move subscribe's customization point into stdexec as __subscribe

include/exec/sequence_senders.hpp

@@ -35,6 +35,7 @@ import stdexec;
 #  include "../stdexec/__detail/__receivers.hpp"
 #  include "../stdexec/__detail/__senders.hpp"
 #  include "../stdexec/__detail/__stop_token.hpp"
+#  include "../stdexec/__detail/__subscribe.hpp"
 #  include "../stdexec/__detail/__transform_sender.hpp"
 #  include "../stdexec/__detail/__type_traits.hpp"
 #  include "../stdexec/__detail/__utility.hpp"
@@ -880,18 +881,6 @@ namespace experimental::execution
       && sequence_receiver_of<_Receiver, item_types<STDEXEC::__decay_t<_Sender>>>
       && sender_to<next_sender_of_t<_Receiver, _Sender>, __stopped_means_break<_Receiver>>;
 
-    template <class _Sequence, class _Receiver>
-    using __subscribe_member_result_t = decltype(__declval<_Sequence>().subscribe(
-      __declval<_Receiver>()));
-
-    template <class _Sequence, class _Receiver>
-    concept __subscribable_with_member =
-      __minvocable_q<__subscribe_member_result_t, _Sequence, _Receiver>;
-
-    template <class _Sequence, class _Receiver>
-    using __subscribe_static_member_result_t = decltype(STDEXEC_REMOVE_REFERENCE(
-      _Sequence)::__static_subscribe(__declval<_Sequence>(), __declval<_Receiver>()));
-
     // Receiver wrapper used to subscribe to the child of a transparent
     // sequence adaptor (see `__sequence_adaptor_traits` above): it forwards
     // all completions and `set_next` calls to the wrapped receiver, but
@@ -939,10 +928,6 @@ namespace experimental::execution
       _Env      __env_;
     };
 
-    template <class _Sequence, class _Receiver>
-    concept __subscribable_with_static_member =
-      __minvocable_q<__subscribe_static_member_result_t, _Sequence, _Receiver>;
-
     struct subscribe_t
     {
      private:
@@ -1030,21 +1015,12 @@ namespace experimental::execution
           return __declfn<__result_t,
                           __nothrow_subscribe && __nothrow_child_env && __nothrow_tfx_seq>();
         }
-        else if constexpr (__subscribable_with_static_member<__tfx_seq_t, _Receiver>)
+        else if constexpr (STDEXEC::__subscribable<__tfx_seq_t, _Receiver>)
         {
-          using __result_t = decltype(STDEXEC_REMOVE_REFERENCE(
-            __tfx_seq_t)::__static_subscribe(__declval<__tfx_seq_t>(), __declval<_Receiver>()));
+          using __result_t = STDEXEC::__subscribe_result_t<__tfx_seq_t, _Receiver>;
           __check_operation_state<__result_t>();
-          constexpr bool __nothrow_subscribe = noexcept(STDEXEC_REMOVE_REFERENCE(
-            __tfx_seq_t)::__static_subscribe(__declval<__tfx_seq_t>(), __declval<_Receiver>()));
-          return __declfn<__result_t, __nothrow_subscribe && __nothrow_tfx_seq>();
-        }
-        else if constexpr (__subscribable_with_member<__tfx_seq_t, _Receiver>)
-        {
-          using __result_t = decltype(__declval<__tfx_seq_t>().subscribe(__declval<_Receiver>()));
-          __check_operation_state<__result_t>();
-          constexpr bool __nothrow_subscribe = noexcept(
-            __declval<__tfx_seq_t>().subscribe(__declval<_Receiver>()));
+          constexpr bool __nothrow_subscribe =
+            __nothrow_callable<decltype(STDEXEC::__subscribe), __tfx_seq_t, _Receiver>;
           return __declfn<__result_t, __nothrow_subscribe && __nothrow_tfx_seq>();
         }
         else if constexpr (__is_debug_env<env_of_t<_Receiver>>)
@@ -1103,14 +1079,10 @@ namespace experimental::execution
                            __xform_t{}(static_cast<decltype(__env)&&>(__env),
                                        STDEXEC::__forward_like<__tfx_seq_t>(__data))});
         }
-        else if constexpr (__subscribable_with_static_member<__tfx_seq_t, _Receiver>)
-        {  // NOLINT(bugprone-branch-clone)
-          return __tfx_seq.__static_subscribe(static_cast<__tfx_seq_t&&>(__tfx_seq),
-                                              static_cast<_Receiver&&>(__rcvr));
-        }
-        else if constexpr (__subscribable_with_member<__tfx_seq_t, _Receiver>)
+        else if constexpr (STDEXEC::__subscribable<__tfx_seq_t, _Receiver>)
         {
-          return static_cast<__tfx_seq_t&&>(__tfx_seq).subscribe(static_cast<_Receiver&&>(__rcvr));
+          return STDEXEC::__subscribe(static_cast<__tfx_seq_t&&>(__tfx_seq),
+                                      static_cast<_Receiver&&>(__rcvr));
         }
         else if constexpr (enable_sequence_sender<STDEXEC::__decay_t<__tfx_seq_t>>)
         {

include/stdexec/__detail/__subscribe.hpp

@@ -0,0 +1,91 @@
+/*
+ * Copyright (c) 2026 NVIDIA Corporation
+ *
+ * Licensed under the Apache License Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include "__config.hpp"
+
+#if STDEXEC_USE_MODULES() && !defined(STDEXEC_IN_MODULE_PURVIEW)
+
+import stdexec;
+
+#else
+
+#  include "__execution_fwd.hpp"
+
+#  include "__concepts.hpp"
+#  include "__type_traits.hpp"
+#  include "__utility.hpp"
+
+#  include "__prologue.hpp"
+
+// NOLINTNEXTLINE(bugprone-std-namespace-modification, modernize-concat-nested-namespaces)
+namespace STDEXEC
+{
+  namespace __subscribe_
+  {
+    template <class _Sender, class _Receiver>
+    concept __has_static_memfn = requires(_Sender && (*__sndr)(), _Receiver && (*__rcvr)()) {
+      STDEXEC_REMOVE_REFERENCE(_Sender)::__static_subscribe(__sndr(), __rcvr());
+    };
+
+    template <class _Sender, class _Receiver>
+    concept __has_memfn = requires(_Sender && (*__sndr)(), _Receiver && (*__rcvr)()) {
+      __sndr().subscribe(__rcvr());
+    };
+
+    // The customization point behind exec::subscribe. A sender hooks it with a
+    // static __static_subscribe(sndr, rcvr) member or a subscribe(rcvr) member
+    // function, the static one taking precedence. Senders defined in stdexec can
+    // use this to behave as sequence senders without depending on exec.
+    struct __subscribe_t
+    {
+      template <class _Sender, class _Receiver>
+        requires __has_static_memfn<_Sender, _Receiver>
+      STDEXEC_ATTRIBUTE(always_inline)
+      constexpr auto operator()(_Sender&& __sndr, _Receiver __rcvr) const
+        noexcept(noexcept(__sndr.__static_subscribe(__declval<_Sender>(), __declval<_Receiver>())))
+          -> decltype(__sndr.__static_subscribe(__declval<_Sender>(), __declval<_Receiver>()))
+      {
+        return __sndr.__static_subscribe(static_cast<_Sender&&>(__sndr),
+                                         static_cast<_Receiver&&>(__rcvr));
+      }
+
+      template <class _Sender, class _Receiver>
+        requires(!__has_static_memfn<_Sender, _Receiver>) && __has_memfn<_Sender, _Receiver>
+      STDEXEC_ATTRIBUTE(always_inline)
+      constexpr auto operator()(_Sender&& __sndr, _Receiver __rcvr) const
+        noexcept(noexcept(__declval<_Sender>().subscribe(__declval<_Receiver>())))
+          -> decltype(__declval<_Sender>().subscribe(__declval<_Receiver>()))
+      {
+        return static_cast<_Sender&&>(__sndr).subscribe(static_cast<_Receiver&&>(__rcvr));
+      }
+    };
+  }  // namespace __subscribe_
+
+  STDEXEC_MODULE_EXPORT_AUTHORING
+  inline constexpr __subscribe_::__subscribe_t __subscribe{};
+
+  STDEXEC_MODULE_EXPORT_AUTHORING
+  template <class _Sender, class _Receiver>
+  using __subscribe_result_t = __call_result_t<__subscribe_::__subscribe_t, _Sender, _Receiver>;
+
+  STDEXEC_MODULE_EXPORT_AUTHORING
+  template <class _Sender, class _Receiver>
+  concept __subscribable = __callable<__subscribe_::__subscribe_t, _Sender, _Receiver>;
+}  // namespace STDEXEC
+
+#  include "__epilogue.hpp"
+#endif  // !STDEXEC_USE_MODULES() || defined(STDEXEC_IN_MODULE_PURVIEW)

include/stdexec/execution.hpp

@@ -63,6 +63,7 @@ import stdexec;
 #  include "__detail/__stopped_as_error.hpp"
 #  include "__detail/__stopped_as_optional.hpp"
 #  include "__detail/__submit.hpp"
+#  include "__detail/__subscribe.hpp"
 #  include "__detail/__sync_wait.hpp"
 #  include "__detail/__task.hpp"
 #  include "__detail/__task_scheduler.hpp"

test/exec/test_sequence_senders.cpp

@@ -151,4 +151,44 @@ namespace
     STATIC_REQUIRE(sequence_sender_to<seq_sender_t, next_receiver_t>);
     STATIC_REQUIRE(sequence_sender_to<some_sequence_sender_of<set_value_t(int)>, next_receiver_t>);
   }
+
+  struct tagged_operation
+  {
+    int tag;
+
+    void start() & noexcept {}
+  };
+
+  template <__completion_signature... _Sigs>
+  struct static_subscribe_sequence_of : some_sequence_sender_of<_Sigs...>
+  {
+    template <class Self, receiver R>
+    static auto __static_subscribe(Self&&, R&&) noexcept -> tagged_operation
+    {
+      return {1};
+    }
+
+    template <receiver R>
+    auto subscribe(R&&) const -> tagged_operation
+    {
+      return {2};
+    }
+  };
+
+  TEST_CASE("sequence_senders - __subscribe hooks", "[sequence_senders]")
+  {
+    using next_receiver_t = next_receiver<set_value_t(int), set_stopped_t()>;
+    using member_seq_t    = some_sequence_sender_of<set_value_t(int), set_stopped_t()>;
+    using static_seq_t    = static_subscribe_sequence_of<set_value_t(int), set_stopped_t()>;
+
+    STATIC_REQUIRE(__subscribable<member_seq_t, next_receiver_t>);
+    STATIC_REQUIRE(__same_as<__subscribe_result_t<member_seq_t, next_receiver_t>, nop_operation>);
+    STATIC_REQUIRE_FALSE(__subscribable<int, next_receiver_t>);
+
+    // the static hook wins over the member function
+    STATIC_REQUIRE(__subscribable<static_seq_t, next_receiver_t>);
+    STATIC_REQUIRE(noexcept(STDEXEC::__subscribe(static_seq_t{}, next_receiver_t{})));
+    CHECK(STDEXEC::__subscribe(static_seq_t{}, next_receiver_t{}).tag == 1);
+    CHECK(exec::subscribe(static_seq_t{}, next_receiver_t{}).tag == 1);
+  }
 }  // namespace