fixed how when_all determines its error completions

Author: dietmarkuehl

examples/sender-demo.cpp

@@ -46,16 +46,18 @@ static_assert(ex::sender<just_sender<std::pmr::string>>);
 static_assert(ex::sender_in<just_sender<std::pmr::string>>);
 
 int main() {
-    auto j  = just_sender{std::pmr::string("value")};
-    auto t  = std::move(j) | ex::then([](std::pmr::string v) { return v + " then"; });
-    auto w  = ex::when_all(std::move(t));
-    auto t2 = std::move(w) | ex::then([](auto&& v) { std::cout << "got " << v << "\n"; });
-    auto e  = ex::detail::write_env(std::move(t2),
+    auto j = just_sender{std::pmr::string("value")};
+    auto t = std::move(j) | ex::then([](std::pmr::string v) { return v + " then"; });
+    auto w = ex::when_all(std::move(t));
+    auto e = ex::detail::write_env(std::move(w),
                                    ex::detail::make_env(ex::get_allocator, std::pmr::polymorphic_allocator<>()));
 
-    auto o = ex::connect(std::move(e), test_receiver{});
     std::cout << "before start\n";
-    ex::start(o);
-    // auto r = ex::sync_wait(std::move(w) | ex::then([](auto&&){}));
+    auto r = ex::sync_wait(std::move(e));
+    if (r) {
+        auto [v] = *r;
+        std::cout << "produced='" << v << "'\n";
+    } else
+        std::cout << "operation was cancelled\n";
     std::cout << "after start\n";
 }

include/beman/execution26/detail/when_all.hpp

@@ -231,13 +231,15 @@ template <typename Data, typename Env, typename... Sender>
 struct completion_signatures_for_impl<
     ::beman::execution26::detail::basic_sender<::beman::execution26::detail::when_all_t, Data, Sender...>,
     Env> {
+    template <typename... E>
+    using error_comps = ::beman::execution26::completion_signatures<::beman::execution26::set_error_t(E)...>;
     using value_types =
         typename ::beman::execution26::detail::when_all_value_types<::beman::execution26::detail::meta::combine<
             ::beman::execution26::
                 value_types_of_t<Sender, Env, ::beman::execution26::detail::type_list, ::std::type_identity_t>...>>::
             type;
     using error_types = ::beman::execution26::detail::meta::unique<::beman::execution26::detail::meta::combine<
-        ::beman::execution26::error_types_of_t<Sender, Env, ::beman::execution26::completion_signatures>...>>;
+        ::beman::execution26::error_types_of_t<Sender, Env, error_comps>...>>;
     using type        = ::beman::execution26::detail::meta::combine<value_types, error_types>;
 };
 } // namespace beman::execution26::detail