libstdc++-v3/include/experimental/internet

// <experimental/internet> -*- C++ -*-

// Copyright (C) 2015-2026 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file experimental/internet
 *  This is a TS C++ Library header.
 *  @ingroup networking-ts
 */

#ifndef _GLIBCXX_EXPERIMENTAL_INTERNET
#define _GLIBCXX_EXPERIMENTAL_INTERNET

#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif

#include <bits/requires_hosted.h> // experimental is currently omitted

#if __cplusplus >= 201402L

#include <experimental/netfwd>
#include <experimental/io_context>
#include <experimental/bits/net.h>
#include <array>
#include <forward_list>
#include <sstream>
#include <cstdint>
#include <experimental/string_view>
#include <bits/charconv.h>
#ifdef _GLIBCXX_HAVE_UNISTD_H
# include <unistd.h>
#endif
#ifdef _GLIBCXX_HAVE_SYS_SOCKET_H
# include <sys/socket.h>	// AF_INET, AF_INET6, SOCK_DGRAM, SOCK_STREAM
#endif
#ifdef _GLIBCXX_HAVE_ARPA_INET_H
# include <arpa/inet.h>		// inet_ntop
#endif
#ifdef _GLIBCXX_HAVE_NETINET_IN_H
# include <netinet/in.h>	// IPPROTO_IP, IPPROTO_IPV6, in_addr, in6_addr
#endif
#ifdef _GLIBCXX_HAVE_NETINET_TCP_H
# include <netinet/tcp.h>	// TCP_NODELAY
#endif
#ifdef _GLIBCXX_HAVE_NETDB_H
# include <netdb.h>		// getaddrinfo etc.
#endif

#if defined _WIN32 && __has_include(<ws2tcpip.h>)
# include <ws2tcpip.h>
#endif

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
namespace experimental
{
namespace net
{
inline namespace v1
{
namespace ip
{
  /** @addtogroup networking-ts
   *  @{
   */

  /** Error codes for resolver errors.
   * @{
   */

  enum class resolver_errc : int {
#ifdef _GLIBCXX_HAVE_NETDB_H
    host_not_found = EAI_NONAME,
    host_not_found_try_again = EAI_AGAIN,
    service_not_found = EAI_SERVICE
    // N.B. POSIX defines additional errors that have no enumerator here:
    // EAI_BADFLAGS, EAI_FAIL, EAI_FAMILY, EAI_MEMORY, EAI_SOCKTYPE, EAI_SYSTEM
    // Some C libraries define additional errors:
    // EAI_BADHINTS, EAI_OVERFLOW, EAI_PROTOCOL
    // Some C libraries define additional (obsolete?) errors:
    // EAI_ADDRFAMILY, EAI_NODATA
#endif
  };

  /// Error category for resolver errors.
  inline const error_category& resolver_category() noexcept // TODO non-inline
  {
    struct __cat : error_category
    {
      const char* name() const noexcept { return "resolver"; }
      std::string message(int __e) const {
#ifdef _GLIBCXX_HAVE_NETDB_H
	  return ::gai_strerror(__e);
#else
	  return "name resolution requires <netdb.h>";
#endif
      }
      virtual void __message(int) { } // TODO dual ABI XXX
    };
    static __cat __c;
    return __c;
  }

  inline error_code make_error_code(resolver_errc __e) noexcept
  { return error_code(static_cast<int>(__e), resolver_category()); }

  inline error_condition make_error_condition(resolver_errc __e) noexcept
  { return error_condition(static_cast<int>(__e), resolver_category()); }

  /// @cond undocumented
  inline error_code
  __make_resolver_error_code(int __ai_err,
			     [[__maybe_unused__]] int __sys_err) noexcept
  {
#ifdef EAI_SYSTEM
    if (__builtin_expect(__ai_err == EAI_SYSTEM, 0))
      return error_code(__sys_err, std::generic_category());
#endif
    return error_code(__ai_err, resolver_category());
  }
  /// @endcond

  /// @}

  using port_type = uint_least16_t;	///< Type used for port numbers.
  using scope_id_type = uint_least32_t;	///< Type used for IPv6 scope IDs.

  /// Convenience alias for constraining allocators for strings.
  template<typename _Alloc>
    using __string_with
      = enable_if_t<std::is_same<typename _Alloc::value_type, char>::value,
		    std::basic_string<char, std::char_traits<char>, _Alloc>>;

  constexpr errc
  __unsupported_err() noexcept
  {
#if defined EAFNOSUPPORT
    return std::errc::address_family_not_supported;
#else
    return std::errc::operation_not_supported;
#endif
  }

  /** Tag indicating conversion between IPv4 and IPv4-mapped IPv6 addresses.
   * @{
   */

  struct v4_mapped_t {};
  constexpr v4_mapped_t v4_mapped;

  /// @}

  /// An IPv4 address.
  class address_v4
  {
  public:
    // types:
    using uint_type = uint_least32_t;

    struct bytes_type : array<unsigned char, 4>
    {
      template<typename... _Tp>
	explicit constexpr
	bytes_type(_Tp... __tp)
	: array<unsigned char, 4>{{static_cast<unsigned char>(__tp)...}}
	{
#if UCHAR_MAX > 0xFF
	  for (auto __b : *this)
	    if (__b > 0xFF)
	      __throw_out_of_range("invalid address_v4::bytes_type value");
#endif
	}
    };

    // constructors:
    constexpr address_v4() noexcept : _M_addr(0) { }

    constexpr address_v4(const address_v4& a) noexcept = default;

    constexpr
    address_v4(const bytes_type& __b)
#if __has_builtin(__builtin_bit_cast)
    : _M_addr(__builtin_bit_cast(uint_type, __b))
#else
    : _M_addr(_S_hton_32((__b[0] << 24) | (__b[1] << 16)
			   | (__b[2] << 8) | __b[3]))
#endif
    { }

    explicit constexpr
    address_v4(uint_type __val) : _M_addr(_S_hton_32(__val))
    {
#if UINT_LEAST32_MAX > 0xFFFFFFFF
      if (__val > 0xFFFFFFFF)
	__throw_out_of_range("invalid address_v4::uint_type value");
#endif
    }

    // assignment:
    address_v4& operator=(const address_v4& a) noexcept = default;

    // members:
    constexpr bool is_unspecified() const noexcept { return to_uint() == 0; }

    constexpr bool
    is_loopback() const noexcept
    { return (to_uint() & 0xFF000000) == 0x7F000000; }

    constexpr bool
    is_multicast() const noexcept
    { return (to_uint() & 0xF0000000) == 0xE0000000; }

    constexpr bytes_type
    to_bytes() const noexcept
    {
#if __has_builtin(__builtin_bit_cast)
      return __builtin_bit_cast(bytes_type, _M_addr);
#else
      auto __host = to_uint();
      return bytes_type{
	(__host >> 24) & 0xFF,
	(__host >> 16) & 0xFF,
	(__host >> 8) & 0xFF,
	__host & 0xFF
      };
#endif
    }

    constexpr uint_type
    to_uint() const noexcept { return _S_ntoh_32(_M_addr); }

    template<typename _Allocator = allocator<char>>
      __string_with<_Allocator>
      to_string(const _Allocator& __a = _Allocator()) const
      {
	auto __write = [__addr = to_uint()](char* __p, size_t) {
	  auto __to_chars = [](char* __p, uint8_t __v) {
	    unsigned __n = __v >= 100u ? 3 : __v >= 10u ? 2 : 1;
	    std::__detail::__to_chars_10_impl(__p, __n, __v);
	    return __p + __n;
	  };
	  const auto __begin = __p;
	  __p = __to_chars(__p, uint8_t(__addr >> 24));
	  for (int __i = 2; __i >= 0; __i--) {
	    *__p++ = '.';
	    __p = __to_chars(__p, uint8_t(__addr >> (__i * 8)));
	  }
	  return __p - __begin;
	};
	__string_with<_Allocator> __str(__a);
#if __cpp_lib_string_resize_and_overwrite
	__str.resize_and_overwrite(15, __write);
#else
	__str.resize(15);
	__str.resize(__write(&__str.front(), 15));
#endif
	return __str;
      }

    // static members:
    static constexpr address_v4 any() noexcept { return address_v4{}; }

    static constexpr
    address_v4 loopback() noexcept { return address_v4{0x7F000001}; }

    static constexpr
    address_v4 broadcast() noexcept { return address_v4{0xFFFFFFFF}; }

  private:
    template<typename _InternetProtocol>
      friend class basic_endpoint;

    friend address_v4 make_address_v4(const char*, error_code&) noexcept;

#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
    static constexpr uint16_t _S_hton_16(uint16_t __h) { return __h; }
    static constexpr uint16_t _S_ntoh_16(uint16_t __n) { return __n; }
    static constexpr uint32_t _S_hton_32(uint32_t __h) { return __h; }
    static constexpr uint32_t _S_ntoh_32(uint32_t __n) { return __n; }
#else
    static constexpr uint16_t
    _S_hton_16(uint16_t __h) { return __builtin_bswap16(__h); }

    static constexpr uint16_t
    _S_ntoh_16(uint16_t __n) { return __builtin_bswap16(__n); }

    static constexpr uint32_t
    _S_hton_32(uint32_t __h) { return __builtin_bswap32(__h); }

    static constexpr uint32_t
    _S_ntoh_32(uint32_t __n) { return __builtin_bswap32(__n); }
#endif

#ifdef _GLIBCXX_HAVE_ARPA_INET_H
    in_addr_t _M_addr; // network byte order
#else
    uint32_t _M_addr;
#endif
  };

  /// An IPv6 address.
  class address_v6
  {
  public:
    // types:
    struct bytes_type : array<unsigned char, 16>
    {
      template<typename... _Tp>
	explicit constexpr
	bytes_type(_Tp... __t)
	: array<unsigned char, 16>{{static_cast<unsigned char>(__t)...}}
	{ }
    };

    // constructors:
    constexpr address_v6() noexcept : _M_bytes(), _M_scope_id() { }

    constexpr address_v6(const address_v6& __a) noexcept = default;

    constexpr
    address_v6(const bytes_type& __bytes, scope_id_type __scope = 0)
    : _M_bytes(__bytes), _M_scope_id(__scope)
    { }

    // assignment:
    address_v6& operator=(const address_v6& __a) noexcept = default;

    // members:
    void scope_id(scope_id_type __id) noexcept { _M_scope_id = __id; }

    constexpr scope_id_type scope_id() const noexcept { return _M_scope_id; }

    constexpr bool
    is_unspecified() const noexcept
    {
      for (int __i = 0; __i < 16; ++__i)
	if (_M_bytes[__i] != 0x00)
	  return false;
      return _M_scope_id == 0;
    }

    constexpr bool
    is_loopback() const noexcept
    {
      for (int __i = 0; __i < 15; ++__i)
	if (_M_bytes[__i] != 0x00)
	  return false;
      return _M_bytes[15] == 0x01 && _M_scope_id == 0;
    }

    constexpr bool
    is_multicast() const noexcept { return _M_bytes[0] == 0xFF; }

    constexpr bool
    is_link_local() const noexcept
    { return _M_bytes[0] == 0xFE && (_M_bytes[1] & 0xC0) == 0x80; }

    constexpr bool
    is_site_local() const noexcept
    { return _M_bytes[0] == 0xFE && (_M_bytes[1] & 0xC0) == 0xC0; }

    constexpr bool
    is_v4_mapped() const noexcept
    {
      const bytes_type& __b = _M_bytes;
      return __b[0] == 0 && __b[1] == 0 && __b[ 2] == 0    && __b[ 3] == 0
	  && __b[4] == 0 && __b[5] == 0 && __b[ 6] == 0    && __b[ 7] == 0
	  && __b[8] == 0 && __b[9] == 0 && __b[10] == 0xFF && __b[11] == 0xFF;
    }

    constexpr bool
    is_multicast_node_local() const noexcept
    { return is_multicast() && (_M_bytes[1] & 0x0F) == 0x01; }

    constexpr bool
    is_multicast_link_local() const noexcept
    { return is_multicast() && (_M_bytes[1] & 0x0F) == 0x02; }

    constexpr bool
    is_multicast_site_local() const noexcept
    { return is_multicast() && (_M_bytes[1] & 0x0F) == 0x05; }

    constexpr bool
    is_multicast_org_local() const noexcept
    { return is_multicast() && (_M_bytes[1] & 0x0F) == 0x08; }

    constexpr bool
    is_multicast_global() const noexcept
    { return is_multicast() && (_M_bytes[1] & 0x0F) == 0x0b; }

    constexpr bytes_type to_bytes() const noexcept { return _M_bytes; }

    template<typename _Allocator = allocator<char>>
      __string_with<_Allocator>
      to_string(const _Allocator& __a = _Allocator()) const
      {
#ifdef _GLIBCXX_HAVE_ARPA_INET_H
	__string_with<_Allocator> __str(__a);
	__str.resize(INET6_ADDRSTRLEN + (_M_scope_id ? 11 : 0));
	char* const __p = &__str.front();
	if (inet_ntop(AF_INET6, &_M_bytes, __p, __str.size()))
	  {
	    auto __end = __str.find('\0');
	    if (unsigned long __scope = _M_scope_id)
	      {
		__end +=
#if _GLIBCXX_USE_C99_STDIO
		  __builtin_snprintf(__p + __end, __str.size() - __end,
				     "%%%lu", __scope);
#else
		  __builtin_sprintf(__p + __end, "%%%lu", __scope);
#endif
	      }
	    __str.erase(__end);
	  }
	else
	  __str.resize(0);
	return __str;
#else
	std::__throw_system_error((int)__unsupported_err());
#endif
      }

    // static members:

    static constexpr address_v6
    any() noexcept
    {
      return {};
    }

    static constexpr address_v6
    loopback() noexcept
    {
      return {bytes_type{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}};
    }

  private:
    template<typename _InternetProtocol>
      friend class basic_endpoint;

    friend constexpr bool
    operator==(const address_v6&, const address_v6&) noexcept;

    friend constexpr bool
    operator< (const address_v6&, const address_v6&) noexcept;

    bytes_type _M_bytes;
    scope_id_type _M_scope_id;
  };

  /// Exception type thrown on misuse of IPv4 addresses as IPv6 or vice versa.
  class bad_address_cast : public bad_cast
  {
  public:
    bad_address_cast() { }

    const char* what() const noexcept { return "bad address cast"; }
  };

  /// An IPv4 or IPv6 address.
  class address
  {
  public:
    // constructors:
    constexpr address() noexcept : _M_v4(), _M_is_v4(true) { }

#if __cpp_constexpr_dynamic_alloc
    constexpr
#endif
    address(const address& __a) noexcept : _M_uninit(), _M_is_v4(__a._M_is_v4)
    {
      if (_M_is_v4)
	std::_Construct(std::addressof(_M_v4), __a.to_v4());
      else
	std::_Construct(std::addressof(_M_v6), __a.to_v6());
    }

    constexpr
    address(const address_v4& __a) noexcept : _M_v4(__a), _M_is_v4(true) { }

    constexpr
    address(const address_v6& __a) noexcept : _M_v6(__a), _M_is_v4(false) { }

    // assignment:
    address&
    operator=(const address& __a) noexcept
    {
      if (__a._M_is_v4)
	*this = __a.to_v4();
      else
	*this = __a.to_v6();
      return *this;
    }

    address&
    operator=(const address_v4& __a) noexcept
    {
      std::_Construct(std::addressof(_M_v4), __a);
      _M_is_v4 = true;
      return *this;
    }

    address&
    operator=(const address_v6& __a) noexcept
    {
      std::_Construct(std::addressof(_M_v6), __a);
      _M_is_v4 = false;
      return *this;
    }

    // members:

    constexpr bool is_v4() const noexcept { return _M_is_v4; }
    constexpr bool is_v6() const noexcept { return !_M_is_v4; }

    constexpr address_v4
    to_v4() const
    {
      if (!is_v4())
	_GLIBCXX_THROW_OR_ABORT(bad_address_cast());
      return _M_v4;
    }

    constexpr address_v6
    to_v6() const
    {
      if (!is_v6())
	_GLIBCXX_THROW_OR_ABORT(bad_address_cast());
      return _M_v6;
    }

    constexpr bool
    is_unspecified() const noexcept
    { return _M_is_v4 ? _M_v4.is_unspecified() : _M_v6.is_unspecified(); }

    constexpr bool
    is_loopback() const noexcept
    { return _M_is_v4 ? _M_v4.is_loopback() : _M_v6.is_loopback(); }

    constexpr bool
    is_multicast() const noexcept
    { return _M_is_v4 ? _M_v4.is_multicast() : _M_v6.is_multicast(); }

    template<typename _Allocator = allocator<char>>
      __string_with<_Allocator>
      to_string(const _Allocator& __a = _Allocator()) const
      {
	if (_M_is_v4)
	  return to_v4().to_string(__a);
	return to_v6().to_string(__a);
      }

  private:
    template<typename _InternetProtocol>
      friend class basic_endpoint;

    friend constexpr bool
    operator==(const address&, const address&) noexcept;

    friend constexpr bool
    operator<(const address&, const address&) noexcept;

    union {
      address_v4 _M_v4;
      address_v6 _M_v6;
      bool	 _M_uninit;
    };
    bool _M_is_v4;
  };

  /** ip::address_v4 comparisons
   * @{
   */

  constexpr bool
  operator==(const address_v4& __a, const address_v4& __b) noexcept
  { return __a.to_uint() == __b.to_uint(); }

  constexpr bool
  operator!=(const address_v4& __a, const address_v4& __b) noexcept
  { return !(__a == __b); }

  constexpr bool
  operator< (const address_v4& __a, const address_v4& __b) noexcept
  { return __a.to_uint() < __b.to_uint(); }

  constexpr bool
  operator> (const address_v4& __a, const address_v4& __b) noexcept
  { return __b < __a; }

  constexpr bool
  operator<=(const address_v4& __a, const address_v4& __b) noexcept
  { return !(__b < __a); }

  constexpr bool
  operator>=(const address_v4& __a, const address_v4& __b) noexcept
  { return !(__a < __b); }

  /// @}

  /** ip::address_v6 comparisons
   * @{
   */

  constexpr bool
  operator==(const address_v6& __a, const address_v6& __b) noexcept
  {
    const auto& __aa = __a._M_bytes;
    const auto& __bb = __b._M_bytes;
    int __i = 0;
    for (; __i < 16 && __aa[__i] == __bb[__i]; ++__i)
      ;
    return __i == 16 ? __a.scope_id() == __b.scope_id() : false;
  }

  constexpr bool
  operator!=(const address_v6& __a, const address_v6& __b) noexcept
  { return !(__a == __b); }

  constexpr bool
  operator< (const address_v6& __a, const address_v6& __b) noexcept
  {
    const auto& __aa = __a._M_bytes;
    const auto& __bb = __b._M_bytes;
    int __i = 0;
    for (; __i < 16 && __aa[__i] == __bb[__i]; ++__i)
      ;
    return __i == 16 ? __a.scope_id() < __b.scope_id() : __aa[__i] < __bb[__i];
  }

  constexpr bool
  operator> (const address_v6& __a, const address_v6& __b) noexcept
  { return __b < __a; }

  constexpr bool
  operator<=(const address_v6& __a, const address_v6& __b) noexcept
  { return !(__b < __a); }

  constexpr bool
  operator>=(const address_v6& __a, const address_v6& __b) noexcept
  { return !(__a < __b); }

  /// @}

  /** ip::address comparisons
   * @{
   */

  constexpr bool
  operator==(const address& __a, const address& __b) noexcept
  {
    if (__a.is_v4())
      return __b.is_v4() ? __a._M_v4 == __b._M_v4 : false;
    return __b.is_v4() ? false : __a._M_v6 == __b._M_v6;
  }

  constexpr bool
  operator!=(const address& __a, const address& __b) noexcept
  { return !(__a == __b); }

  constexpr bool
  operator< (const address& __a, const address& __b) noexcept
  {
    if (__a.is_v4())
      return __b.is_v4() ? __a._M_v4 < __b._M_v4 : true;
    return __b.is_v4() ? false : __a._M_v6 < __b._M_v6;
  }

  constexpr bool
  operator> (const address& __a, const address& __b) noexcept
  { return __b < __a; }

  constexpr bool
  operator<=(const address& __a, const address& __b) noexcept
  { return !(__b < __a); }

  constexpr bool
  operator>=(const address& __a, const address& __b) noexcept
  { return !(__a < __b); }

  /// @}

  /** ip::address_v4 creation
   * @{
   */

  constexpr address_v4
  make_address_v4(const address_v4::bytes_type& __b)
  { return address_v4{__b}; }

  constexpr address_v4
  make_address_v4(address_v4::uint_type __val)
  { return address_v4{__val}; }

  constexpr address_v4
  make_address_v4(v4_mapped_t, const address_v6& __a)
  {
    if (!__a.is_v4_mapped())
      _GLIBCXX_THROW_OR_ABORT(bad_address_cast());

    const auto __v6b = __a.to_bytes();
    return address_v4::bytes_type(__v6b[12], __v6b[13], __v6b[14], __v6b[15]);
  }

  inline address_v4
  make_address_v4(const char* __str, error_code& __ec) noexcept
  {
#ifdef _GLIBCXX_HAVE_ARPA_INET_H
    address_v4 __a;
    const int __res = ::inet_pton(AF_INET, __str, &__a._M_addr);
    if (__res == 1)
      {
	__ec.clear();
	return __a;
      }
    if (__res == 0)
      __ec = std::make_error_code(std::errc::invalid_argument);
    else
      __ec.assign(errno, generic_category());
#else
    __ec = std::make_error_code(__unsupported_err());
#endif
    return {};
  }

  inline address_v4
  make_address_v4(const char* __str)
  { return make_address_v4(__str, __throw_on_error{"make_address_v4"}); }

  inline address_v4
  make_address_v4(const string& __str, error_code& __ec) noexcept
  { return make_address_v4(__str.c_str(), __ec); }

  inline address_v4
  make_address_v4(const string& __str)
  { return make_address_v4(__str.c_str()); }

  inline address_v4
  make_address_v4(string_view __str, error_code& __ec) noexcept
  {
    char __buf[16]; // INET_ADDRSTRLEN isn't defined on Windows
    auto __len = __str.copy(__buf, sizeof(__buf));
    if (__len == sizeof(__buf))
      {
	__ec = std::make_error_code(std::errc::invalid_argument);
	return {};
      }
    __ec.clear();
    __buf[__len] = '\0';
    return make_address_v4(__buf, __ec);
  }

  inline address_v4
  make_address_v4(string_view __str)
  { return make_address_v4(__str, __throw_on_error{"make_address_v4"}); }

  /// @}

  /** ip::address_v6 creation
   * @{
   */

  constexpr address_v6
  make_address_v6(const address_v6::bytes_type& __b, scope_id_type __scope = 0)
  { return address_v6{__b, __scope}; }

  constexpr address_v6
  make_address_v6(v4_mapped_t, const address_v4& __a) noexcept
  {
    const address_v4::bytes_type __v4b = __a.to_bytes();
    address_v6::bytes_type __v6b(0, 0, 0, 0, 0, 0, 0, 0,
				 0, 0, 0xFF, 0xFF,
				 __v4b[0], __v4b[1], __v4b[2], __v4b[3]);
    return address_v6(__v6b);
  }

  inline address_v6
  __make_address_v6(const char* __addr, const char* __scope, error_code& __ec)
  {
#ifdef _GLIBCXX_HAVE_ARPA_INET_H
    address_v6::bytes_type __b;
    const int __res = ::inet_pton(AF_INET6, __addr, __b.data());
    if (__res == 1)
      {
	__ec.clear();
	if (!__scope)
	  {
	    return { __b };
	  }

	char* __eptr;
	unsigned long __val = std::strtoul(__scope, &__eptr, 10);
	if (__eptr != __scope && !*__eptr
	    && __val <= numeric_limits<scope_id_type>::max())
	  {
	    return { __b, static_cast<scope_id_type>(__val) };
	  }
	__ec = std::make_error_code(std::errc::invalid_argument);
      }
    else if (__res == 0)
      __ec = std::make_error_code(std::errc::invalid_argument);
    else
      __ec.assign(errno, generic_category());
#else
    __ec = std::make_error_code(__unsupported_err());
#endif
    return {};
  }

  inline address_v6
  make_address_v6(const char* __str, error_code& __ec) noexcept
  {
    auto __p = __builtin_strchr(__str, '%');
    if (__p == nullptr)
      return __make_address_v6(__str, nullptr, __ec);
    char __buf[64];
    char* __out = __buf;
    bool __skip_leading_zero = true;
    while (__str < __p && __out < std::end(__buf))
      {
	if (!__skip_leading_zero || *__str != '0')
	  {
	    if (*__str == ':' || *__str == '.')
	      __skip_leading_zero = true;
	    else
	      __skip_leading_zero = false;
	    *__out = *__str;
	  }
	__str++;
      }
    if (__out == std::end(__buf))
      {
	__ec = std::make_error_code(std::errc::invalid_argument);
	return {};
      }
    else
      {
	*__out = '\0';
	return __make_address_v6(__buf, __p + 1, __ec);
      }
  }

  inline address_v6
  make_address_v6(const char* __str)
  { return make_address_v6(__str, __throw_on_error{"make_address_v6"}); }

  inline address_v6
  make_address_v6(const string& __str, error_code& __ec) noexcept
  {
    auto __pos = __str.find('%');
    if (__pos == string::npos)
      return __make_address_v6(__str.c_str(), nullptr, __ec);
    char __buf[64];
    char* __out = __buf;
    bool __skip_leading_zero = true;
    size_t __n = 0;
    while (__n < __pos && __out < std::end(__buf))
      {
	if (!__skip_leading_zero || __str[__n] != '0')
	  {
	    if (__str[__n] == ':' || __str[__n] == '.')
	      __skip_leading_zero = true;
	    else
	      __skip_leading_zero = false;
	    *__out = __str[__n];
	  }
	__n++;
      }
    if (__out == std::end(__buf))
      {
	__ec = std::make_error_code(std::errc::invalid_argument);
	return {};
      }
    else
      {
	*__out = '\0';
	return __make_address_v6(__buf, __str.c_str() + __pos + 1, __ec);
      }
  }

  inline address_v6
  make_address_v6(const string& __str)
  { return make_address_v6(__str, __throw_on_error{"make_address_v6"}); }

  inline address_v6
  make_address_v6(string_view __str, error_code& __ec) noexcept
  {
    char __buf[64];
    char* __out = __buf;
    char* __scope = nullptr;
    bool __skip_leading_zero = true;
    size_t __n = 0;
    while (__n < __str.length() && __out < std::end(__buf))
      {
	if (__str[__n] == '%')
	  {
	    if (__scope)
	      __out = std::end(__buf);
	    else
	      {
		*__out = '\0';
		__scope = ++__out;
		__skip_leading_zero = true;
	      }
	  }
	else if (!__skip_leading_zero || __str[__n] != '0')
	  {
	    if (__str[__n] == ':' || __str[__n] == '.')
	      __skip_leading_zero = true;
	    else
	      __skip_leading_zero = false;
	    *__out = __str[__n];
	    __out++;
	  }
	__n++;
      }
    if (__out == std::end(__buf))
      {
	__ec = std::make_error_code(std::errc::invalid_argument);
	return {};
      }
    else
      {
	*__out = '\0';
	return __make_address_v6(__buf, __scope, __ec);
      }
  }

  inline address_v6
  make_address_v6(string_view __str)
  { return make_address_v6(__str, __throw_on_error{"make_address_v6"}); }

  /// @}

  /** ip::address creation
   * @{
   */

  inline address
  make_address(const char* __str, error_code& __ec) noexcept
  {
    address __a;
    address_v6 __v6a = make_address_v6(__str, __ec);
    if (!__ec)
      __a = __v6a;
    else
    {
      address_v4 __v4a = make_address_v4(__str, __ec);
      if (!__ec)
	__a = __v4a;
    }
    return __a;
  }

  inline address
  make_address(const char* __str)
  { return make_address(__str, __throw_on_error{"make_address"}); }

  inline address
  make_address(const string& __str, error_code& __ec) noexcept
  { return make_address(__str.c_str(), __ec); }

  inline address
  make_address(const string& __str)
  { return make_address(__str, __throw_on_error{"make_address"}); }

  inline address
  make_address(string_view __str, error_code& __ec) noexcept
  {
    if (__str.rfind('\0') != string_view::npos)
      return make_address(__str.data(), __ec);
    return make_address(__str.to_string(), __ec); // TODO don't allocate
  }

  inline address
  make_address(string_view __str)
  { return make_address(__str, __throw_on_error{"make_address"}); }

  /// @}

  /// ip::address I/O
  template<typename _CharT, typename _Traits>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>& __os, const address& __a)
    { return __os << __a.to_string(); }

  /// ip::address_v4 I/O
  template<typename _CharT, typename _Traits>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>& __os, const address_v4& __a)
    { return __os << __a.to_string(); }

  /// ip::address_v6 I/O
  template<typename _CharT, typename _Traits>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>& __os, const address_v6& __a)
    { return __os << __a.to_string(); }

  template<typename> class basic_address_iterator; // not defined

  template<> class basic_address_iterator<address_v4>
  {
  public:
    // types:
    using value_type = address_v4;
    using difference_type = ptrdiff_t;
    using pointer = const address_v4*;
    using reference = const address_v4&;
    using iterator_category = input_iterator_tag;

    // constructors:
    basic_address_iterator(const address_v4& __a) noexcept
    : _M_address(__a) { }

    // members:
    reference operator*() const noexcept { return _M_address; }
    pointer operator->() const noexcept { return &_M_address; }

    basic_address_iterator&
    operator++() noexcept
    {
      _M_address = value_type(_M_address.to_uint() + 1);
      return *this;
    }

    basic_address_iterator operator++(int) noexcept
    {
      auto __tmp = *this;
      ++*this;
      return __tmp;
    }

    basic_address_iterator& operator--() noexcept
    {
      _M_address = value_type(_M_address.to_uint() - 1);
      return *this;
    }

    basic_address_iterator
    operator--(int) noexcept
    {
      auto __tmp = *this;
      --*this;
      return __tmp;
    }

    bool
    operator==(const basic_address_iterator& __rhs) const noexcept
    { return _M_address == __rhs._M_address; }

    bool
    operator!=(const basic_address_iterator& __rhs) const noexcept
    { return _M_address != __rhs._M_address; }

  private:
    address_v4 _M_address;
  };

  using address_v4_iterator = basic_address_iterator<address_v4>;

  template<> class basic_address_iterator<address_v6>
  {
  public:
    // types:
    using value_type = address_v6;
    using difference_type = ptrdiff_t;
    using pointer = const address_v6*;
    using reference = const address_v6&;
    using iterator_category = input_iterator_tag;

    // constructors:
    basic_address_iterator(const address_v6& __a) noexcept
    : _M_address(__a) { }

    // members:
    reference operator*() const noexcept { return _M_address; }
    pointer operator->() const noexcept { return &_M_address; }

    basic_address_iterator&
    operator++() noexcept; // TODO

    basic_address_iterator
    operator++(int) noexcept
    {
      auto __tmp = *this;
      ++*this;
      return __tmp;
    }

    basic_address_iterator&
    operator--() noexcept; // TODO

    basic_address_iterator
    operator--(int) noexcept
    {
      auto __tmp = *this;
      --*this;
      return __tmp;
    }

    bool
    operator==(const basic_address_iterator& __rhs) const noexcept
    { return _M_address == __rhs._M_address; }

    bool
    operator!=(const basic_address_iterator& __rhs) const noexcept
    { return _M_address != __rhs._M_address; }

  private:
    address_v6 _M_address;
  };

  using address_v6_iterator = basic_address_iterator<address_v6>;

  template<typename> class basic_address_range; // not defined

  /** An IPv6 address range.
   * @{
   */

  template<> class basic_address_range<address_v4>
  {
  public:
    // types:

    using iterator = basic_address_iterator<address_v4>;

    // constructors:

    basic_address_range() noexcept : _M_begin({}), _M_end({}) { }

    basic_address_range(const address_v4& __first,
                        const address_v4& __last) noexcept
    : _M_begin(__first), _M_end(__last) { }

    // members:

    iterator begin() const noexcept { return _M_begin; }
    iterator end() const noexcept { return _M_end; }
    _GLIBCXX_NODISCARD bool empty() const noexcept { return _M_begin == _M_end; }

    size_t
    size() const noexcept { return _M_end->to_uint() - _M_begin->to_uint(); }

    iterator
    find(const address_v4& __addr) const noexcept
    {
      if (*_M_begin <= __addr && __addr < *_M_end)
	return iterator{__addr};
      return end();
    }

  private:
    iterator _M_begin;
    iterator _M_end;
  };

  using address_v4_range = basic_address_range<address_v4>;

  /// @}

  /** An IPv6 address range.
   * @{
   */

  template<> class basic_address_range<address_v6>
  {
  public:
    // types:

    using iterator = basic_address_iterator<address_v6>;

    // constructors:

    basic_address_range() noexcept : _M_begin({}), _M_end({}) { }
    basic_address_range(const address_v6& __first,
                        const address_v6& __last) noexcept
    : _M_begin(__first), _M_end(__last) { }

    // members:

    iterator begin() const noexcept { return _M_begin; }
    iterator end() const noexcept { return _M_end; }
    _GLIBCXX_NODISCARD bool empty() const noexcept { return _M_begin == _M_end; }

    iterator
    find(const address_v6& __addr) const noexcept
    {
      if (*_M_begin <= __addr && __addr < *_M_end)
	return iterator{__addr};
      return end();
    }

  private:
    iterator _M_begin;
    iterator _M_end;
  };

  using address_v6_range = basic_address_range<address_v6>;

  /// @}

  constexpr bool
  operator==(const network_v4& __a, const network_v4& __b) noexcept;

  constexpr bool
  operator==(const network_v6& __a, const network_v6& __b) noexcept;


  /// An IPv4 network address.
  class network_v4
  {
  public:
    // constructors:
    constexpr network_v4() noexcept : _M_addr(), _M_prefix_len(0) { }

    constexpr
    network_v4(const address_v4& __addr, int __prefix_len)
    : _M_addr(__addr), _M_prefix_len(__prefix_len)
    {
      if (_M_prefix_len < 0 || _M_prefix_len > 32)
	__throw_out_of_range("network_v4: invalid prefix length");
    }

    constexpr
    network_v4(const address_v4& __addr, const address_v4& __mask)
    : _M_addr(__addr), _M_prefix_len(__builtin_popcount(__mask.to_uint()))
    {
      if (_M_prefix_len != 0)
	{
	  address_v4::uint_type __mask_uint = __mask.to_uint();
	  if (__builtin_ctz(__mask_uint) != (32 - _M_prefix_len))
	    __throw_invalid_argument("network_v4: invalid mask");
	  if ((__mask_uint & 0x80000000) == 0)
	    __throw_invalid_argument("network_v4: invalid mask");
	}
    }

    // members:

    constexpr address_v4 address() const noexcept { return _M_addr; }
    constexpr int prefix_length() const noexcept { return _M_prefix_len; }

    constexpr address_v4
    netmask() const noexcept
    {
      address_v4 __m;
      if (_M_prefix_len)
	__m = address_v4(0xFFFFFFFFu << (32 - _M_prefix_len));
      return __m;
    }

    constexpr address_v4
    network() const noexcept
    { return address_v4{_M_addr.to_uint() & netmask().to_uint()}; }

    constexpr address_v4
    broadcast() const noexcept
    {
      auto __b = _M_addr.to_uint();
      if (_M_prefix_len < 32)
	__b |= 0xFFFFFFFFu >> _M_prefix_len;
      return address_v4{__b};
    }

    address_v4_range
    hosts() const noexcept
    {
      if (is_host())
	return { address(), *++address_v4_iterator(address()) };
      return { network(), broadcast() };
    }

    constexpr network_v4
    canonical() const noexcept
    { return network_v4(network(), prefix_length()); }

    constexpr bool is_host() const noexcept { return _M_prefix_len == 32; }

    constexpr bool
    is_subnet_of(const network_v4& __other) const noexcept
    {
      if (__other.prefix_length() < prefix_length())
	{
	  network_v4 __net(address(), __other.prefix_length());
	  return __net.canonical() == __other.canonical();
	}
      return false;
    }

    template<typename _Allocator = allocator<char>>
      __string_with<_Allocator>
      to_string(const _Allocator& __a = _Allocator()) const
      {
	auto __str = address().to_string(__a);
	const unsigned __addrlen = __str.length();
	const unsigned __preflenlen = _M_prefix_len >= 10 ? 2 : 1;
	__str.resize(__addrlen + 1 + __preflenlen);
	__str[__addrlen] = '/';
	std::__detail::__to_chars_10_impl(&__str.front() + __addrlen + 1,
					  __preflenlen,
					  (unsigned char)_M_prefix_len);
	return __str;
      }

  private:
    address_v4 _M_addr;
    int _M_prefix_len;
  };

  /// An IPv6 network address.
  class network_v6
  {
  public:
    // constructors:
    constexpr network_v6() noexcept : _M_addr(), _M_prefix_len(0) { }

    constexpr
    network_v6(const address_v6& __addr, int __prefix_len)
    : _M_addr(__addr), _M_prefix_len(__prefix_len)
    {
      if (_M_prefix_len < 0 || _M_prefix_len > 128)
	__throw_out_of_range("network_v6: invalid prefix length");
    }

    // members:
    constexpr address_v6 address() const noexcept { return _M_addr; }
    constexpr int prefix_length() const noexcept { return _M_prefix_len; }

    _GLIBCXX17_CONSTEXPR address_v6
    network() const noexcept
    {
      address_v6::bytes_type __bytes = _M_addr.to_bytes();
      int __nbytes = (_M_prefix_len + 7) / 8;
      for (int __n = __nbytes; __n < 16; ++__n)
	__bytes[__n] = 0;
      if (int __zbits = (__nbytes * 8) - _M_prefix_len)
	__bytes[__nbytes - 1] &= 0xFF << __zbits;
      return address_v6(__bytes, _M_addr.scope_id());
    }

    address_v6_range
    hosts() const noexcept
    {
      if (is_host())
	return { address(), *++address_v6_iterator(address()) };

      address_v6::bytes_type __bytes = _M_addr.to_bytes();
      int __nbytes = (_M_prefix_len + 7) / 8;
      for (int __n = __nbytes; __n < 16; ++__n)
	__bytes[__n] = 0xFF;
      if (int __bits = (__nbytes * 8) - _M_prefix_len)
	__bytes[__nbytes - 1] |= (1 << __bits) - 1;
      address_v6 __last(__bytes, _M_addr.scope_id());
      return { network(), *++address_v6_iterator(__last) };
    }

    _GLIBCXX17_CONSTEXPR network_v6
    canonical() const noexcept
    { return network_v6{network(), prefix_length()}; }

    constexpr bool is_host() const noexcept { return _M_prefix_len == 128; }

    constexpr bool
    is_subnet_of(const network_v6& __other) const noexcept
    {
      if (__other.prefix_length() < prefix_length())
	{
	  network_v6 __net(address(), __other.prefix_length());
	  return __net.canonical() == __other.canonical();
	}
      return false;
    }

    template<typename _Allocator = allocator<char>>
      __string_with<_Allocator>
      to_string(const _Allocator& __a = _Allocator()) const
      {
	return address().to_string(__a) + '/'
		 + std::to_string(prefix_length()).c_str();
      }

  private:
    address_v6 _M_addr;
    int _M_prefix_len;
  };


  /** ip::network_v4 comparisons
   * @{
   */

  constexpr bool
  operator==(const network_v4& __a, const network_v4& __b) noexcept
  {
    return __a.address() == __b.address()
      && __a.prefix_length() == __b.prefix_length();
  }

  constexpr bool
  operator!=(const network_v4& __a, const network_v4& __b) noexcept
  { return !(__a == __b); }

  /// @}

  /** ip::network_v6 comparisons
   * @{
   */

  constexpr bool
  operator==(const network_v6& __a, const network_v6& __b) noexcept
  {
    return __a.address() == __b.address()
      && __a.prefix_length() == __b.prefix_length();
  }

  constexpr bool
  operator!=(const network_v6& __a, const network_v6& __b) noexcept
  { return !(__a == __b); }

  /// @}

  /** ip::network_v4 creation
   * @{
   */

  inline network_v4
  make_network_v4(const address_v4& __a, int __prefix_len)
  { return network_v4{__a, __prefix_len}; }

  inline network_v4
  make_network_v4(const address_v4& __a, const address_v4& __mask)
  { return network_v4{ __a, __mask }; }

  network_v4 make_network_v4(const char*, error_code&) noexcept; // TODO

  inline network_v4
  make_network_v4(const char* __str)
  { return make_network_v4(__str, __throw_on_error{"make_network_v4"}); }

  network_v4 make_network_v4(const string&, error_code&) noexcept; // TODO

  inline network_v4
  make_network_v4(const string& __str)
  { return make_network_v4(__str, __throw_on_error{"make_network_v4"}); }

  network_v4 make_network_v4(string_view, error_code&) noexcept; // TODO

  inline network_v4
  make_network_v4(string_view __str)
  { return make_network_v4(__str, __throw_on_error{"make_network_v4"}); }

  /// @}

  /** ip::network_v6 creation
   * @{
   */

  inline network_v6
  make_network_v6(const address_v6& __a, int __prefix_len)
  { return network_v6{__a, __prefix_len}; }

  network_v6 make_network_v6(const char*, error_code&) noexcept; // TODO

  inline network_v6
  make_network_v6(const char* __str)
  { return make_network_v6(__str, __throw_on_error{"make_network_v6"}); }

  network_v6 make_network_v6(const string&, error_code&) noexcept; // TODO

  inline network_v6
  make_network_v6(const string& __str)
  { return make_network_v6(__str, __throw_on_error{"make_network_v6"}); }

  network_v6 make_network_v6(string_view, error_code&) noexcept; // TODO

  inline network_v6
  make_network_v6(string_view __str)
  { return make_network_v6(__str, __throw_on_error{"make_network_v6"}); }

  /// @}

  /// ip::network_v4 I/O
  template<typename _CharT, typename _Traits>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>& __os, const network_v4& __net)
    { return __os << __net.to_string(); }

  /// ip::network_v6 I/O
  template<typename _CharT, typename _Traits>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>& __os, const network_v6& __net)
    { return __os << __net.to_string(); }

#if defined IPPROTO_TCP || defined  IPPROTO_UDP
  /// An IP endpoint.
  template<typename _InternetProtocol>
    class basic_endpoint
    {
    public:
      // types:
      using protocol_type = _InternetProtocol;

      // constructors:

      _GLIBCXX20_CONSTEXPR
      basic_endpoint() noexcept : _M_data()
      {
	_M_data._M_v4.sin_family = protocol_type::v4().family();
	// If in_addr contains a union, make the correct member active:
	if (std::__is_constant_evaluated())
	  std::_Construct(&_M_data._M_v4.sin_addr.s_addr);
      }

      _GLIBCXX20_CONSTEXPR
      basic_endpoint(const protocol_type& __proto,
		     port_type __port_num) noexcept
      : _M_data()
      {
	if (__proto == protocol_type::v4())
	  {
	    _M_data._M_v4.sin_family = protocol_type::v4().family();
	    _M_data._M_v4.sin_port = address_v4::_S_hton_16(__port_num);
	    if (std::__is_constant_evaluated())
	      std::_Construct(&_M_data._M_v4.sin_addr.s_addr);
	  }
	else if (__proto == protocol_type::v6())
	  {
	    std::_Construct(&_M_data._M_v6);
	    _M_data._M_v6.sin6_family = __proto.family();
	    _M_data._M_v6.sin6_port = address_v4::_S_hton_16(__port_num);
	    _M_data._M_v6.sin6_scope_id = 0;
	    if (std::__is_constant_evaluated())
	      std::_Construct(&_M_data._M_v6.sin6_addr.s6_addr);
	  }
	else
	  {
	    __glibcxx_assert(__proto == protocol_type::v4()
			       || __proto == protocol_type::v6());

	  }
      }

      _GLIBCXX20_CONSTEXPR
      basic_endpoint(const ip::address& __addr,
		     port_type __port_num) noexcept
      : _M_data()
      {
	if (__addr.is_v4())
	  {
	    _M_data._M_v4.sin_family = protocol_type::v4().family();
	    _M_data._M_v4.sin_port = address_v4::_S_hton_16(__port_num);
	    std::_Construct(&_M_data._M_v4.sin_addr.s_addr,
			    __addr._M_v4._M_addr);
	  }
	else
	  {
	    std::_Construct(&_M_data._M_v6);
	    _M_data._M_v6.sin6_family = protocol_type::v6().family();
	    _M_data._M_v6.sin6_port = address_v4::_S_hton_16(__port_num);
	    if (std::__is_constant_evaluated())
	      std::_Construct(&_M_data._M_v6.sin6_addr.s6_addr);
	    uint8_t* __s6a = _M_data._M_v6.sin6_addr.s6_addr;
	    for (int __i = 0; __i < 16; ++__i)
	      __s6a[__i] = __addr._M_v6._M_bytes[__i];
	    _M_data._M_v6.sin6_scope_id = __addr._M_v6._M_scope_id;
	  }
      }

      // members:

      constexpr protocol_type protocol() const noexcept
      {
	return _M_is_v6() ? protocol_type::v6() : protocol_type::v4();
      }

      constexpr ip::address
      address() const noexcept
      {
	if (_M_is_v6())
	  {
	    address_v6 __v6;
	    const uint8_t* __s6a = _M_data._M_v6.sin6_addr.s6_addr;
	    for (int __i = 0; __i < 16; ++__i)
	      __v6._M_bytes[__i] = __s6a[__i];
	    __v6._M_scope_id = _M_data._M_v6.sin6_scope_id;
	    return __v6;
	  }
	else
	  {
	    address_v4 __v4;
	    __v4._M_addr = _M_data._M_v4.sin_addr.s_addr;
	    return __v4;
	  }
      }

      void
      address(const ip::address& __addr) noexcept
      {
	if (__addr.is_v6())
	  {
	    std::_Construct(&_M_data._M_v6);
	    _M_data._M_v6.sin6_family = protocol_type::v6().family();
	    __builtin_memcpy(_M_data._M_v6.sin6_addr.s6_addr,
			     __addr._M_v6._M_bytes.data(), 16);
	    _M_data._M_v6.sin6_scope_id = __addr._M_v6._M_scope_id;
	  }
	else
	  {
	    std::_Construct(&_M_data._M_v4);
	    _M_data._M_v4.sin_family = protocol_type::v4().family();
	    _M_data._M_v4.sin_addr.s_addr = __addr._M_v4._M_addr;
	  }
      }

      constexpr port_type
      port() const noexcept
      {
	port_type __p = 0;
	if (_M_is_v6())
	  __p = _M_data._M_v6.sin6_port;
	else
	  __p = _M_data._M_v4.sin_port;
	return address_v4::_S_ntoh_16(__p);
      }

      void
      port(port_type __port_num) noexcept
      {
	__port_num = address_v4::_S_hton_16(__port_num);
	if (_M_is_v6())
	  _M_data._M_v6.sin6_port = __port_num;
	else
	  _M_data._M_v4.sin_port = __port_num;
      }

      void* data() noexcept { return &_M_data; }

      const void* data() const noexcept { return &_M_data; }

      constexpr size_t
      size() const noexcept
      { return _M_is_v6() ? sizeof(sockaddr_in6) : sizeof(sockaddr_in); }

      void
      resize(size_t __s)
      {
	if (__s != size())
	  __throw_length_error("net::ip::basic_endpoint::resize");
      }

      constexpr size_t capacity() const noexcept { return sizeof(_M_data); }

    private:
      union
      {
	sockaddr_in	_M_v4;
	sockaddr_in6	_M_v6;
      } _M_data;

      constexpr bool
      _M_is_v6() const noexcept
      {
	// For constexpr eval we can just detect which union member is active.
	// i.e. emulate P2641R1's std::is_active_member(&_M_data._M_v6)).
	if (std::__is_constant_evaluated())
	  return __builtin_constant_p(_M_data._M_v6.sin6_family);
	return _M_data._M_v6.sin6_family == AF_INET6;
      }
    };

  /** basic_endpoint comparisons
   * @{
   */

  template<typename _InternetProtocol>
    constexpr bool
    operator==(const basic_endpoint<_InternetProtocol>& __a,
	       const basic_endpoint<_InternetProtocol>& __b)
    { return __a.address() == __b.address() && __a.port() == __b.port(); }

  template<typename _InternetProtocol>
    constexpr bool
    operator!=(const basic_endpoint<_InternetProtocol>& __a,
	       const basic_endpoint<_InternetProtocol>& __b)
    { return !(__a == __b); }

  template<typename _InternetProtocol>
    constexpr bool
    operator< (const basic_endpoint<_InternetProtocol>& __a,
	       const basic_endpoint<_InternetProtocol>& __b)
    {
      return __a.address() < __b.address()
	|| (!(__b.address() < __a.address()) && __a.port() < __b.port());
    }

  template<typename _InternetProtocol>
    constexpr bool
    operator> (const basic_endpoint<_InternetProtocol>& __a,
	       const basic_endpoint<_InternetProtocol>& __b)
    { return __b < __a; }

  template<typename _InternetProtocol>
    constexpr bool
    operator<=(const basic_endpoint<_InternetProtocol>& __a,
	       const basic_endpoint<_InternetProtocol>& __b)
    { return !(__b < __a); }

  template<typename _InternetProtocol>
    constexpr bool
    operator>=(const basic_endpoint<_InternetProtocol>& __a,
	       const basic_endpoint<_InternetProtocol>& __b)
    { return !(__a < __b); }

  /// @}

  /// basic_endpoint I/O
  template<typename _CharT, typename _Traits, typename _InternetProtocol>
    inline basic_ostream<_CharT, _Traits>&
    operator<<(basic_ostream<_CharT, _Traits>& __os,
	       const basic_endpoint<_InternetProtocol>& __ep)
    {
      basic_ostringstream<_CharT, _Traits> __ss;
      if (__ep.protocol()
	  == basic_endpoint<_InternetProtocol>::protocol_type::v6())
	__ss << '[' << __ep.address() << ']';
      else
	__ss << __ep.address();
      __ss << ':' << __ep.port();
      __os << __ss.str();
      return __os;
    }

  /** Type representing a single result of name/address resolution.
   * @{
   */

  template<typename _InternetProtocol>
    class basic_resolver_entry
    {
    public:
      // types:
      using protocol_type = _InternetProtocol;
      using endpoint_type = typename _InternetProtocol::endpoint;

      // constructors:
      basic_resolver_entry() { }

      basic_resolver_entry(const endpoint_type& __ep,
			   string_view __h, string_view __s)
      : _M_ep(__ep), _M_host(__h), _M_svc(__s) { }

      // members:
      endpoint_type endpoint() const { return _M_ep; }
      operator endpoint_type() const { return _M_ep; }

      template<typename _Allocator = allocator<char>>
	__string_with<_Allocator>
	host_name(const _Allocator& __a = _Allocator()) const
	{ return { _M_host, __a }; }

      template<typename _Allocator = allocator<char>>
	__string_with<_Allocator>
	service_name(const _Allocator& __a = _Allocator()) const
	{ return { _M_svc, __a }; }

    private:
      basic_endpoint<_InternetProtocol> _M_ep;
      string _M_host;
      string _M_svc;
    };

  template<typename _InternetProtocol>
    inline bool
    operator==(const basic_resolver_entry<_InternetProtocol>& __a,
	       const basic_resolver_entry<_InternetProtocol>& __b)
    {
      return __a.endpoint() == __b.endpoint()
	&& __a.host_name() == __b.host_name()
	&& __a.service_name() == __b.service_name();
    }

  template<typename _InternetProtocol>
    inline bool
    operator!=(const basic_resolver_entry<_InternetProtocol>& __a,
	       const basic_resolver_entry<_InternetProtocol>& __b)
    { return !(__a == __b); }

  /// @}

  /** Base class defining flags for name/address resolution.
   * @{
   */

  class resolver_base
  {
  public:
    enum flags : int { };
    static constexpr flags passive		= (flags)AI_PASSIVE;
    static constexpr flags canonical_name	= (flags)AI_CANONNAME;
    static constexpr flags numeric_host		= (flags)AI_NUMERICHOST;
#ifdef AI_NUMERICSERV
    static constexpr flags numeric_service	= (flags)AI_NUMERICSERV;
#endif
#ifdef AI_V4MAPPED
    static constexpr flags v4_mapped		= (flags)AI_V4MAPPED;
#endif
#ifdef AI_ALL
    static constexpr flags all_matching		= (flags)AI_ALL;
#endif
#ifdef AI_ADDRCONFIG
    static constexpr flags address_configured	= (flags)AI_ADDRCONFIG;
#endif

    friend constexpr flags
    operator&(flags __f1, flags __f2) noexcept
    { return flags( int(__f1) & int(__f2) ); }

    friend constexpr flags
    operator|(flags __f1, flags __f2) noexcept
    { return flags( int(__f1) | int(__f2) ); }

    friend constexpr flags
    operator^(flags __f1, flags __f2) noexcept
    { return flags( int(__f1) ^ int(__f2) ); }

    friend constexpr flags
    operator~(flags __f) noexcept
    { return flags( ~int(__f) ); }

    friend constexpr flags&
    operator&=(flags& __f1, flags __f2) noexcept
    { return __f1 = (__f1 & __f2); }

    friend constexpr flags&
    operator|=(flags& __f1, flags __f2) noexcept
    { return __f1 = (__f1 | __f2); }

    friend constexpr flags&
    operator^=(flags& __f1, flags __f2) noexcept
    { return __f1 = (__f1 ^ __f2); }

  protected:
    resolver_base() = default;
    ~resolver_base() = default;
  };

  // TODO define resolver_base::flags static constants in .so for C++14 mode

  /// @}

  /** Container for results of name/address resolution.
   * @{
   */

  template<typename _InternetProtocol>
    class basic_resolver_results
    {
    public:
      // types:
      using protocol_type = _InternetProtocol;
      using endpoint_type = typename protocol_type::endpoint;
      using value_type = basic_resolver_entry<protocol_type>;
      using const_reference = const value_type&;
      using reference = value_type&;
      using const_iterator = typename forward_list<value_type>::const_iterator;
      using iterator = const_iterator;
      using difference_type = ptrdiff_t;
      using size_type = size_t;

      // construct / copy / destroy:

      basic_resolver_results() = default;

      basic_resolver_results(const basic_resolver_results&) = default;

      basic_resolver_results(basic_resolver_results&&) noexcept = default;

      basic_resolver_results&
      operator=(const basic_resolver_results&) = default;

      basic_resolver_results&
      operator=(basic_resolver_results&&) = default;

      ~basic_resolver_results() = default;

      // size:
      size_type size() const noexcept { return _M_size; }
      size_type max_size() const noexcept { return _M_results.max_size(); }

      _GLIBCXX_NODISCARD bool
      empty() const noexcept { return _M_results.empty(); }

      // element access:
      const_iterator begin() const { return _M_results.begin(); }
      const_iterator end() const { return _M_results.end(); }
      const_iterator cbegin() const { return _M_results.begin(); }
      const_iterator cend() const { return _M_results.end(); }

      // swap:
      void
      swap(basic_resolver_results& __that) noexcept
      { _M_results.swap(__that._M_results); }

    private:
      friend class basic_resolver<protocol_type>;

      basic_resolver_results(string_view, string_view, resolver_base::flags,
			     error_code&, protocol_type* = nullptr);

      basic_resolver_results(const endpoint_type&, error_code&);

      forward_list<value_type> _M_results;
      size_t _M_size = 0;
    };

  template<typename _InternetProtocol>
    inline bool
    operator==(const basic_resolver_results<_InternetProtocol>& __a,
	       const basic_resolver_results<_InternetProtocol>& __b)
    {
      return __a.size() == __b.size()
	&& std::equal(__a.begin(), __a.end(), __b.begin());
    }

  template<typename _InternetProtocol>
    inline bool
    operator!=(const basic_resolver_results<_InternetProtocol>& __a,
	       const basic_resolver_results<_InternetProtocol>& __b)
    { return !(__a == __b); }

  /// @}

  /// Perform name/address resolution.
  template<typename _InternetProtocol>
    class basic_resolver : public resolver_base
    {
    public:
      // types:

      using executor_type = io_context::executor_type;
      using protocol_type = _InternetProtocol;
      using endpoint_type = typename _InternetProtocol::endpoint;
      using results_type = basic_resolver_results<_InternetProtocol>;

      // construct / copy / destroy:

      explicit basic_resolver(io_context& __ctx) : _M_ctx(&__ctx) { }

      basic_resolver(const basic_resolver&) = delete;

      basic_resolver(basic_resolver&& __rhs) noexcept
      : _M_ctx(__rhs._M_ctx)
      { } // TODO move state/tasks etc.

      ~basic_resolver() { cancel(); }

      basic_resolver& operator=(const basic_resolver&) = delete;

      basic_resolver& operator=(basic_resolver&& __rhs)
      {
	cancel();
	_M_ctx = __rhs._M_ctx;
	// TODO move state/tasks etc.
	return *this;
      }

      // basic_resolver operations:

      executor_type get_executor() noexcept { return _M_ctx->get_executor(); }

      void cancel() { } // TODO

      results_type
      resolve(string_view __host_name, string_view __service_name)
      {
	return resolve(__host_name, __service_name, resolver_base::flags(),
		       __throw_on_error{"basic_resolver::resolve"});
      }

      results_type
      resolve(string_view __host_name, string_view __service_name,
	      error_code& __ec)
      {
	return resolve(__host_name, __service_name, resolver_base::flags(),
		       __ec);
      }

      results_type
      resolve(string_view __host_name, string_view __service_name, flags __f)
      {
	return resolve(__host_name, __service_name, __f,
		       __throw_on_error{"basic_resolver::resolve"});
      }

      results_type
      resolve(string_view __host_name, string_view __service_name, flags __f,
	      error_code& __ec)
      { return {__host_name, __service_name, __f, __ec}; }

      template<typename _CompletionToken>
	__deduced_t<_CompletionToken, void(error_code, results_type)>
	async_resolve(string_view __host_name, string_view __service_name,
		      _CompletionToken&& __token)
	{
	  return async_resolve(__host_name, __service_name,
			       resolver_base::flags(),
			       forward<_CompletionToken>(__token));
	}

      template<typename _CompletionToken>
	__deduced_t<_CompletionToken, void(error_code, results_type)>
	async_resolve(string_view __host_name, string_view __service_name,
		      flags __f, _CompletionToken&& __token); // TODO

      results_type
      resolve(const protocol_type& __protocol,
	      string_view __host_name, string_view __service_name)
      {
	return resolve(__protocol, __host_name, __service_name,
		       resolver_base::flags(),
		       __throw_on_error{"basic_resolver::resolve"});
      }

      results_type
      resolve(const protocol_type& __protocol,
	      string_view __host_name, string_view __service_name,
	      error_code& __ec)
      {
	return resolve(__protocol, __host_name, __service_name,
		       resolver_base::flags(), __ec);
      }

      results_type
      resolve(const protocol_type& __protocol,
	      string_view __host_name, string_view __service_name, flags __f)
      {
	return resolve(__protocol, __host_name, __service_name, __f,
		       __throw_on_error{"basic_resolver::resolve"});
      }

      results_type
      resolve(const protocol_type& __protocol,
	      string_view __host_name, string_view __service_name,
	      flags __f, error_code& __ec)
      { return {__host_name, __service_name, __f, __ec, &__protocol}; }

      template<typename _CompletionToken>
	__deduced_t<_CompletionToken, void(error_code, results_type)>
	async_resolve(const protocol_type& __protocol,
		      string_view __host_name, string_view __service_name,
		      _CompletionToken&& __token)
	{
	  return async_resolve(__protocol, __host_name, __service_name,
			       resolver_base::flags(),
			       forward<_CompletionToken>(__token));
	}

      template<typename _CompletionToken>
	__deduced_t<_CompletionToken, void(error_code, results_type)>
	async_resolve(const protocol_type& __protocol,
		      string_view __host_name, string_view __service_name,
		      flags __f, _CompletionToken&& __token); // TODO

      results_type
      resolve(const endpoint_type& __ep)
      { return resolve(__ep, __throw_on_error{"basic_resolver::resolve"}); }

      results_type
      resolve(const endpoint_type& __ep, error_code& __ec)
      { return { __ep, __ec }; }

      template<typename _CompletionToken> // TODO
	__deduced_t<_CompletionToken, void(error_code, results_type)>
	async_resolve(const endpoint_type& __ep, _CompletionToken&& __token);

    private:
      io_context* _M_ctx;
    };

  /// Private constructor to synchronously resolve host and service names.
  template<typename _InternetProtocol>
    basic_resolver_results<_InternetProtocol>::
    basic_resolver_results(string_view __host_name, string_view __service_name,
			   resolver_base::flags __f, error_code& __ec,
			   protocol_type* __protocol)
    {
#ifdef _GLIBCXX_HAVE_NETDB_H
      string __host;
      const char* __h = __host_name.data()
	? (__host = __host_name.to_string()).c_str()
	: nullptr;
      string __svc;
      const char* __s = __service_name.data()
	? (__svc = __service_name.to_string()).c_str()
	: nullptr;

      ::addrinfo __hints{ };
      __hints.ai_flags = static_cast<int>(__f);
      if (__protocol)
	{
	  __hints.ai_family = __protocol->family();
	  __hints.ai_socktype = __protocol->type();
	  __hints.ai_protocol = __protocol->protocol();
	}
      else
	{
	  auto __p = endpoint_type{}.protocol();
	  __hints.ai_family = AF_UNSPEC;
	  __hints.ai_socktype = __p.type();
	  __hints.ai_protocol = __p.protocol();
	}

      struct __scoped_addrinfo
      {
	~__scoped_addrinfo() { if (_M_p) ::freeaddrinfo(_M_p); }
	::addrinfo* _M_p = nullptr;
      } __sai;

      if (int __err = ::getaddrinfo(__h, __s, &__hints, &__sai._M_p))
	{
	  __ec = ip::__make_resolver_error_code(__err, errno);
	  return;
	}
      __ec.clear();

      endpoint_type __ep;
      auto __tail = _M_results.before_begin();
      for (auto __ai = __sai._M_p; __ai != nullptr; __ai = __ai->ai_next)
	{
	  if (__ai->ai_family == AF_INET || __ai->ai_family == AF_INET6)
	    {
	      if (__ai->ai_addrlen <= __ep.capacity())
		__builtin_memcpy(__ep.data(), __ai->ai_addr, __ai->ai_addrlen);
	      __ep.resize(__ai->ai_addrlen);
	      __tail = _M_results.emplace_after(__tail, __ep, __host, __svc);
	      _M_size++;
	    }
	}
#else
      __ec = std::make_error_code(errc::operation_not_supported);
#endif
    }

  /// Private constructor to synchronously resolve an endpoint.
  template<typename _InternetProtocol>
    basic_resolver_results<_InternetProtocol>::
    basic_resolver_results(const endpoint_type& __ep, error_code& __ec)
    {
#ifdef _GLIBCXX_HAVE_NETDB_H
      char __host_name[1025];	// glibc NI_MAXHOST
      char __service_name[32];  // glibc NI_MAXSERV
      int __flags = 0;
      if (__ep.protocol().type() == SOCK_DGRAM)
	__flags |= NI_DGRAM;
      auto __sa = static_cast<const sockaddr*>(__ep.data());
      int __err = ::getnameinfo(__sa, __ep.size(),
				__host_name, sizeof(__host_name),
				__service_name, sizeof(__service_name),
				__flags);
      if (__err)
	{
	  __flags |= NI_NUMERICSERV;
	  __err = ::getnameinfo(__sa, __ep.size(),
				__host_name, sizeof(__host_name),
				__service_name, sizeof(__service_name),
				__flags);
	}
      if (__err)
	__ec = ip::__make_resolver_error_code(__err, errno);
      else
	{
	  __ec.clear();
	  _M_results.emplace_front(__ep, __host_name, __service_name);
	  _M_size = 1;
	}
#else
      __ec = std::make_error_code(errc::operation_not_supported);
#endif
    }
#endif // IPPROTO_TCP || IPPROTO_UDP

  /** The name of the local host.
   * @{
   */

  template<typename _Allocator>
    __string_with<_Allocator>
    host_name(const _Allocator& __a, error_code& __ec)
    {
#ifdef HOST_NAME_MAX
      constexpr size_t __maxlen = HOST_NAME_MAX;
#else
      constexpr size_t __maxlen = 256;
#endif
      char __buf[__maxlen + 1];
      if (::gethostname(__buf, __maxlen) == -1)
	__ec.assign(errno, generic_category());
      __buf[__maxlen] = '\0';
      return { __buf, __a };
    }

  template<typename _Allocator>
    inline __string_with<_Allocator>
    host_name(const _Allocator& __a)
    { return host_name(__a, __throw_on_error{"host_name"}); }

  inline string
  host_name(error_code& __ec)
  { return host_name(std::allocator<char>{}, __ec); }

  inline string
  host_name()
  { return host_name(std::allocator<char>{}, __throw_on_error{"host_name"}); }

  /// @}

#ifdef IPPROTO_TCP
  /// The TCP byte-stream protocol.
  class tcp
  {
  public:
    // types:
    using endpoint = basic_endpoint<tcp>;	 ///< A TCP endpoint.
    using resolver = basic_resolver<tcp>;	 ///< A TCP resolver.
    using socket = basic_stream_socket<tcp>;	 ///< A TCP socket.
    using acceptor = basic_socket_acceptor<tcp>; ///< A TCP acceptor.
    using iostream = basic_socket_iostream<tcp>; ///< A TCP iostream.

#ifdef TCP_NODELAY
    /// Disable coalescing of small segments (i.e. the Nagle algorithm).
    struct no_delay : __sockopt_crtp<no_delay, bool>
    {
      using __sockopt_crtp::__sockopt_crtp;
      using __sockopt_crtp::operator=;

      static const int _S_level = IPPROTO_TCP;
      static const int _S_name = TCP_NODELAY;
    };
#endif

    // static members:

    /// A protocol object representing IPv4 TCP.
    static constexpr tcp v4() noexcept { return tcp(AF_INET); }
    /// A protocol object representing IPv6 TCP.
    static constexpr tcp v6() noexcept { return tcp(AF_INET6); }

    tcp() = delete;

    constexpr int family() const noexcept { return _M_family; }
    constexpr int type() const noexcept { return SOCK_STREAM; }
    constexpr int protocol() const noexcept { return IPPROTO_TCP; }

  private:
    constexpr explicit tcp(int __family) : _M_family(__family) { }

    int _M_family;
  };

  /** tcp comparisons
   * @{
   */

  constexpr bool
  operator==(const tcp& __a, const tcp& __b) noexcept
  { return __a.family() == __b.family(); }

  constexpr bool
  operator!=(const tcp& __a, const tcp& __b) noexcept
  { return !(__a == __b); }

  /// @}
#endif // IPPROTO_TCP

#ifdef IPPROTO_UDP
  /// The UDP datagram protocol.
  class udp
  {
  public:
    // types:
    using endpoint = basic_endpoint<udp>;
    using resolver = basic_resolver<udp>;
    using socket = basic_datagram_socket<udp>;

    // static members:
    static constexpr udp v4() noexcept { return udp(AF_INET); }
    static constexpr udp v6() noexcept { return udp(AF_INET6); }

    udp() = delete;

    constexpr int family() const noexcept { return _M_family; }
    constexpr int type() const noexcept { return SOCK_DGRAM; }
    constexpr int protocol() const noexcept { return IPPROTO_UDP; }

  private:
    constexpr explicit udp(int __family) : _M_family(__family) { }

    int _M_family;
  };

  /** udp comparisons
   * @{
   */

  constexpr bool
  operator==(const udp& __a, const udp& __b) noexcept
  { return __a.family() == __b.family(); }

  constexpr bool
  operator!=(const udp& __a, const udp& __b) noexcept
  { return !(__a == __b); }

  /// @}
#endif // IPPROTO_UDP

#if defined IPPROTO_IP && defined IPPROTO_IPV6

  /// Restrict a socket created for an IPv6 protocol to IPv6 only.
  class v6_only : public __sockopt_crtp<v6_only, bool>
  {
  public:
    using __sockopt_crtp::__sockopt_crtp;
    using __sockopt_crtp::operator=;

  private:
    friend __sockopt_crtp<v6_only, bool>;
    static const int _S_level = IPPROTO_IPV6;
    static const int _S_name = IPV6_V6ONLY;
  };

  namespace unicast
  {
    /// Set the default number of hops (TTL) for outbound datagrams.
    class hops : public __sockopt_crtp<hops>
    {
    public:
      using __sockopt_crtp::__sockopt_crtp;
      using __sockopt_crtp::operator=;

      template<typename _Protocol>
	int
	level(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? IPPROTO_IPV6 : IPPROTO_IP; }

      template<typename _Protocol>
	int
	name(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? IPV6_UNICAST_HOPS : IP_TTL; }
    };
  } // namespace unicast

  namespace multicast
  {
    class __mcastopt
    {
    public:
      explicit
      __mcastopt(const address& __grp) noexcept
      : __mcastopt(__grp.is_v4() ? __mcastopt(__grp.to_v4()) : __mcastopt(__grp.to_v6()))
      { }

      explicit
      __mcastopt(const address_v4& __grp,
		 const address_v4& __iface = address_v4::any()) noexcept
      {
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
	_M_v4.imr_multiaddr.s_addr = __grp.to_uint();
	_M_v4.imr_interface.s_addr = __iface.to_uint();
#else
	_M_v4.imr_multiaddr.s_addr = __builtin_bswap32(__grp.to_uint());
	_M_v4.imr_interface.s_addr = __builtin_bswap32(__iface.to_uint());
#endif
      }

      explicit
      __mcastopt(const address_v6& __grp, unsigned int __iface = 0) noexcept
      {
	const auto __addr = __grp.to_bytes();
	__builtin_memcpy(_M_v6.ipv6mr_multiaddr.s6_addr, __addr.data(), 16);
	_M_v6.ipv6mr_interface = __iface;
      }

      template<typename _Protocol>
	int
	level(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? IPPROTO_IPV6 : IPPROTO_IP; }

      template<typename _Protocol>
	const void*
	data(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? &_M_v6 : &_M_v4; }

      template<typename _Protocol>
	size_t
	size(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? sizeof(_M_v6) : sizeof(_M_v4); }

    private:
      ipv6_mreq _M_v6 = {};
      ip_mreq _M_v4 = {};
    };

    /// Request that a socket joins a multicast group.
    class join_group : private __mcastopt
    {
    public:
      using __mcastopt::__mcastopt;
      using __mcastopt::level;
      using __mcastopt::data;
      using __mcastopt::size;

      template<typename _Protocol>
	int
	name(const _Protocol& __p) const noexcept
	{
	  if (__p.family() == AF_INET6)
	    return IPV6_JOIN_GROUP;
	  return IP_ADD_MEMBERSHIP;
	}
    };

    /// Request that a socket leaves a multicast group.
    class leave_group : private __mcastopt
    {
    public:
      using __mcastopt::__mcastopt;
      using __mcastopt::level;
      using __mcastopt::data;
      using __mcastopt::size;

      template<typename _Protocol>
	int
	name(const _Protocol& __p) const noexcept
	{
	  if (__p.family() == AF_INET6)
	    return IPV6_LEAVE_GROUP;
	  return IP_DROP_MEMBERSHIP;
	}
    };

    /// Specify the network interface for outgoing multicast datagrams.
    class outbound_interface
    {
    public:
      explicit
      outbound_interface(const address_v4& __v4) noexcept
      {
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
	_M_v4.s_addr = __v4.to_uint();
#else
	_M_v4.s_addr = __builtin_bswap32(__v4.to_uint());
#endif
      }

      explicit
      outbound_interface(unsigned int __v6) noexcept
      : _M_v4(), _M_v6(__v6)
      { }

      template<typename _Protocol>
	int
	level(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? IPPROTO_IPV6 : IPPROTO_IP; }

      template<typename _Protocol>
	int
	name(const _Protocol& __p) const noexcept
	{
	  return __p.family() == AF_INET6
	    ? IPV6_MULTICAST_IF : IP_MULTICAST_IF;
	}

      template<typename _Protocol>
	const void*
	data(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? &_M_v6 : &_M_v4; }

      template<typename _Protocol>
	size_t
	size(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? sizeof(_M_v6) : sizeof(_M_v4); }

    private:
      in_addr _M_v4;
      unsigned _M_v6 = 0;
    };

    /// Set the default number of hops (TTL) for outbound datagrams.
    class hops : public __sockopt_crtp<hops>
    {
    public:
      using __sockopt_crtp::__sockopt_crtp;
      using __sockopt_crtp::operator=;

      template<typename _Protocol>
	int
	level(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? IPPROTO_IPV6 : IPPROTO_IP; }

      template<typename _Protocol>
	int
	name(const _Protocol& __p) const noexcept
	{
	  return __p.family() == AF_INET6
	    ? IPV6_MULTICAST_HOPS : IP_MULTICAST_TTL;
	}
    };

    /// Set whether datagrams are delivered back to the local application.
    class enable_loopback : public __sockopt_crtp<enable_loopback, bool>
    {
    public:
      using __sockopt_crtp::__sockopt_crtp;
      using __sockopt_crtp::operator=;

      template<typename _Protocol>
	int
	level(const _Protocol& __p) const noexcept
	{ return __p.family() == AF_INET6 ? IPPROTO_IPV6 : IPPROTO_IP; }

      template<typename _Protocol>
	int
	name(const _Protocol& __p) const noexcept
	{
	  return __p.family() == AF_INET6
	    ? IPV6_MULTICAST_LOOP : IP_MULTICAST_LOOP;
	}
    };

  } // namespace multicast

#endif // IPPROTO_IP && IPPROTO_IPV6

  /// @}

} // namespace ip
} // namespace v1
} // namespace net
} // namespace experimental

  template<>
    struct is_error_condition_enum<experimental::net::v1::ip::resolver_errc>
    : public true_type {};

  // hash support
  template<typename _Tp> struct hash;
  template<>
    struct hash<experimental::net::v1::ip::address>
    : __hash_base<size_t, experimental::net::v1::ip::address>
    {
      size_t
      operator()(const experimental::net::v1::ip::address& __a) const
      {
	if (__a.is_v4())
	  return _Hash_impl::hash(__a.to_v4());
	else
	  return _Hash_impl::hash(__a.to_v6());
      }
    };

  template<>
    struct hash<experimental::net::v1::ip::address_v4>
    : __hash_base<size_t, experimental::net::v1::ip::address_v4>
    {
      size_t
      operator()(const experimental::net::v1::ip::address_v4& __a) const
      { return _Hash_impl::hash(__a.to_bytes()); }
    };

  template<> struct hash<experimental::net::v1::ip::address_v6>
    : __hash_base<size_t, experimental::net::v1::ip::address_v6>
    {
      size_t
      operator()(const experimental::net::v1::ip::address_v6& __a) const
      { return _Hash_impl::hash(__a.to_bytes()); }
    };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++14

#endif // _GLIBCXX_EXPERIMENTAL_INTERNET