add a get_completion_domain<void> query

Author: ericniebler

include/stdexec/__detail/__completion_signatures.hpp

@@ -371,7 +371,7 @@ namespace stdexec {
 
   namespace __detail {
     template <class _Tag, class _Sigs>
-    extern const std::size_t __count_of;
+    inline constexpr std::size_t __count_of = 0;
 
     template <class _Sigs>
     inline constexpr std::size_t __count_of<set_value_t, _Sigs> =

include/stdexec/__detail/__continues_on.hpp

@@ -261,7 +261,7 @@ namespace stdexec {
       }
 
       //! @overload
-      template <__not_same_as<set_value_t> _SetTag, class... _Env>
+      template <__one_of<set_error_t, set_stopped_t> _SetTag, class... _Env>
         requires(!__has_decay_copy_errors<_SetTag, _Env...>())
       [[nodiscard]]
       constexpr auto

include/stdexec/__detail/__domain.hpp

@@ -285,7 +285,7 @@ namespace stdexec {
     //! custom implementation of a sender algorithm.
     //!
     //! @tparam _Tag one of set_value_t, set_error_t, or set_stopped_t
-    template <__completion_tag _Tag>
+    template <class _Tag>
     struct get_completion_domain_t {
       template <class Sig>
       static inline constexpr get_completion_domain_t<_Tag> (*signature)(Sig) = nullptr;
@@ -320,13 +320,15 @@ namespace stdexec {
       static consteval auto __check_domain(_Domain) noexcept -> _Domain {
         // Sanity check: if a completion scheduler can be determined from the attributes
         // (not the environment), then its domain must match the domain returned by the attributes.
-        if constexpr (__callable<get_completion_scheduler_t<_Tag>, const _Attrs&, const _Env&...>) {
-          using __sch_t =
-            __call_result_t<get_completion_scheduler_t<_Tag>, const _Attrs&, const _Env&...>;
-          // Skip check if the "scheduler" is the same as the domain or the attributes
-          // (this can happen with __prop_like which answers any query with the same type)
-          if constexpr (!__same_as<__sch_t, _Attrs>) {
-            __check_domain_<__sch_t, _Env...>(_Domain{});
+        if constexpr (!__same_as<_Tag, void>) {
+          if constexpr (__callable<get_completion_scheduler_t<_Tag>, const _Attrs&, const _Env&...>) {
+            using __sch_t =
+              __call_result_t<get_completion_scheduler_t<_Tag>, const _Attrs&, const _Env&...>;
+            // Skip check if the "scheduler" is the same as the domain or the attributes
+            // (this can happen with __prop_like which answers any query with the same type)
+            if constexpr (!__same_as<__sch_t, _Attrs>) {
+              __check_domain_<__sch_t, _Env...>(_Domain{});
+            }
           }
         }
         return {};
@@ -338,6 +340,16 @@ namespace stdexec {
         if constexpr (__callable<__read_query_t, const _Attrs&, const _Env&...>) {
           using __domain_t = __call_result_t<__read_query_t, const _Attrs&, const _Env&...>;
           return __check_domain<_Attrs, _Env...>(__domain_t{});
+          // Otherwise, if _Tag is void, fall back to querying for the set_value_t completion domain:
+        } else if constexpr (__same_as<_Tag, void>) {
+          if constexpr (
+            __callable<get_completion_domain_t<set_value_t>, const _Attrs&, const _Env&...>) {
+            using __domain_t =
+              __call_result_t<get_completion_domain_t<set_value_t>, const _Attrs&, const _Env&...>;
+            return __check_domain<_Attrs, _Env...>(__domain_t{});
+          } else {
+            return void();
+          }
         }
         // Otherwise, if __attrs has a completion scheduler, we can ask that scheduler for its
         // completion domain.
@@ -433,9 +445,11 @@ namespace stdexec {
   using __queries::get_domain_t;
 
 #if !STDEXEC_GCC() || defined(__OPTIMIZE_SIZE__)
-  template <__completion_tag _Query>
-  inline constexpr get_completion_domain_t<_Query> get_completion_domain{};
+  template <class _Tag>
+  inline constexpr get_completion_domain_t<_Tag> get_completion_domain{};
 #else
+  template <>
+  inline constexpr get_completion_domain_t<> get_completion_domain<>{};
   template <>
   inline constexpr get_completion_domain_t<set_value_t> get_completion_domain<set_value_t>{};
   template <>

include/stdexec/__detail/__execution_fwd.hpp

@@ -129,7 +129,7 @@ namespace stdexec {
     struct get_stop_token_t;
     template <__completion_tag _CPO>
     struct get_completion_scheduler_t;
-    template <__completion_tag _CPO>
+    template <class _CPO = void>
     struct get_completion_domain_t;
     template <__completion_tag _CPO>
     struct get_completion_behavior_t;
@@ -157,7 +157,7 @@ namespace stdexec {
   extern const get_stop_token_t get_stop_token;
   template <__completion_tag _CPO>
   extern const get_completion_scheduler_t<_CPO> get_completion_scheduler;
-  template <__completion_tag _CPO>
+  template <class _CPO>
   extern const get_completion_domain_t<_CPO> get_completion_domain;
   extern const get_domain_t get_domain;
 

include/stdexec/__detail/__transform_sender.hpp

@@ -53,7 +53,7 @@ namespace stdexec {
           return __get_declfn<__result_t, _Env, (_Nothrow && __is_nothrow)>();
         } else {
           using __transform_recurse_t =
-            __transform_sender_t<__completing_domain_t<set_value_t, __result_t, _Env>, set_value_t>;
+            __transform_sender_t<__completing_domain_t<void, __result_t, _Env>, set_value_t>;
           return __transform_recurse_t::template __get_declfn<
             __result_t,
             _Env,
@@ -83,7 +83,7 @@ namespace stdexec {
             __domain_t().transform_sender(_OpTag(), static_cast<_Sndr&&>(__sndr), __env), __env);
         } else {
           using __transform_recurse_t =
-            __transform_sender_t<__completing_domain_t<set_value_t, __result_t, _Env>, set_value_t>;
+            __transform_sender_t<__completing_domain_t<void, __result_t, _Env>, set_value_t>;
           return __transform_recurse_t()(
             __domain_t().transform_sender(_OpTag(), static_cast<_Sndr&&>(__sndr), __env), __env);
         }
@@ -110,10 +110,7 @@ namespace stdexec {
     template <class _Sndr, class _Env>
     using __impl_fn_t = __compose<
       __detail::__transform_sender_t<__detail::__starting_domain_t<_Env>, start_t>,
-      __detail::__transform_sender_t<
-        __detail::__completing_domain_t<set_value_t, _Sndr, _Env>,
-        set_value_t
-      >
+      __detail::__transform_sender_t<__detail::__completing_domain_t<void, _Sndr, _Env>, set_value_t>
     >;
 
    public: