change how completion_signatures_for works, making it customizable

Author: dietmarkuehl

include/beman/execution26/detail/basic_sender.hpp

@@ -65,7 +65,6 @@ namespace beman::execution26::detail
             return { ::std::forward<Self>(self), ::std::move(receiver) };
         }
 #endif
-#if 1
 #if __cpp_explicit_this_parameter < 202110L
         template <typename Env>
         auto get_completion_signatures(Env&&) &&
@@ -99,7 +98,6 @@ namespace beman::execution26::detail
         {
             return {};
         }
-#endif
 #endif
     };
 }

include/beman/execution26/detail/completion_signatures_for.hpp

@@ -8,6 +8,7 @@
 #include <beman/execution26/detail/get_completion_signatures.hpp>
 #include <functional>
 #include <type_traits>
+#include <concepts>
 
 // ----------------------------------------------------------------------------
 
@@ -16,15 +17,31 @@ namespace beman::execution26::detail
     struct no_completion_signatures_defined_in_sender {};
 
     template <typename Sender, typename Env>
-    using completion_signatures_for
-        = decltype(::std::invoke([]{
+    struct completion_signatures_for_impl
+    {
+        using type = decltype(::std::invoke([]{
             if constexpr (::beman::execution26::sender_in<Sender, Env>)
                 return decltype(::beman::execution26::get_completion_signatures(
                     ::std::declval<Sender>(), ::std::declval<Env>())
                 ){};
             else
                 return ::beman::execution26::detail::no_completion_signatures_defined_in_sender{};
         }));
+    };
+
+    template <typename Sender, typename Env>
+    using completion_signatures_for
+        = ::std::conditional_t<
+            ::std::same_as<
+                beman::execution26::detail::no_completion_signatures_defined_in_sender,
+                    typename ::beman::execution26::detail::completion_signatures_for_impl<
+                        Sender, Env>::type
+                >,
+            typename ::beman::execution26::detail::completion_signatures_for_impl<
+                ::std::remove_cvref_t<Sender>, Env>::type,
+            typename ::beman::execution26::detail::completion_signatures_for_impl<
+                Sender, Env>::type
+        >;
 }
 
 // ----------------------------------------------------------------------------

include/beman/execution26/detail/write_env.hpp

@@ -11,6 +11,7 @@
 #include <beman/execution26/detail/make_sender.hpp>
 #include <beman/execution26/detail/queryable.hpp>
 #include <beman/execution26/detail/sender.hpp>
+#include <type_traits>
 #include <utility>
 
 // ----------------------------------------------------------------------------
@@ -30,6 +31,19 @@ namespace beman::execution26::detail
         static auto name() { return "write_env_t"; }
     };
 
+    template <typename NewEnv, typename Child, typename Env>
+    struct completion_signatures_for_impl<
+        ::beman::execution26::detail::basic_sender<
+            ::beman::execution26::detail::write_env_t, NewEnv, Child>
+        , Env>
+    {
+        using type = decltype(::beman::execution26::get_completion_signatures(
+            ::std::declval<Child>(), 
+            ::beman::execution26::detail::join_env(
+                ::std::declval<NewEnv>(), ::std::declval<Env>())
+            ));
+    };
+
     template <>
     struct impls_for<write_env_t>
         : ::beman::execution26::detail::default_impls

src/beman/execution26/tests/exec-snd-expos.pass.cpp

@@ -1404,13 +1404,25 @@ namespace
             test_std::completion_signatures<test_std::set_value_t(bool)>,
             decltype(test_std::get_completion_signatures(write_env_sender{}, write_env_env{}))
         >);
-#if 0
+
+        static_assert(std::same_as<
+            test_detail::completion_signatures_for<decltype(we_sender), write_env_env>,
+            test_std::completion_signatures<test_std::set_value_t(bool)>
+        >);
+        static_assert(std::same_as<
+            test_detail::completion_signatures_for<decltype(we_sender), test_std::empty_env>,
+            test_std::completion_signatures<test_std::set_value_t(bool)>
+        >);
+        static_assert(std::same_as<
+            test_detail::completion_signatures_for<decltype(we_sender)&, test_std::empty_env>,
+            test_std::completion_signatures<test_std::set_value_t(bool)>
+        >);
         static_assert(test_std::sender_in<decltype(we_sender)>);
         static_assert(std::same_as<
             test_std::completion_signatures<test_std::set_value_t(bool)>,
             decltype(test_std::get_completion_signatures(we_sender, write_env_env{}))
         >);
-#endif
+
         static_assert(test_std::sender<decltype(we_sender)>);
         static_assert(std::same_as<
             test_detail::write_env_t,