fix the attributes of the let_ senders

include/exec/env.hpp

@@ -269,23 +269,36 @@ namespace experimental::execution
     {
       template <class _Sender, class _Attrs>
       STDEXEC_ATTRIBUTE(host, device)
-      constexpr auto operator()(_Sender snd, _Attrs __attrs_) const -> __sender<_Sender, _Attrs>
+      constexpr auto operator()(_Sender snd, _Attrs __attrs) const -> __sender<_Sender, _Attrs>
       {
         return __sender<_Sender, _Attrs>{static_cast<_Sender&&>(snd),
-                                         static_cast<_Attrs&&>(__attrs_)};
+                                         static_cast<_Attrs&&>(__attrs)};
       }
 
       template <class _Attrs>
       STDEXEC_ATTRIBUTE(host, device)
-      constexpr auto operator()(_Attrs __attrs_) const
+      constexpr auto operator()(_Attrs __attrs) const
       {
-        return STDEXEC::__closure(*this, static_cast<_Attrs&&>(__attrs_));
+        return STDEXEC::__closure(*this, static_cast<_Attrs&&>(__attrs));
       }
     };
   }  // namespace __write_attrs
 
   inline constexpr __write_attrs::__write_attrs_t write_attrs{};
 
+  template <class Tag = STDEXEC::set_value_t, STDEXEC::scheduler Scheduler>
+  STDEXEC_ATTRIBUTE(host, device)
+  constexpr auto completes_on(Scheduler sched) noexcept
+  {
+    return exec::write_attrs(STDEXEC::prop{STDEXEC::get_completion_scheduler<Tag>, sched});
+  }
+
+  template <class Tag = STDEXEC::set_value_t, STDEXEC::sender Sender, STDEXEC::scheduler Scheduler>
+  STDEXEC_ATTRIBUTE(host, device)
+  constexpr auto completes_on(Sender sndr, Scheduler sched) noexcept
+  {
+    return exec::completes_on<Tag>(sched)(std::move(sndr));
+  }
 }  // namespace experimental::execution
 
 namespace exec = experimental::execution;

include/nvexec/stream/common.cuh

@@ -445,7 +445,8 @@ namespace nv::execution
         return get_stream_provider(env)->own_stream_.value();
       }
 
-      STDEXEC_ATTRIBUTE(host, device) auto operator()() const noexcept
+      STDEXEC_ATTRIBUTE(host, device)
+      auto operator()() const noexcept
       {
         return STDEXEC::read_env(*this);
       }

include/stdexec/__detail/__completion_behavior.hpp

@@ -133,11 +133,17 @@ 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 | ...);
+        }
       }
     };