fix the attributes of the let_ senders

include/nvexec/stream/common.cuh

@@ -429,10 +429,11 @@ namespace nv::execution
     using decayed_tuple_t = ::cuda::std::tuple<__decay_t<Ts>...>;
 
     template <class Sender, class Env>
-    using variant_storage_t =
-      __for_each_completion_signature_t<__completion_signatures_of_t<Sender, Env>,
-                                        decayed_tuple_t,
-                                        __munique<__q<_nullable_variant_t>>::__f>;
+    using variant_storage_t = __for_each_completion_signature_t<
+      __completion_signatures_of_t<Sender, Env>,
+      decayed_tuple_t,
+      __munique<
+        __mbind_back_q<_nullable_variant_t, ::cuda::std::tuple<set_error_t, cudaError_t>>>::__f>;
 
     inline constexpr get_stream_provider_t get_stream_provider{};
 

include/nvexec/stream/continues_on.cuh

@@ -239,9 +239,10 @@ namespace nv::execution::_strm
 
       template <__decay_copyable Self, class... Env>
       static consteval auto get_completion_signatures()
-        -> __completion_signatures_of_t<__copy_cvref_t<Self, Sender>, Env...>
       {
-        return {};
+        return exec::concat_completion_signatures(
+          STDEXEC::get_completion_signatures<__copy_cvref_t<Self, Sender>, Env...>(),
+          STDEXEC::completion_signatures<set_error_t(cudaError_t)>());
       }
 
       [[nodiscard]]

include/stdexec/__detail/__completion_behavior.hpp

@@ -134,11 +134,17 @@ namespace STDEXEC
     struct __common_t
     {
       template <__behavior... _CSs>
-        requires(sizeof...(_CSs) > 0)
       STDEXEC_ATTRIBUTE(nodiscard, host, device)
       constexpr auto operator()(__constant_t<_CSs>... __cbs) const noexcept
       {
-        return (__cbs | ...);
+        if constexpr (sizeof...(_CSs) == 0)
+        {
+          return __completion_behavior::__unknown;
+        }
+        else
+        {
+          return (__cbs | ...);
+        }
       }
     };