<E>, std::exception>
+        void return_value(E& err) = delete;
+
         void return_value(auto&& val) {
             bool success = State_->TrySetValue(std::forward<decltype(val)>(val), /* deferCallbacks */ true);
             Y_ASSERT(success && "value already set");
diff --git a/library/cpp/threading/future/ut_gtest/coroutine_traits_ut.cpp b/library/cpp/threading/future/ut_gtest/coroutine_traits_ut.cpp
index a9b0176920e..0f902b9b998 100644
--- a/library/cpp/threading/future/ut_gtest/coroutine_traits_ut.cpp
+++ b/library/cpp/threading/future/ut_gtest/coroutine_traits_ut.cpp
@@ -149,7 +149,12 @@ TEST(TestFutureTraits, ErrorViaReturnException) {
         co_return std::runtime_error("exception_to_return");
 
         static std::runtime_error another("another_exception_not_to_return");
-        co_return another;
+
+        // should not compile
+        // co_return another;
+
+        // explicit slicing is allowed
+        co_return std::exception(another);
     };
 
     auto coroutineReturnValueReturnException = [&]() -> NThreading::TFuture<size_t> {