//
// prefer.hpp
// ~~~~~~~~~~
//
// Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// 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)
//

#ifndef BOOST_ASIO_PREFER_HPP
#define BOOST_ASIO_PREFER_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include <boost/asio/detail/config.hpp>
#include <boost/asio/detail/type_traits.hpp>
#include <boost/asio/is_applicable_property.hpp>
#include <boost/asio/traits/prefer_free.hpp>
#include <boost/asio/traits/prefer_member.hpp>
#include <boost/asio/traits/require_free.hpp>
#include <boost/asio/traits/require_member.hpp>
#include <boost/asio/traits/static_require.hpp>

#include <boost/asio/detail/push_options.hpp>

#if defined(GENERATING_DOCUMENTATION)

namespace boost {
namespace asio {

/// A customisation point that attempts to apply a property to an object.
/**
 * The name <tt>prefer</tt> denotes a customisation point object. The
 * expression <tt>boost::asio::prefer(E, P0, Pn...)</tt> for some subexpressions
 * <tt>E</tt> and <tt>P0</tt>, and where <tt>Pn...</tt> represents <tt>N</tt>
 * subexpressions (where <tt>N</tt> is 0 or more, and with types <tt>T =
 * decay_t<decltype(E)></tt> and <tt>Prop0 = decay_t<decltype(P0)></tt>) is
 * expression-equivalent to:
 *
 * @li If <tt>is_applicable_property_v<T, Prop0> && Prop0::is_preferable</tt> is
 *   not a well-formed constant expression with value <tt>true</tt>,
 *   <tt>boost::asio::prefer(E, P0, Pn...)</tt> is ill-formed.
 *
 * @li Otherwise, <tt>E</tt> if <tt>N == 0</tt> and the expression
 *   <tt>Prop0::template static_query_v<T> == Prop0::value()</tt> is a
 *   well-formed constant expression with value <tt>true</tt>.
 *
 * @li Otherwise, <tt>(E).require(P0)</tt> if <tt>N == 0</tt> and the expression
 *   <tt>(E).require(P0)</tt> is a valid expression.
 *
 * @li Otherwise, <tt>require(E, P0)</tt> if <tt>N == 0</tt> and the expression
 *   <tt>require(E, P0)</tt> is a valid expression with overload resolution
 *   performed in a context that does not include the declaration of the
 *   <tt>require</tt> customization point object.
 *
 * @li Otherwise, <tt>(E).prefer(P0)</tt> if <tt>N == 0</tt> and the expression
 *   <tt>(E).prefer(P0)</tt> is a valid expression.
 *
 * @li Otherwise, <tt>prefer(E, P0)</tt> if <tt>N == 0</tt> and the expression
 *   <tt>prefer(E, P0)</tt> is a valid expression with overload resolution
 *   performed in a context that does not include the declaration of the
 *   <tt>prefer</tt> customization point object.
 *
 * @li Otherwise, <tt>E</tt> if <tt>N == 0</tt>.
 *
 * @li Otherwise,
 *   <tt>boost::asio::prefer(boost::asio::prefer(E, P0), Pn...)</tt>
 *   if <tt>N > 0</tt> and the expression
 *   <tt>boost::asio::prefer(boost::asio::prefer(E, P0), Pn...)</tt>
 *   is a valid expression.
 *
 * @li Otherwise, <tt>boost::asio::prefer(E, P0, Pn...)</tt> is ill-formed.
 */
inline constexpr unspecified prefer = unspecified;

/// A type trait that determines whether a @c prefer expression is well-formed.
/**
 * Class template @c can_prefer is a trait that is derived from
 * @c true_type if the expression <tt>boost::asio::prefer(std::declval<T>(),
 * std::declval<Properties>()...)</tt> is well formed; otherwise @c false_type.
 */
template <typename T, typename... Properties>
struct can_prefer :
  integral_constant<bool, automatically_determined>
{
};

/// A type trait that determines whether a @c prefer expression will not throw.
/**
 * Class template @c is_nothrow_prefer is a trait that is derived from
 * @c true_type if the expression <tt>boost::asio::prefer(std::declval<T>(),
 * std::declval<Properties>()...)</tt> is @c noexcept; otherwise @c false_type.
 */
template <typename T, typename... Properties>
struct is_nothrow_prefer :
  integral_constant<bool, automatically_determined>
{
};

/// A type trait that determines the result type of a @c prefer expression.
/**
 * Class template @c prefer_result is a trait that determines the result
 * type of the expression <tt>boost::asio::prefer(std::declval<T>(),
 * std::declval<Properties>()...)</tt>.
 */
template <typename T, typename... Properties>
struct prefer_result
{
  /// The result of the @c prefer expression.
  typedef automatically_determined type;
};

} // namespace asio
} // namespace boost

#else // defined(GENERATING_DOCUMENTATION)

namespace boost_asio_prefer_fn {

using boost::asio::conditional;
using boost::asio::decay;
using boost::asio::declval;
using boost::asio::enable_if;
using boost::asio::is_applicable_property;
using boost::asio::traits::prefer_free;
using boost::asio::traits::prefer_member;
using boost::asio::traits::require_free;
using boost::asio::traits::require_member;
using boost::asio::traits::static_require;

void prefer();
void require();

enum overload_type
{
  identity,
  call_require_member,
  call_require_free,
  call_prefer_member,
  call_prefer_free,
  two_props,
  n_props,
  ill_formed
};

template <typename Impl, typename T, typename Properties,
    typename = void, typename = void, typename = void, typename = void,
    typename = void, typename = void, typename = void>
struct call_traits
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_noexcept = false);
  typedef void result_type;
};

template <typename Impl, typename T, typename Property>
struct call_traits<Impl, T, void(Property),
  typename enable_if<
    is_applicable_property<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value
  >::type,
  typename enable_if<
    decay<Property>::type::is_preferable
  >::type,
  typename enable_if<
    static_require<T, Property>::is_valid
  >::type>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

#if defined(BOOST_ASIO_HAS_MOVE)
  typedef BOOST_ASIO_MOVE_ARG(T) result_type;
#else // defined(BOOST_ASIO_HAS_MOVE)
  typedef BOOST_ASIO_MOVE_ARG(typename decay<T>::type) result_type;
#endif // defined(BOOST_ASIO_HAS_MOVE)
};

template <typename Impl, typename T, typename Property>
struct call_traits<Impl, T, void(Property),
  typename enable_if<
    is_applicable_property<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value
  >::type,
  typename enable_if<
    decay<Property>::type::is_preferable
  >::type,
  typename enable_if<
    !static_require<T, Property>::is_valid
  >::type,
  typename enable_if<
    require_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type> :
  require_member<typename Impl::template proxy<T>::type, Property>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = call_require_member);
};

template <typename Impl, typename T, typename Property>
struct call_traits<Impl, T, void(Property),
  typename enable_if<
    is_applicable_property<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value
  >::type,
  typename enable_if<
    decay<Property>::type::is_preferable
  >::type,
  typename enable_if<
    !static_require<T, Property>::is_valid
  >::type,
  typename enable_if<
    !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type,
  typename enable_if<
    require_free<T, Property>::is_valid
  >::type> :
  require_free<T, Property>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = call_require_free);
};

template <typename Impl, typename T, typename Property>
struct call_traits<Impl, T, void(Property),
  typename enable_if<
    is_applicable_property<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value
  >::type,
  typename enable_if<
    decay<Property>::type::is_preferable
  >::type,
  typename enable_if<
    !static_require<T, Property>::is_valid
  >::type,
  typename enable_if<
    !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type,
  typename enable_if<
    !require_free<T, Property>::is_valid
  >::type,
  typename enable_if<
    prefer_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type> :
  prefer_member<typename Impl::template proxy<T>::type, Property>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = call_prefer_member);
};

template <typename Impl, typename T, typename Property>
struct call_traits<Impl, T, void(Property),
  typename enable_if<
    is_applicable_property<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value
  >::type,
  typename enable_if<
    decay<Property>::type::is_preferable
  >::type,
  typename enable_if<
    !static_require<T, Property>::is_valid
  >::type,
  typename enable_if<
    !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type,
  typename enable_if<
    !require_free<T, Property>::is_valid
  >::type,
  typename enable_if<
    !prefer_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type,
  typename enable_if<
    prefer_free<T, Property>::is_valid
  >::type> :
  prefer_free<T, Property>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = call_prefer_free);
};

template <typename Impl, typename T, typename Property>
struct call_traits<Impl, T, void(Property),
  typename enable_if<
    is_applicable_property<
      typename decay<T>::type,
      typename decay<Property>::type
    >::value
  >::type,
  typename enable_if<
    decay<Property>::type::is_preferable
  >::type,
  typename enable_if<
    !static_require<T, Property>::is_valid
  >::type,
  typename enable_if<
    !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type,
  typename enable_if<
    !require_free<T, Property>::is_valid
  >::type,
  typename enable_if<
    !prefer_member<typename Impl::template proxy<T>::type, Property>::is_valid
  >::type,
  typename enable_if<
    !prefer_free<T, Property>::is_valid
  >::type>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

#if defined(BOOST_ASIO_HAS_MOVE)
  typedef BOOST_ASIO_MOVE_ARG(T) result_type;
#else // defined(BOOST_ASIO_HAS_MOVE)
  typedef BOOST_ASIO_MOVE_ARG(typename decay<T>::type) result_type;
#endif // defined(BOOST_ASIO_HAS_MOVE)
};

template <typename Impl, typename T, typename P0, typename P1>
struct call_traits<Impl, T, void(P0, P1),
  typename enable_if<
    call_traits<Impl, T, void(P0)>::overload != ill_formed
  >::type,
  typename enable_if<
    call_traits<
      Impl,
      typename call_traits<Impl, T, void(P0)>::result_type,
      void(P1)
    >::overload != ill_formed
  >::type>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = two_props);

  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
    (
      call_traits<Impl, T, void(P0)>::is_noexcept
      &&
      call_traits<
        Impl,
        typename call_traits<Impl, T, void(P0)>::result_type,
        void(P1)
      >::is_noexcept
    ));

  typedef typename decay<
    typename call_traits<
      Impl,
      typename call_traits<Impl, T, void(P0)>::result_type,
      void(P1)
    >::result_type
  >::type result_type;
};

template <typename Impl, typename T, typename P0,
    typename P1, typename BOOST_ASIO_ELLIPSIS PN>
struct call_traits<Impl, T, void(P0, P1, PN BOOST_ASIO_ELLIPSIS),
  typename enable_if<
    call_traits<Impl, T, void(P0)>::overload != ill_formed
  >::type,
  typename enable_if<
    call_traits<
      Impl,
      typename call_traits<Impl, T, void(P0)>::result_type,
      void(P1, PN BOOST_ASIO_ELLIPSIS)
    >::overload != ill_formed
  >::type>
{
  BOOST_ASIO_STATIC_CONSTEXPR(overload_type, overload = n_props);

  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
    (
      call_traits<Impl, T, void(P0)>::is_noexcept
      &&
      call_traits<
        Impl,
        typename call_traits<Impl, T, void(P0)>::result_type,
        void(P1, PN BOOST_ASIO_ELLIPSIS)
      >::is_noexcept
    ));

  typedef typename decay<
    typename call_traits<
      Impl,
      typename call_traits<Impl, T, void(P0)>::result_type,
      void(P1, PN BOOST_ASIO_ELLIPSIS)
    >::result_type
  >::type result_type;
};

struct impl
{
  template <typename T>
  struct proxy
  {
#if defined(BOOST_ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT) \
  && defined(BOOST_ASIO_HAS_DEDUCED_PREFER_MEMBER_TRAIT)
    struct type
    {
      template <typename P>
      auto require(BOOST_ASIO_MOVE_ARG(P) p)
        noexcept(
          noexcept(
            declval<typename conditional<true, T, P>::type>().require(
              BOOST_ASIO_MOVE_CAST(P)(p))
          )
        )
        -> decltype(
          declval<typename conditional<true, T, P>::type>().require(
            BOOST_ASIO_MOVE_CAST(P)(p))
        );

      template <typename P>
      auto prefer(BOOST_ASIO_MOVE_ARG(P) p)
        noexcept(
          noexcept(
            declval<typename conditional<true, T, P>::type>().prefer(
              BOOST_ASIO_MOVE_CAST(P)(p))
          )
        )
        -> decltype(
          declval<typename conditional<true, T, P>::type>().prefer(
            BOOST_ASIO_MOVE_CAST(P)(p))
        );
    };
#else // defined(BOOST_ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT)
      //   && defined(BOOST_ASIO_HAS_DEDUCED_PREFER_MEMBER_TRAIT)
    typedef T type;
#endif // defined(BOOST_ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT)
       //   && defined(BOOST_ASIO_HAS_DEDUCED_PREFER_MEMBER_TRAIT)
  };

  template <typename T, typename Property>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T, void(Property)>::overload == identity,
    typename call_traits<impl, T, void(Property)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(Property)) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(Property)>::is_noexcept))
  {
    return BOOST_ASIO_MOVE_CAST(T)(t);
  }

  template <typename T, typename Property>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T, void(Property)>::overload == call_require_member,
    typename call_traits<impl, T, void(Property)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(Property) p) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(Property)>::is_noexcept))
  {
    return BOOST_ASIO_MOVE_CAST(T)(t).require(
        BOOST_ASIO_MOVE_CAST(Property)(p));
  }

  template <typename T, typename Property>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T, void(Property)>::overload == call_require_free,
    typename call_traits<impl, T, void(Property)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(Property) p) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(Property)>::is_noexcept))
  {
    return require(
        BOOST_ASIO_MOVE_CAST(T)(t),
        BOOST_ASIO_MOVE_CAST(Property)(p));
  }

  template <typename T, typename Property>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T, void(Property)>::overload == call_prefer_member,
    typename call_traits<impl, T, void(Property)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(Property) p) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(Property)>::is_noexcept))
  {
    return BOOST_ASIO_MOVE_CAST(T)(t).prefer(
        BOOST_ASIO_MOVE_CAST(Property)(p));
  }

  template <typename T, typename Property>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T, void(Property)>::overload == call_prefer_free,
    typename call_traits<impl, T, void(Property)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(Property) p) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(Property)>::is_noexcept))
  {
    return prefer(
        BOOST_ASIO_MOVE_CAST(T)(t),
        BOOST_ASIO_MOVE_CAST(Property)(p));
  }

  template <typename T, typename P0, typename P1>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T, void(P0, P1)>::overload == two_props,
    typename call_traits<impl, T, void(P0, P1)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(P0) p0,
      BOOST_ASIO_MOVE_ARG(P1) p1) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(P0, P1)>::is_noexcept))
  {
    return (*this)(
        (*this)(
          BOOST_ASIO_MOVE_CAST(T)(t),
          BOOST_ASIO_MOVE_CAST(P0)(p0)),
        BOOST_ASIO_MOVE_CAST(P1)(p1));
  }

  template <typename T, typename P0, typename P1,
    typename BOOST_ASIO_ELLIPSIS PN>
  BOOST_ASIO_NODISCARD BOOST_ASIO_CONSTEXPR typename enable_if<
    call_traits<impl, T,
      void(P0, P1, PN BOOST_ASIO_ELLIPSIS)>::overload == n_props,
    typename call_traits<impl, T,
      void(P0, P1, PN BOOST_ASIO_ELLIPSIS)>::result_type
  >::type
  operator()(
      BOOST_ASIO_MOVE_ARG(T) t,
      BOOST_ASIO_MOVE_ARG(P0) p0,
      BOOST_ASIO_MOVE_ARG(P1) p1,
      BOOST_ASIO_MOVE_ARG(PN) BOOST_ASIO_ELLIPSIS pn) const
    BOOST_ASIO_NOEXCEPT_IF((
      call_traits<impl, T, void(P0, P1, PN BOOST_ASIO_ELLIPSIS)>::is_noexcept))
  {
    return (*this)(
        (*this)(
          BOOST_ASIO_MOVE_CAST(T)(t),
          BOOST_ASIO_MOVE_CAST(P0)(p0)),
        BOOST_ASIO_MOVE_CAST(P1)(p1),
        BOOST_ASIO_MOVE_CAST(PN)(pn) BOOST_ASIO_ELLIPSIS);
  }
};

template <typename T = impl>
struct static_instance
{
  static const T instance;
};

template <typename T>
const T static_instance<T>::instance = {};

} // namespace boost_asio_prefer_fn
namespace boost {
namespace asio {
namespace {

static BOOST_ASIO_CONSTEXPR const boost_asio_prefer_fn::impl&
  prefer = boost_asio_prefer_fn::static_instance<>::instance;

} // namespace

typedef boost_asio_prefer_fn::impl prefer_t;

#if defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

template <typename T, typename... Properties>
struct can_prefer :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(Properties...)>::overload
        != boost_asio_prefer_fn::ill_formed>
{
};

#else // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

template <typename T, typename P0 = void,
    typename P1 = void, typename P2 = void>
struct can_prefer :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0, P1, P2)>::overload
        != boost_asio_prefer_fn::ill_formed>
{
};

template <typename T, typename P0, typename P1>
struct can_prefer<T, P0, P1> :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0, P1)>::overload
        != boost_asio_prefer_fn::ill_formed>
{
};

template <typename T, typename P0>
struct can_prefer<T, P0> :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0)>::overload
        != boost_asio_prefer_fn::ill_formed>
{
};

template <typename T>
struct can_prefer<T> :
  false_type
{
};

#endif // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

#if defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename BOOST_ASIO_ELLIPSIS Properties>
constexpr bool can_prefer_v
  = can_prefer<T, Properties BOOST_ASIO_ELLIPSIS>::value;

#endif // defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

#if defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

template <typename T, typename... Properties>
struct is_nothrow_prefer :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(Properties...)>::is_noexcept>
{
};

#else // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

template <typename T, typename P0 = void,
    typename P1 = void, typename P2 = void>
struct is_nothrow_prefer :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0, P1, P2)>::is_noexcept>
{
};

template <typename T, typename P0, typename P1>
struct is_nothrow_prefer<T, P0, P1> :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0, P1)>::is_noexcept>
{
};

template <typename T, typename P0>
struct is_nothrow_prefer<T, P0> :
  integral_constant<bool,
    boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0)>::is_noexcept>
{
};

template <typename T>
struct is_nothrow_prefer<T> :
  false_type
{
};

#endif // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

#if defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename BOOST_ASIO_ELLIPSIS Properties>
constexpr bool is_nothrow_prefer_v
  = is_nothrow_prefer<T, Properties BOOST_ASIO_ELLIPSIS>::value;

#endif // defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

#if defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

template <typename T, typename... Properties>
struct prefer_result
{
  typedef typename boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(Properties...)>::result_type type;
};

#else // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

template <typename T, typename P0 = void,
    typename P1 = void, typename P2 = void>
struct prefer_result
{
  typedef typename boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0, P1, P2)>::result_type type;
};

template <typename T, typename P0, typename P1>
struct prefer_result<T, P0, P1>
{
  typedef typename boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0, P1)>::result_type type;
};

template <typename T, typename P0>
struct prefer_result<T, P0>
{
  typedef typename boost_asio_prefer_fn::call_traits<
      prefer_t, T, void(P0)>::result_type type;
};

template <typename T>
struct prefer_result<T>
{
};

#endif // defined(BOOST_ASIO_HAS_VARIADIC_TEMPLATES)

} // namespace asio
} // namespace boost

#endif // defined(GENERATING_DOCUMENTATION)

#include <boost/asio/detail/pop_options.hpp>

#endif // BOOST_ASIO_PREFER_HPP