added a version of into_variant

docs/questions.md

@@ -24,4 +24,8 @@ likely observable.
 - [exec.just] Otherwise after p2.3 is missing <ts...>
 - [exec.run.loop.types] p9.1: "refers remains" -> "refers to remains"
 - [exec.run.loop.types] p9.2: "get_stop_token(REC(o))": REC is a receiver, any
-     environment would be attached to get_env(REC(o)).
+     environment would be attached to get_env(REC(o)).
+- [exec.into.variant] p2: the sender argument to into_variant really needs to
+    have completion_signatures defined - should the constaint be sender_in
+    instead of sender? oh, actually, only the result of transform_sender needs
+    to be a sender_in!

include/beman/execution26/detail/child_type.hpp

@@ -0,0 +1,20 @@
+// include/beman/execution26/detail/child_type.hpp                    -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef INCLUDED_BEMAN_EXECUTION26_DETAIL_CHILD_TYPE
+#define INCLUDED_BEMAN_EXECUTION26_DETAIL_CHILD_TYPE
+
+#include <cstddef>
+#include <type_traits>
+
+// ----------------------------------------------------------------------------
+
+namespace beman::execution26::detail
+{
+    template <typename Sender, ::std::size_t I = 0u>
+    using child_type = decltype(::std::declval<Sender>().template get<I + 2>());
+}
+
+// ----------------------------------------------------------------------------
+
+#endif

include/beman/execution26/detail/into_variant.hpp

@@ -0,0 +1,152 @@
+// include/beman/execution26/detail/into_variant.hpp                  -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef INCLUDED_BEMAN_EXECUTION26_DETAIL_INTO_VARIANT
+#define INCLUDED_BEMAN_EXECUTION26_DETAIL_INTO_VARIANT
+
+#include <beman/execution26/detail/child_type.hpp>
+#include <beman/execution26/detail/completion_signatures_for.hpp>
+#include <beman/execution26/detail/default_impls.hpp>
+#include <beman/execution26/detail/decayed_tuple.hpp>
+#include <beman/execution26/detail/env_of_t.hpp>
+#include <beman/execution26/detail/error_types_of_t.hpp>
+#include <beman/execution26/detail/get_domain_early.hpp>
+#include <beman/execution26/detail/impls_for.hpp>
+#include <beman/execution26/detail/make_sender.hpp>
+#include <beman/execution26/detail/meta_combine.hpp>
+#include <beman/execution26/detail/sender.hpp>
+#include <beman/execution26/detail/sender_in.hpp>
+#include <beman/execution26/detail/sends_stopped.hpp>
+#include <beman/execution26/detail/set_error.hpp>
+#include <beman/execution26/detail/set_value.hpp>
+#include <beman/execution26/detail/transform_sender.hpp>
+#include <beman/execution26/detail/value_types_of_t.hpp>
+
+#include <concepts>
+#include <exception>
+#include <type_traits>
+#include <utility>
+
+// ----------------------------------------------------------------------------
+
+namespace beman::execution26::detail
+{
+    struct into_variant_t
+    {
+        template <::beman::execution26::sender Sender>
+        auto operator()(Sender&& sender) const
+        {
+            auto domain{::beman::execution26::detail::get_domain_early(sender)};
+            (void)domain;
+            return 
+                ::beman::execution26::detail::make_sender(*this, {}, ::std::forward<Sender>(sender))
+                ;
+            //return ::beman::execution26::transform_sender(
+            //    ::std::move(domain),
+            //    ::beman::execution26::detail::make_sender(*this, {}, ::std::forward<Sender>(sender))
+            //);
+        }
+    };
+
+    template<>
+    struct impls_for<::beman::execution26::detail::into_variant_t>
+        : ::beman::execution26::detail::default_impls
+    {
+        static constexpr auto get_state
+            = []<typename Sender, typename Receiver>(Sender&&, Receiver&&) noexcept
+                -> ::std::type_identity<
+                        ::beman::execution26::value_types_of_t<
+                            ::beman::execution26::detail::child_type<Sender>,
+                            ::beman::execution26::env_of_t<Receiver>
+                        >
+                    >
+            {
+                return {};
+            };
+        static constexpr auto complete
+            = []<typename State, typename Tag, typename... Args>
+              (auto, State, auto& receiver, Tag, Args&&... args) noexcept
+                -> void
+            {
+                if constexpr (::std::same_as<Tag, ::beman::execution26::set_value_t>)
+                {
+                    using variant_type = typename State::type;
+                    using tuple_type = ::beman::execution26::detail::decayed_tuple<Args...>;
+                    try
+                    {
+                        if constexpr (sizeof...(Args) == 0u)
+                            ::beman::execution26::set_value(
+                                ::std::move(receiver)
+                            );
+                        else
+                            ::beman::execution26::set_value(
+                                ::std::move(receiver),
+                                variant_type(tuple_type{::std::forward<Args>(args)...})
+                            );
+                    }
+                    catch(...)
+                    {
+                        ::beman::execution26::set_error(
+                            ::std::move(receiver),
+                            ::std::current_exception()
+                            );
+                    }
+
+                }
+                else
+                {
+                    Tag()(::std::move(receiver), ::std::forward<Args>(args)...);
+                }
+            };
+    };
+
+    template <typename Sender, typename State, typename Env>
+    struct completion_signatures_for_impl<
+        ::beman::execution26::detail::basic_sender<
+            ::beman::execution26::detail::into_variant_t,
+            State,
+            Sender
+            >,
+        Env
+        >
+    {
+        using variant_type = ::beman::execution26::value_types_of_t<Sender, Env>;
+        using value_types = ::std::conditional_t<
+                ::std::same_as<variant_type, ::beman::execution26::detail::empty_variant>,
+                ::beman::execution26::completion_signatures<>,
+                ::beman::execution26::completion_signatures<
+                    ::beman::execution26::set_value_t(variant_type)
+                >
+            >;
+        template <typename... E>
+        using make_error_types
+            = ::beman::execution26::completion_signatures<::beman::execution26::set_error_t(E)...>;
+
+        using error_types
+        = ::beman::execution26::error_types_of_t<
+            Sender,
+            Env,
+            make_error_types
+            >;
+        using stopped_types
+            = ::std::conditional_t< 
+                ::beman::execution26::sends_stopped<Sender, Env>,
+                ::beman::execution26::completion_signatures<::beman::execution26::set_stopped_t()>,
+                ::beman::execution26::completion_signatures<>
+                >;
+        using type = ::beman::execution26::detail::meta::combine<
+            value_types,
+            ::beman::execution26::detail::meta::combine<error_types, stopped_types>
+            >;
+    };
+}
+
+namespace beman::execution26
+{
+    using into_variant_t = ::beman::execution26::detail::into_variant_t;
+    inline constexpr into_variant_t into_variant{};
+}
+
+// ----------------------------------------------------------------------------
+
+#endif

include/beman/execution26/detail/meta_combine.hpp

@@ -0,0 +1,33 @@
+// include/beman/execution26/detail/meta_combine.hpp                  -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef INCLUDED_BEMAN_EXECUTION26_DETAIL_META_COMBINE
+#define INCLUDED_BEMAN_EXECUTION26_DETAIL_META_COMBINE
+
+#include <beman/execution26/detail/type_list.hpp>
+#include <type_traits>
+
+// ----------------------------------------------------------------------------
+
+namespace beman::execution26::detail::meta::detail
+{
+    template <typename, typename> struct combine;
+
+    template <template <typename...> class L0, typename... T0,
+              template <typename...> class L1, typename... T1>
+    struct combine<L0<T0...>, L1<T1...>>
+    {
+        using type = L0<T0..., T1...>;
+    };
+}
+
+namespace beman::execution26::detail::meta
+{
+    template <typename L0, typename L1>
+    using combine
+        = ::beman::execution26::detail::meta::detail::combine<L0, L1>::type;
+}
+
+// ----------------------------------------------------------------------------
+
+#endif

include/beman/execution26/detail/product_type.hpp

@@ -38,7 +38,7 @@ namespace beman::execution26::detail
         static auto element_get(
             ::beman::execution26::detail::product_type_element<J, S>&& self)
             noexcept
-            -> S&&
+            -> S
         {
             return ::std::move(self.value);
         }

src/beman/execution26/CMakeLists.txt

@@ -31,6 +31,7 @@ target_sources(${TARGET_LIBRARY}
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/call_result_t.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/callable.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/check_type_alias_exist.hpp
+    ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/child_type.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/common.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/completion_domain.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/completion_signature.hpp
@@ -71,11 +72,13 @@ target_sources(${TARGET_LIBRARY}
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/indices_for.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/indirect_meta_apply.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/inplace_stop_source.hpp
+    ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/into_variant.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/join_env.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/just.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/make_env.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/make_sender.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/matching_sig.hpp
+    ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/meta_combine.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/meta_contains.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/meta_filter.hpp
     ${PROJECT_SOURCE_DIR}/include/beman/execution26/detail/meta_prepend.hpp

src/beman/execution26/tests/CMakeLists.txt

@@ -2,6 +2,8 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 list(APPEND execution_tests
+    meta-combine.pass
+    exec-into-variant.pass
     exec-run-loop-types.pass
     exec-run-loop-general.pass
     exec-just.pass