From 84f7ebedb4333043f2c2abc54af8fe4197c12241 Mon Sep 17 00:00:00 2001
From: Cra3z <CrazyCra3z@outlook.com>
Date: Wed, 23 Sep 2026 17:22:28 +0800
Subject: [PATCH 1/3] Refactor completion-signatures of `let` and
 `continues_on`

---
 .../beman/execution/detail/continues_on.hpp   | 63 ++++++++-------
 include/beman/execution/detail/let.hpp        | 79 +++++++++++--------
 tests/beman/execution/issue-186.test.cpp      |  2 +-
 3 files changed, 81 insertions(+), 63 deletions(-)

diff --git a/include/beman/execution/detail/continues_on.hpp b/include/beman/execution/detail/continues_on.hpp
index c5105e04..cac25058 100644
--- a/include/beman/execution/detail/continues_on.hpp
+++ b/include/beman/execution/detail/continues_on.hpp
@@ -29,6 +29,7 @@ import beman.execution.detail.env;
 import beman.execution.detail.env_of_t;
 import beman.execution.detail.error_types_of_t;
 import beman.execution.detail.fwd_env;
+import beman.execution.detail.gather_signatures;
 import beman.execution.detail.get_completion_domain;
 import beman.execution.detail.get_completion_scheduler;
 import beman.execution.detail.get_completion_signatures;
@@ -48,9 +49,7 @@ import beman.execution.detail.schedule_result_t;
 import beman.execution.detail.scheduler;
 import beman.execution.detail.sender;
 import beman.execution.detail.sender_adaptor_closure;
-import beman.execution.detail.sender_for;
 import beman.execution.detail.sender_in;
-import beman.execution.detail.sends_stopped;
 import beman.execution.detail.set_error;
 import beman.execution.detail.set_stopped;
 import beman.execution.detail.set_value;
@@ -66,6 +65,7 @@ import beman.execution.detail.start;
 #include <beman/execution/detail/env_of_t.hpp>
 #include <beman/execution/detail/error_types_of_t.hpp>
 #include <beman/execution/detail/fwd_env.hpp>
+#include <beman/execution/detail/gather_signatures.hpp>
 #include <beman/execution/detail/get_completion_domain.hpp>
 #include <beman/execution/detail/get_completion_scheduler.hpp>
 #include <beman/execution/detail/get_env.hpp>
@@ -83,9 +83,7 @@ import beman.execution.detail.start;
 #include <beman/execution/detail/scheduler.hpp>
 #include <beman/execution/detail/sender.hpp>
 #include <beman/execution/detail/sender_adaptor.hpp>
-#include <beman/execution/detail/sender_for.hpp>
 #include <beman/execution/detail/sender_in.hpp>
-#include <beman/execution/detail/sends_stopped.hpp>
 #include <beman/execution/detail/set_error.hpp>
 #include <beman/execution/detail/set_stopped.hpp>
 #include <beman/execution/detail/set_value.hpp>
@@ -113,6 +111,20 @@ struct continues_on_t {
     }
 
   private:
+    template <typename Child, typename Env>
+    using variant_type = ::beman::execution::detail::meta::unique<::beman::execution::detail::meta::prepend<
+        ::std::monostate,
+        ::beman::execution::detail::meta::transform<
+            ::beman::execution::detail::as_tuple_t,
+            ::beman::execution::detail::meta::to<::std::variant,
+                                                 ::beman::execution::completion_signatures_of_t<Child, Env>>>>>;
+
+    template <typename... E>
+    using as_set_error = ::beman::execution::completion_signatures<::beman::execution::set_error_t(E)...>;
+
+    template <typename Tag>
+    using is_not_set_value = ::std::negation<::beman::execution::detail::is_set_value<Tag>>;
+
     template <typename, typename...>
     struct get_signatures;
     template <typename Sender>
@@ -121,18 +133,21 @@ struct continues_on_t {
     struct get_signatures<
         ::beman::execution::detail::basic_sender<::beman::execution::detail::continues_on_t, Scheduler, Sender>,
         Env> {
-        using scheduler_sender      = ::beman::execution::schedule_result_t<Scheduler>;
-        using additional_signatures = ::std::conditional_t<
-            ::beman::execution::sends_stopped<scheduler_sender, Env>,
-            ::beman::execution::completion_signatures<::beman::execution::set_error_t(::std::exception_ptr),
-                                                      ::beman::execution::set_stopped_t()>,
+        using exception_signature = ::std::conditional_t<
+            ::std::is_nothrow_move_constructible_v<variant_type<Sender, Env>>,
+            ::beman::execution::completion_signatures<>,
             ::beman::execution::completion_signatures<::beman::execution::set_error_t(::std::exception_ptr)>>;
-        template <typename... E>
-        using as_set_error = ::beman::execution::completion_signatures<::beman::execution::set_error_t(E)...>;
-        using type         = ::beman::execution::detail::meta::unique<::beman::execution::detail::meta::combine<
-            decltype(::beman::execution::get_completion_signatures<Sender, Env>()),
-            ::beman::execution::error_types_of_t<scheduler_sender, Env, as_set_error>,
-            additional_signatures>>;
+
+        static consteval auto get() {
+            [[maybe_unused]] auto child_sigs = ::beman::execution::get_completion_signatures<Sender, Env>();
+            [[maybe_unused]] auto sched_sigs =
+                ::beman::execution::get_completion_signatures<::beman::execution::schedule_result_t<Scheduler>, Env>();
+            using child_sigs_t = decltype(child_sigs);
+            using extra_sigs_t = ::beman::execution::detail::meta::filter<is_not_set_value, decltype(sched_sigs)>;
+            using type         = ::beman::execution::detail::meta::unique<
+                        ::beman::execution::detail::meta::combine<child_sigs_t, exception_signature, extra_sigs_t>>;
+            return type{};
+        }
     };
 
     template <typename Scheduler, typename ChildAttrs>
@@ -170,8 +185,8 @@ struct continues_on_t {
 
   public:
     template <typename Sender, typename... Env>
-    static consteval auto get_completion_signatures() noexcept {
-        return typename get_signatures<::std::remove_cvref_t<Sender>, Env...>::type{};
+    static consteval auto get_completion_signatures() {
+        return get_signatures<::std::remove_cvref_t<Sender>, Env...>::get();
     }
 
     struct impls_for : ::beman::execution::detail::default_impls {
@@ -190,7 +205,7 @@ struct continues_on_t {
             State* state;
 
             auto set_value() && noexcept -> void {
-                constexpr bool nothrow = std::is_nothrow_move_constructible_v<decltype(state->async_result)>;
+                constexpr bool nothrow = ::std::is_nothrow_move_constructible_v<decltype(state->async_result)>;
                 try {
                     ::std::visit(
                         [this]<typename Tuple>(Tuple& result) noexcept -> void {
@@ -252,14 +267,8 @@ struct continues_on_t {
                 auto sch{sender.template get<1>()};
 
                 using sched_t   = ::std::remove_cvref_t<decltype(sch)>;
-                using variant_t = ::beman::execution::detail::meta::unique<::beman::execution::detail::meta::prepend<
-                    ::std::monostate,
-                    ::beman::execution::detail::meta::transform<
-                        ::beman::execution::detail::as_tuple_t,
-                        ::beman::execution::detail::meta::to<::std::variant,
-                                                             ::beman::execution::completion_signatures_of_t<
-                                                                 ::beman::execution::detail::child_type<Sender>,
-                                                                 ::beman::execution::env_of_t<Receiver>>>>>>;
+                using variant_t = variant_type<::beman::execution::detail::child_type<Sender>,
+                                               ::beman::execution::env_of_t<Receiver>>;
 
                 return state_type<Receiver, sched_t, variant_t>(sch, receiver);
             };
@@ -270,7 +279,7 @@ struct continues_on_t {
             template <typename Tag, typename... Args>
             auto operator()(auto, auto& state, auto& receiver, Tag, Args&&... args) const noexcept -> void {
                 using result_t         = ::beman::execution::detail::decayed_tuple<Tag, Args...>;
-                constexpr bool nothrow = ::std::is_nothrow_constructible_v<result_t, Tag, Args...>;
+                constexpr bool nothrow = ::std::is_nothrow_move_constructible_v<decltype(state.async_result)>;
 
                 try {
                     [&]() noexcept(nothrow) {
diff --git a/include/beman/execution/detail/let.hpp b/include/beman/execution/detail/let.hpp
index 97f8a14b..a83d5efc 100644
--- a/include/beman/execution/detail/let.hpp
+++ b/include/beman/execution/detail/let.hpp
@@ -170,48 +170,52 @@ struct let_t {
     using let_env_t = decltype(let_env(::std::declval<Sndr>(), ::std::declval<Env>()));
 
   private:
+    template <typename, typename>
+    struct apply_decayed;
+    template <typename Fun, typename... Args>
+    struct apply_decayed<Fun, Completion(Args...)> {
+        static_assert(::std::invocable<Fun, ::std::decay_t<Args>&...>,
+                      "The callback function object of the `let` adapter shall be invocable with `Args&...`");
+        using sender_type = ::beman::execution::detail::call_result_t<Fun, ::std::decay_t<Args>&...>;
+        static_assert(::beman::execution::sender<sender_type>, "`Fun` shall return a sender");
+        using completions = ::std::conditional_t<
+            noexcept(::std::declval<Fun>()(::std::declval<::std::decay_t<Args>&>()...)),
+            ::beman::execution::completion_signatures<>,
+            ::beman::execution::completion_signatures<::beman::execution::set_error_t(::std::exception_ptr)>>;
+    };
+
     template <typename, typename...>
     struct get_signatures;
 
-    template <typename Comp, typename Fun, typename Child>
+    template <typename Fun, typename Child>
         requires ::beman::execution::detail::non_dependent_successor<Completion, Child, Fun>::value
     struct get_signatures<
-        ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Comp>, Fun, Child>>
-        : get_signatures<::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Comp>, Fun, Child>,
-                         ::beman::execution::env<>> {};
+        ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Completion>, Fun, Child>>
+        : get_signatures<
+              ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Completion>, Fun, Child>,
+              ::beman::execution::env<>> {};
 
-    template <typename Comp, typename Fun, typename Child, typename Env>
+    template <typename Fun, typename Child, typename Env>
     struct get_signatures<
-        ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Comp>, Fun, Child>,
+        ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Completion>, Fun, Child>,
         Env> {
         template <typename T>
-        using other_completion = let_other_completion<Comp, T>;
+        using other_completion = let_other_completion<Completion, T>;
         template <typename T>
-        using matching_completion = let_matching_completion<Comp, T>;
-
-        template <typename>
-        struct apply_decayed;
-        template <typename C, typename... A>
-        struct apply_decayed<C(A...)> {
-            using sender_type = ::beman::execution::detail::call_result_t<Fun, ::std::decay_t<A>...>;
-            using completions = ::std::conditional_t<
-                noexcept(::std::declval<Fun>()(::std::declval<::std::decay_t<A>>()...)),
-                ::beman::execution::completion_signatures<>,
-                ::beman::execution::completion_signatures<::beman::execution::set_error_t(::std::exception_ptr)>>;
-        };
+        using matching_completion = let_matching_completion<Completion, T>;
 
         using successor_env =
             ::beman::execution::detail::join_env<let_env_t<Child, Env>, ::beman::execution::detail::fwd_env<Env>>;
 
         template <typename>
         struct get_completions;
-        template <template <typename...> class L, typename... C>
-        struct get_completions<L<C...>> {
+        template <typename... Sigs>
+        struct get_completions<::beman::execution::completion_signatures<Sigs...>> {
             using type = ::beman::execution::detail::meta::unique<::beman::execution::detail::meta::combine<
                 ::beman::execution::completion_signatures<>,
-                ::beman::execution::completion_signatures_of_t<typename apply_decayed<C>::sender_type,
+                ::beman::execution::completion_signatures_of_t<typename apply_decayed<Fun, Sigs>::sender_type,
                                                                successor_env>...,
-                typename apply_decayed<C>::completions...>>;
+                typename apply_decayed<Fun, Sigs>::completions...>>;
         };
 
         using upstream_completions = ::beman::execution::completion_signatures_of_t<Child, Env>;
@@ -271,7 +275,7 @@ struct let_t {
         struct to_state {
             template <typename Tuple>
             using trans =
-                decltype(::beman::execution::connect(::std::apply(::std::declval<Fun>(), ::std::declval<Tuple>()),
+                decltype(::beman::execution::connect(::std::apply(::std::declval<Fun>(), ::std::declval<Tuple&>()),
                                                      ::std::declval<let_receiver<Receiver, Env>>()));
         };
 
@@ -308,28 +312,33 @@ struct let_t {
                 {},
                 {}};
         }};
+
+        template <typename Receiver, typename Env, typename Fun, typename... Args>
+        static constexpr bool nothrow =
+            ::std::is_nothrow_constructible_v<::beman::execution::detail::decayed_tuple<Args...>, Args...> &&
+            noexcept(::beman::execution::connect(
+                ::std::invoke(::std::move(::std::declval<Fun>()), ::std::declval<::std::decay_t<Args>&>()...),
+                ::std::declval<let_receiver<Receiver, Env>>()));
+
         template <typename Receiver, typename... Args>
-        static auto let_bind(auto& state, Receiver& receiver, Args&&... args) noexcept(
-            noexcept(::beman::execution::connect(::std::invoke(::std::move(state.fun), ::std::forward<Args>(args)...),
-                                                 let_receiver<Receiver, decltype(state.env)>{receiver, state.env}))) {
+        static auto let_bind(auto& state, Receiver& receiver, Args&&... args) {
             using args_t = ::beman::execution::detail::decayed_tuple<Args...>;
             auto mkop{[&] {
-                return ::beman::execution::connect(
-                    ::std::apply(::std::move(state.fun),
-                                 ::std::move(state.args.template emplace<args_t>(::std::forward<Args>(args)...))),
-                    let_receiver<Receiver, decltype(state.env)>{receiver, state.env});
+                auto& tpl = state.args.template emplace<args_t>(::std::forward<Args>(args)...);
+                return ::beman::execution::connect(::std::apply(::std::move(state.fun), tpl),
+                                                   let_receiver<Receiver, decltype(state.env)>{receiver, state.env});
             }};
             ::beman::execution::start(
                 state.ops2.template emplace<decltype(mkop())>(beman::execution::detail::emplace_from{mkop}));
         }
         struct complete_impl {
-            template <class Tag, class... Args>
-            auto operator()(auto, auto& state, auto& receiver, Tag, Args&&... args) const {
+            template <typename Tag, typename Receiver, typename... Args>
+            auto operator()(auto, auto& state, Receiver& receiver, Tag, Args&&... args) const {
                 if constexpr (::std::same_as<Tag, Completion>) {
                     try {
-                        let_bind(state, receiver, ::std::forward<Args>(args)...);
+                        (let_bind)(state, receiver, ::std::forward<Args>(args)...);
                     } catch (...) {
-                        if constexpr (not noexcept(let_bind(state, receiver, ::std::forward<Args>(args)...))) {
+                        if constexpr (not nothrow<Receiver, decltype(state.env), decltype(state.fun), Args...>) {
                             ::beman::execution::set_error(::std::move(receiver), ::std::current_exception());
                         }
                     }
diff --git a/tests/beman/execution/issue-186.test.cpp b/tests/beman/execution/issue-186.test.cpp
index f9729b86..0b67abfb 100644
--- a/tests/beman/execution/issue-186.test.cpp
+++ b/tests/beman/execution/issue-186.test.cpp
@@ -35,7 +35,7 @@ TEST(issue186) {
     // NOTE: Compile time error with move_only_type
     auto snd2   = ex::just(move_only_type(1))                                                     //
                   | ex::then([](move_only_type v) noexcept { return move_only_type{v.val * 2}; }) //
-                  | ex::let_value([](move_only_type v) noexcept { return ex::just(v.val * 3); }); // move_only_type
+                  | ex::let_value([](move_only_type& v) noexcept { return ex::just(v.val * 3); }); // move_only_type
     auto [ret2] = ex::sync_wait(std::move(snd2)).value();
     assert(ret2 == 6);
 }

From 7a4ff57132a2a8e0a6bcd0342b5c2f35304adff4 Mon Sep 17 00:00:00 2001
From: Cra3z <CrazyCra3z@outlook.com>
Date: Wed, 23 Sep 2026 17:31:32 +0800
Subject: [PATCH 2/3] Add missing headers

---
 .../beman/execution/detail/continues_on.hpp    |  3 +++
 include/beman/execution/detail/let.hpp         | 18 ++++++++----------
 src/beman/execution/completion_signature.cppm  |  3 +++
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/include/beman/execution/detail/continues_on.hpp b/include/beman/execution/detail/continues_on.hpp
index cac25058..97cbd1b2 100644
--- a/include/beman/execution/detail/continues_on.hpp
+++ b/include/beman/execution/detail/continues_on.hpp
@@ -18,6 +18,7 @@ import std;
 import beman.execution.detail.as_tuple;
 import beman.execution.detail.basic_sender;
 import beman.execution.detail.child_type;
+import beman.execution.detail.completion_signature;
 import beman.execution.detail.completion_signatures;
 import beman.execution.detail.completion_signatures_for;
 import beman.execution.detail.completion_signatures_of_t;
@@ -38,6 +39,7 @@ import beman.execution.detail.impls_for;
 import beman.execution.detail.join_env;
 import beman.execution.detail.make_sender;
 import beman.execution.detail.meta.combine;
+import beman.execution.detail.meta.filter;
 import beman.execution.detail.meta.prepend;
 import beman.execution.detail.meta.to;
 import beman.execution.detail.meta.transform;
@@ -73,6 +75,7 @@ import beman.execution.detail.start;
 #include <beman/execution/detail/join_env.hpp>
 #include <beman/execution/detail/make_sender.hpp>
 #include <beman/execution/detail/meta_combine.hpp>
+#include <beman/execution/detail/meta_filter.hpp>
 #include <beman/execution/detail/meta_prepend.hpp>
 #include <beman/execution/detail/meta_to.hpp>
 #include <beman/execution/detail/meta_transform.hpp>
diff --git a/include/beman/execution/detail/let.hpp b/include/beman/execution/detail/let.hpp
index a83d5efc..36831ce2 100644
--- a/include/beman/execution/detail/let.hpp
+++ b/include/beman/execution/detail/let.hpp
@@ -184,26 +184,24 @@ struct let_t {
             ::beman::execution::completion_signatures<::beman::execution::set_error_t(::std::exception_ptr)>>;
     };
 
+    template <typename T>
+    using other_completion = let_other_completion<Completion, T>;
+
+    template <typename T>
+    using matching_completion = let_matching_completion<Completion, T>;
+
     template <typename, typename...>
     struct get_signatures;
 
     template <typename Fun, typename Child>
         requires ::beman::execution::detail::non_dependent_successor<Completion, Child, Fun>::value
-    struct get_signatures<
-        ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Completion>, Fun, Child>>
-        : get_signatures<
-              ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Completion>, Fun, Child>,
-              ::beman::execution::env<>> {};
+    struct get_signatures<::beman::execution::detail::basic_sender<let_t, Fun, Child>>
+        : get_signatures<::beman::execution::detail::basic_sender<let_t, Fun, Child>, ::beman::execution::env<>> {};
 
     template <typename Fun, typename Child, typename Env>
     struct get_signatures<
         ::beman::execution::detail::basic_sender<::beman::execution::detail::let_t<Completion>, Fun, Child>,
         Env> {
-        template <typename T>
-        using other_completion = let_other_completion<Completion, T>;
-        template <typename T>
-        using matching_completion = let_matching_completion<Completion, T>;
-
         using successor_env =
             ::beman::execution::detail::join_env<let_env_t<Child, Env>, ::beman::execution::detail::fwd_env<Env>>;
 
diff --git a/src/beman/execution/completion_signature.cppm b/src/beman/execution/completion_signature.cppm
index d395fafd..94f5de33 100644
--- a/src/beman/execution/completion_signature.cppm
+++ b/src/beman/execution/completion_signature.cppm
@@ -8,4 +8,7 @@ export module beman.execution.detail.completion_signature;
 
 namespace beman::execution::detail {
 export using beman::execution::detail::completion_signature;
+export using beman::execution::detail::is_set_value;
+export using beman::execution::detail::is_set_error;
+export using beman::execution::detail::is_set_stopped;
 } // namespace beman::execution::detail

From 866def9cfaec4e27d2cedc6723d9e1e28df26bd1 Mon Sep 17 00:00:00 2001
From: Cra3z <CrazyCra3z@outlook.com>
Date: Wed, 23 Sep 2026 17:59:57 +0800
Subject: [PATCH 3/3] Format code

---
 include/beman/execution/detail/continues_on.hpp | 2 +-
 tests/beman/execution/issue-186.test.cpp        | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/beman/execution/detail/continues_on.hpp b/include/beman/execution/detail/continues_on.hpp
index 97cbd1b2..9bfeed3f 100644
--- a/include/beman/execution/detail/continues_on.hpp
+++ b/include/beman/execution/detail/continues_on.hpp
@@ -148,7 +148,7 @@ struct continues_on_t {
             using child_sigs_t = decltype(child_sigs);
             using extra_sigs_t = ::beman::execution::detail::meta::filter<is_not_set_value, decltype(sched_sigs)>;
             using type         = ::beman::execution::detail::meta::unique<
-                        ::beman::execution::detail::meta::combine<child_sigs_t, exception_signature, extra_sigs_t>>;
+                ::beman::execution::detail::meta::combine<child_sigs_t, exception_signature, extra_sigs_t>>;
             return type{};
         }
     };
diff --git a/tests/beman/execution/issue-186.test.cpp b/tests/beman/execution/issue-186.test.cpp
index 0b67abfb..0c2ae821 100644
--- a/tests/beman/execution/issue-186.test.cpp
+++ b/tests/beman/execution/issue-186.test.cpp
@@ -33,8 +33,8 @@ TEST(issue186) {
     assert(ret == 6);
 
     // NOTE: Compile time error with move_only_type
-    auto snd2   = ex::just(move_only_type(1))                                                     //
-                  | ex::then([](move_only_type v) noexcept { return move_only_type{v.val * 2}; }) //
+    auto snd2   = ex::just(move_only_type(1))                                                      //
+                  | ex::then([](move_only_type v) noexcept { return move_only_type{v.val * 2}; })  //
                   | ex::let_value([](move_only_type& v) noexcept { return ex::just(v.val * 3); }); // move_only_type
     auto [ret2] = ex::sync_wait(std::move(snd2)).value();
     assert(ret2 == 6);