execution: Add get_env() CPO

include/async/algorithm.hpp

@@ -94,6 +94,10 @@ struct value_transform_receiver {
 		}
 	}
 
+	auto get_env() {
+		return execution::get_env(dr_);
+	}
+
 private:
 	Receiver dr_; // Downstream receiver.
 	[[no_unique_address]] F f_;
@@ -113,6 +117,10 @@ struct void_transform_receiver {
 		}
 	}
 
+	auto get_env() {
+		return execution::get_env(dr_);
+	}
+
 private:
 	Receiver dr_; // Downstream receiver.
 	[[no_unique_address]] F f_;
@@ -281,6 +289,10 @@ struct [[nodiscard]] repeat_while_operation {
 				execution::set_value(s->dr_);
 		}
 
+		auto get_env() {
+			return execution::get_env(self_->dr_);
+		}
+
 	private:
 		repeat_while_operation *self_;
 	};
@@ -359,6 +371,10 @@ struct race_and_cancel_operation<Receiver, frg::tuple<Functors...>, std::index_s
 				execution::set_value(self_->r_);
 		}
 
+		auto get_env() {
+			return execution::get_env(self_->r_);
+		}
+
 	private:
 		race_and_cancel_operation *self_;
 	};
@@ -595,6 +611,10 @@ struct [[nodiscard]] sequence_operation {
 			execution::set_value(s->dr_, std::move(value));
 		}
 
+		auto get_env() {
+			return execution::get_env(self_->dr_);
+		}
+
 	private:
 		sequence_operation *self_;
 	};
@@ -657,6 +677,10 @@ struct when_all_operation {
 				execution::set_value(self_->dr_);
 		}
 
+		auto get_env() {
+			return execution::get_env(self_->dr_);
+		}
+
 	private:
 		when_all_operation *self_;
 	};

include/async/cancellation.hpp

@@ -387,6 +387,10 @@ struct with_cancel_cb_operation {
 				self->complete_();
 		}
 
+		auto get_env() {
+			return execution::get_env(self->dr_);
+		}
+
 		with_cancel_cb_operation *self;
 	};
 	static_assert(Receives<intermediate_receiver, value_type>);

include/async/execution.hpp

@@ -111,6 +111,24 @@ struct set_value_cpo {
 	}
 };
 
+template<typename T>
+concept get_env_member = requires(T &&obj) {
+	obj.get_env();
+};
+
+struct empty_env { };
+
+struct get_env_cpo {
+	template<typename T>
+	auto operator() (T &&obj) {
+		if constexpr (get_env_member<T>) {
+			return obj.get_env();
+		} else {
+			return empty_env{};
+		}
+	}
+};
+
 } // namespace cpo_types
 
 namespace execution {
@@ -121,6 +139,7 @@ namespace execution {
 	inline cpo_types::start_cpo start;
 	inline cpo_types::start_inline_cpo start_inline;
 	inline cpo_types::set_value_cpo set_value;
+	inline cpo_types::get_env_cpo get_env;
 }
 
 } // namespace async

include/async/post-ack.hpp

@@ -115,6 +115,8 @@ struct post_ack_mechanism {
 	};
 
 	struct [[nodiscard]] post_sender {
+		using value_type = void;
+
 		friend sender_awaiter<post_sender> operator co_await (post_sender sender) {
 			return {std::move(sender)};
 		}

include/async/wait-group.hpp

@@ -197,6 +197,10 @@ struct [[nodiscard]] sender_ {
 					std::forward<Ts>(ts)...
 				);
 			}
+
+			auto get_env() {
+				return execution::get_env(op_.originalr_);
+			}
 		};
 
 		execution::operation_t<OriginalS, receiver_> originalop_;