use consteval to compute completion sigs of when_all

include/nvexec/stream/when_all.cuh

@@ -39,6 +39,9 @@ namespace nv::execution::_strm
 {
   namespace _when_all
   {
+    using STDEXEC::__when_all::_THE_GIVEN_SENDER_CAN_COMPLETE_SUCCESSFULLY_IN_MORE_THAN_ONE_WAY_;
+    using STDEXEC::__when_all::_USE_WHEN_ALL_WITH_VARIANT_INSTEAD_;
+
     enum disposition : std::uint32_t
     {
       started,
@@ -75,14 +78,11 @@ namespace nv::execution::_strm
       requires(too_many_completions_sender<Senders, Env...> || ...)
     struct completions<__mlist<Env...>, Senders...>
     {
-      static constexpr auto position_of() noexcept -> std::size_t
-      {
-        constexpr bool which[] = {too_many_completions_sender<Senders, Env...>...};
-        return __pos_of(which, which + sizeof...(Senders));
-      }
-
-      using invalid_arg_t = __m_at_c<position_of(), Senders...>;
-      using __t = STDEXEC::__when_all::__too_many_value_completions_error_t<invalid_arg_t, Env...>;
+      using __t =
+        __mexception<_WHAT_(_INVALID_ARGUMENT_),
+                     _WHERE_(_IN_ALGORITHM_, when_all_t),
+                     _WHY_(_THE_GIVEN_SENDER_CAN_COMPLETE_SUCCESSFULLY_IN_MORE_THAN_ONE_WAY_),
+                     _TO_FIX_THIS_ERROR_(_USE_WHEN_ALL_WITH_VARIANT_INSTEAD_)>;
     };
 
     template <class... Env, class... Senders>

include/stdexec/__detail/__basic_sender.hpp

@@ -112,9 +112,6 @@ namespace STDEXEC
     template <class _Tag, class _Indices>
     struct __connect;
 
-    template <class _Sexpr>
-    using __connect_t = __connect<tag_of_t<_Sexpr>, __child_indices_t<_Sexpr>>;
-
     template <class _Tag, std::size_t... _Idx>
     struct __connect<_Tag, __indices<_Idx...>>
     {
@@ -129,9 +126,13 @@ namespace STDEXEC
       }
     };
 
+    template <class _Sexpr>
+    using __connect_t = __connect<tag_of_t<_Sexpr>, __child_indices_t<_Sexpr>>;
+
     template <class _Sexpr, class _Receiver>
     concept __connectable_to =
-      __applicable<__connect_t<_Sexpr>, _Sexpr, __state_type_t<_Sexpr, _Receiver>&>;
+      sender_in<_Sexpr, env_of_t<_Receiver>>
+      && __applicable<__connect_t<_Sexpr>, _Sexpr, __state_type_t<_Sexpr, _Receiver>&>;
 
     STDEXEC_MODULE_EXPORT_AUTHORING
     struct __defaults
@@ -283,8 +284,8 @@ namespace STDEXEC
   struct __opstate
   {
     using __tag_t       = __decay_t<_Sexpr>::__tag_t;
+    using __connect_t   = __detail::__connect_t<_Sexpr>;
     using __state_t     = __detail::__state_type_t<_Sexpr, _Receiver>;
-    using __connect_t   = __detail::__connect<__tag_t, __detail::__child_indices_t<_Sexpr>>;
     using __child_ops_t = __apply_result_t<__connect_t, _Sexpr, __state_t&>;
 
     constexpr explicit __opstate(_Sexpr&& __sndr, _Receiver __rcvr)

include/stdexec/__detail/__continues_on.hpp

@@ -332,7 +332,6 @@ namespace STDEXEC
           __nothrow_constructible_from<__state_for_t<_Sender, _Receiver>,
                                        __data_of<_Sender>&,
                                        _Receiver>) -> __state_for_t<_Sender, _Receiver>
-        requires sender_in<__child_of<_Sender>, __fwd_env_t<env_of_t<_Receiver>>>
       {
         static_assert(__sender_for<_Sender, continues_on_t>);
         auto& [__tag, __sched, __child] = __sndr;

include/stdexec/__detail/__transform_completion_signatures.hpp

@@ -372,11 +372,14 @@ namespace STDEXEC
     template <class... _Args>
     consteval auto operator()() const noexcept
     {
-      if constexpr (__decay_copyable<_Args...>)
+      if constexpr (__nothrow_decay_copyable<_Args...>)
       {
-        return __concat_completion_signatures(
-          completion_signatures<_SetTag(__decay_t<_Args>...)>(),
-          __eptr_completion_unless_t<__nothrow_decay_copyable_t<_Args...>>());
+        return completion_signatures<_SetTag(__decay_t<_Args>...)>();
+      }
+      else if constexpr (__decay_copyable<_Args...>)
+      {
+        return completion_signatures<_SetTag(__decay_t<_Args>...),
+                                     set_error_t(std::exception_ptr)>();
       }
       else
       {

include/stdexec/__detail/__tuple.hpp

@@ -328,6 +328,32 @@ namespace STDEXEC
       }
     };
 
+    struct __fold_right_t
+    {
+      template <__std::move_constructible _Init>
+      STDEXEC_ATTRIBUTE(always_inline, host, device)
+      constexpr auto operator()(__ignore, _Init&& __init) const
+        noexcept(__nothrow_move_constructible<_Init>) -> _Init
+      {
+        return static_cast<_Init&&>(__init);
+      }
+
+      template <class _Fn, class _Init, class _Head, class... _Tail>
+        requires __callable<_Fn&, _Init, _Head>
+              && __callable<__fold_right_t, _Fn&, __call_result_t<_Fn&, _Init, _Head>, _Tail...>
+      STDEXEC_ATTRIBUTE(always_inline, host, device)
+      constexpr auto
+      operator()(_Fn&& __fn, _Init&& __init, _Head&& __head, _Tail&&... __tail) const  //
+        noexcept(
+          __nothrow_callable<_Fn&, _Init, _Head>
+          && __nothrow_callable<__fold_right_t, _Fn&, __call_result_t<_Fn&, _Init, _Head>, _Tail...>)
+      {
+        return (*this)(__fn,
+                       __fn(static_cast<_Init&&>(__init), static_cast<_Head&&>(__head)),
+                       static_cast<_Tail&&>(__tail)...);
+      }
+    };
+
 #  undef STDEXEC_TUPLE_GET
   }  // namespace __tup