Add range-based connect composed operation

Free functions in boost/corosio/connect.hpp that try each endpoint in a
range (or iterator pair) until one connects, with optional connect-condition
predicate. Mirrors Boost.Asio's async_connect semantics adapted to
corosio's coroutine model.

Author: mvandeberg

doc/modules/ROOT/pages/4.guide/4d.sockets.adoc

@@ -136,6 +136,78 @@ For simpler code when errors are fatal:
 (co_await s.connect(endpoint)).value();  // Throws on error
 ----
 
+=== Range-Based Connect
+
+When connecting to a hostname, the resolver may return multiple endpoints
+(IPv4, IPv6, multiple A records). The free function `corosio::connect()` tries
+each in order, returning on the first success:
+
+[source,cpp]
+----
+#include <boost/corosio/connect.hpp>
+
+corosio::resolver r(ioc);
+auto [rec, results] = co_await r.resolve("www.boost.org", "80");
+if (rec)
+    co_return;
+
+corosio::tcp_socket s(ioc);
+auto [cec, ep] = co_await corosio::connect(s, results);
+if (cec)
+    co_return;
+// `ep` is the endpoint that accepted the connection.
+----
+
+Between attempts the socket is closed so the next `connect()` auto-opens with
+the correct address family. This lets a single call try IPv4 and IPv6
+candidates transparently.
+
+The signature is generic over any range whose elements convert to the
+socket's endpoint type:
+
+[source,cpp]
+----
+template<class Socket, std::ranges::input_range Range>
+    requires std::convertible_to<
+        std::ranges::range_reference_t<Range>,
+        typename Socket::endpoint_type>
+capy::task<capy::io_result<typename Socket::endpoint_type>>
+connect(Socket& s, Range endpoints);
+----
+
+On success, returns the connected endpoint. On all-fail, returns the error
+from the last attempt. On empty range (or when a connect condition rejects
+every candidate), returns `std::errc::no_such_device_or_address`.
+
+==== Filtering Candidates
+
+A second overload accepts a predicate invoked as `cond(last_ec, ep)` before
+each attempt. Returning `false` skips the candidate:
+
+[source,cpp]
+----
+auto [ec, ep] = co_await corosio::connect(
+    s,
+    results,
+    [](std::error_code const&, corosio::endpoint const& e) {
+        return e.is_v4();  // IPv4 only.
+    });
+----
+
+==== Iterator Overload
+
+An iterator-pair overload returns the iterator to the successful endpoint on
+success, or `end` on failure:
+
+[source,cpp]
+----
+auto [ec, it] = co_await corosio::connect(s, v.begin(), v.end());
+if (!ec)
+    std::cout << "connected to index " << (it - v.begin()) << "\n";
+----
+
+Both overloads accept an optional connect condition as a trailing argument.
+
 == Reading Data
 
 === read_some()

include/boost/corosio/connect.hpp

@@ -0,0 +1,299 @@
+//
+// Copyright (c) 2026 Michael Vandeberg
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/cppalliance/corosio
+//
+
+#ifndef BOOST_COROSIO_CONNECT_HPP
+#define BOOST_COROSIO_CONNECT_HPP
+
+#include <boost/corosio/detail/config.hpp>
+
+#include <boost/capy/cond.hpp>
+#include <boost/capy/io_result.hpp>
+#include <boost/capy/task.hpp>
+
+#include <concepts>
+#include <iterator>
+#include <ranges>
+#include <system_error>
+#include <utility>
+
+/*
+  Range-based composed connect operation.
+
+  These free functions try each endpoint in a range (or iterator pair)
+  in order, returning on the first successful connect. Between attempts
+  the socket is closed so that the next attempt can auto-open with the
+  correct address family (e.g. going from IPv4 to IPv6 candidates).
+
+  The iteration semantics follow Boost.Asio's range/iterator async_connect:
+  on success, the successful endpoint (or its iterator) is returned; on
+  all-fail, the last attempt's error code is returned; on an empty range
+  (or when a connect_condition rejects every candidate),
+  std::errc::no_such_device_or_address is returned, matching the error
+  the resolver uses for "no results" in posix_resolver_service.
+
+  The operation is a plain coroutine; cancellation is propagated to the
+  inner per-endpoint connect via the affine awaitable protocol on io_env.
+*/
+
+namespace boost::corosio {
+
+namespace detail {
+
+/* Always-true connect condition used by the overloads that take no
+   user-supplied predicate. Kept at namespace-detail scope so it has a
+   stable linkage name across translation units. */
+struct default_connect_condition
+{
+    template<class Endpoint>
+    bool operator()(std::error_code const&, Endpoint const&) const noexcept
+    {
+        return true;
+    }
+};
+
+} // namespace detail
+
+/* Forward declarations so the non-condition overloads can delegate
+   to the condition overloads via qualified lookup (qualified calls
+   bind to the overload set visible at definition, not instantiation). */
+
+template<class Socket, std::ranges::input_range Range, class ConnectCondition>
+    requires std::convertible_to<
+                 std::ranges::range_reference_t<Range>,
+                 typename Socket::endpoint_type> &&
+    std::predicate<
+                 ConnectCondition&,
+                 std::error_code const&,
+                 typename Socket::endpoint_type const&>
+capy::task<capy::io_result<typename Socket::endpoint_type>>
+connect(Socket& s, Range endpoints, ConnectCondition cond);
+
+template<class Socket, std::input_iterator Iter, class ConnectCondition>
+    requires std::convertible_to<
+                 std::iter_reference_t<Iter>,
+                 typename Socket::endpoint_type> &&
+    std::predicate<
+                 ConnectCondition&,
+                 std::error_code const&,
+                 typename Socket::endpoint_type const&>
+capy::task<capy::io_result<Iter>>
+connect(Socket& s, Iter begin, Iter end, ConnectCondition cond);
+
+/** Asynchronously connect a socket by trying each endpoint in a range.
+
+    Each candidate is tried in order. Before each attempt the socket is
+    closed (so the next `connect` auto-opens with the candidate's
+    address family). On first successful connect, the operation
+    completes with the connected endpoint.
+
+    @par Cancellation
+    Supports cancellation via the affine awaitable protocol. If a
+    per-endpoint connect completes with `capy::cond::canceled` the
+    operation completes immediately with that error and does not try
+    further endpoints.
+
+    @param s The socket to connect. Must have a `connect(endpoint)`
+        member returning an awaitable, plus `close()` and `is_open()`.
+        If the socket is already open, it will be closed before the
+        first attempt.
+    @param endpoints A range of candidate endpoints. Taken by value
+        so temporaries (e.g. `resolver_results` returned from
+        `resolver::resolve`) remain alive for the coroutine's lifetime.
+
+    @return An awaitable completing with
+        `capy::io_result<typename Socket::endpoint_type>`:
+        - on success: default error_code and the connected endpoint;
+        - on failure of all attempts: the error from the last attempt
+          and a default-constructed endpoint;
+        - on empty range: `std::errc::no_such_device_or_address` and a
+          default-constructed endpoint.
+
+    @throws std::system_error if auto-opening the socket fails during
+        an attempt (inherits the contract of `Socket::connect`).
+
+    @par Example
+    @code
+    resolver r(ioc);
+    auto [rec, results] = co_await r.resolve("www.boost.org", "80");
+    if (rec) co_return;
+    tcp_socket s(ioc);
+    auto [cec, ep] = co_await corosio::connect(s, results);
+    @endcode
+*/
+template<class Socket, std::ranges::input_range Range>
+    requires std::convertible_to<
+        std::ranges::range_reference_t<Range>,
+        typename Socket::endpoint_type>
+capy::task<capy::io_result<typename Socket::endpoint_type>>
+connect(Socket& s, Range endpoints)
+{
+    return corosio::connect(
+        s, std::move(endpoints), detail::default_connect_condition{});
+}
+
+/** Asynchronously connect a socket by trying each endpoint in a range,
+    filtered by a user-supplied condition.
+
+    For each candidate the condition is invoked as
+    `cond(last_ec, ep)` where `last_ec` is the error from the most
+    recent attempt (default-constructed before the first attempt). If
+    the condition returns `false` the candidate is skipped; otherwise a
+    connect is attempted.
+
+    @param s The socket to connect. See the non-condition overload for
+        requirements.
+    @param endpoints A range of candidate endpoints.
+    @param cond A predicate invocable with
+        `(std::error_code const&, typename Socket::endpoint_type const&)`
+        returning a value contextually convertible to `bool`.
+
+    @return Same as the non-condition overload. If every candidate is
+        rejected, completes with `std::errc::no_such_device_or_address`.
+
+    @throws std::system_error if auto-opening the socket fails.
+*/
+template<class Socket, std::ranges::input_range Range, class ConnectCondition>
+    requires std::convertible_to<
+                 std::ranges::range_reference_t<Range>,
+                 typename Socket::endpoint_type> &&
+    std::predicate<
+                 ConnectCondition&,
+                 std::error_code const&,
+                 typename Socket::endpoint_type const&>
+capy::task<capy::io_result<typename Socket::endpoint_type>>
+connect(Socket& s, Range endpoints, ConnectCondition cond)
+{
+    using endpoint_type = typename Socket::endpoint_type;
+
+    std::error_code last_ec;
+
+    for (auto&& e : endpoints)
+    {
+        endpoint_type ep = e;
+
+        if (!cond(static_cast<std::error_code const&>(last_ec),
+                  static_cast<endpoint_type const&>(ep)))
+            continue;
+
+        if (s.is_open())
+            s.close();
+
+        auto [ec] = co_await s.connect(ep);
+
+        if (!ec)
+            co_return {std::error_code{}, std::move(ep)};
+
+        if (ec == capy::cond::canceled)
+            co_return {ec, endpoint_type{}};
+
+        last_ec = ec;
+    }
+
+    if (!last_ec)
+        last_ec = std::make_error_code(std::errc::no_such_device_or_address);
+
+    co_return {last_ec, endpoint_type{}};
+}
+
+/** Asynchronously connect a socket by trying each endpoint in an
+    iterator range.
+
+    Behaves like the range overload, except the return value carries
+    the iterator to the successfully connected endpoint on success, or
+    `end` on failure. This mirrors Boost.Asio's iterator-based
+    `async_connect`.
+
+    @param s The socket to connect.
+    @param begin The first candidate.
+    @param end One past the last candidate.
+
+    @return An awaitable completing with `capy::io_result<Iter>`:
+        - on success: default error_code and the iterator of the
+          successful endpoint;
+        - on failure of all attempts: the error from the last attempt
+          and `end`;
+        - on empty range: `std::errc::no_such_device_or_address` and
+          `end`.
+
+    @throws std::system_error if auto-opening the socket fails.
+*/
+template<class Socket, std::input_iterator Iter>
+    requires std::convertible_to<
+        std::iter_reference_t<Iter>,
+        typename Socket::endpoint_type>
+capy::task<capy::io_result<Iter>>
+connect(Socket& s, Iter begin, Iter end)
+{
+    return corosio::connect(
+        s,
+        std::move(begin),
+        std::move(end),
+        detail::default_connect_condition{});
+}
+
+/** Asynchronously connect a socket by trying each endpoint in an
+    iterator range, filtered by a user-supplied condition.
+
+    @param s The socket to connect.
+    @param begin The first candidate.
+    @param end One past the last candidate.
+    @param cond A predicate invocable with
+        `(std::error_code const&, typename Socket::endpoint_type const&)`.
+
+    @return Same as the plain iterator overload. If every candidate is
+        rejected, completes with `std::errc::no_such_device_or_address`.
+
+    @throws std::system_error if auto-opening the socket fails.
+*/
+template<class Socket, std::input_iterator Iter, class ConnectCondition>
+    requires std::convertible_to<
+                 std::iter_reference_t<Iter>,
+                 typename Socket::endpoint_type> &&
+    std::predicate<
+                 ConnectCondition&,
+                 std::error_code const&,
+                 typename Socket::endpoint_type const&>
+capy::task<capy::io_result<Iter>>
+connect(Socket& s, Iter begin, Iter end, ConnectCondition cond)
+{
+    using endpoint_type = typename Socket::endpoint_type;
+
+    std::error_code last_ec;
+
+    for (Iter it = begin; it != end; ++it)
+    {
+        endpoint_type ep = *it;
+
+        if (!cond(static_cast<std::error_code const&>(last_ec),
+                  static_cast<endpoint_type const&>(ep)))
+            continue;
+
+        if (s.is_open())
+            s.close();
+
+        auto [ec] = co_await s.connect(ep);
+
+        if (!ec)
+            co_return {std::error_code{}, std::move(it)};
+
+        if (ec == capy::cond::canceled)
+            co_return {ec, std::move(end)};
+
+        last_ec = ec;
+    }
+
+    if (!last_ec)
+        last_ec = std::make_error_code(std::errc::no_such_device_or_address);
+
+    co_return {last_ec, std::move(end)};
+}
+
+} // namespace boost::corosio
+
+#endif

include/boost/corosio/local_stream_socket.hpp

@@ -77,6 +77,9 @@ namespace boost::corosio {
 class BOOST_COROSIO_DECL local_stream_socket : public io_stream
 {
 public:
+    /// The endpoint type used by this socket.
+    using endpoint_type = corosio::local_endpoint;
+
     using shutdown_type = corosio::shutdown_type;
     using enum corosio::shutdown_type;
 

include/boost/corosio/tcp_socket.hpp

@@ -78,6 +78,9 @@ namespace boost::corosio {
 class BOOST_COROSIO_DECL tcp_socket : public io_stream
 {
 public:
+    /// The endpoint type used by this socket.
+    using endpoint_type = corosio::endpoint;
+
     using shutdown_type = corosio::shutdown_type;
     using enum corosio::shutdown_type;