relationship.hpp 21.1 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720
//
// execution/relationship.hpp
// ~~~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2020 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 ASIO_EXECUTION_RELATIONSHIP_HPP
#define ASIO_EXECUTION_RELATIONSHIP_HPP

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

#include "asio/detail/config.hpp"
#include "asio/detail/type_traits.hpp"
#include "asio/execution/executor.hpp"
#include "asio/execution/scheduler.hpp"
#include "asio/execution/sender.hpp"
#include "asio/is_applicable_property.hpp"
#include "asio/query.hpp"
#include "asio/traits/query_free.hpp"
#include "asio/traits/query_member.hpp"
#include "asio/traits/query_static_constexpr_member.hpp"
#include "asio/traits/static_query.hpp"
#include "asio/traits/static_require.hpp"

#include "asio/detail/push_options.hpp"

namespace asio {

#if defined(GENERATING_DOCUMENTATION)

namespace execution {

/// A property to describe whether submitted tasks represent continuations of
/// the calling context.
struct relationship_t
{
  /// The relationship_t property applies to executors, senders, and schedulers.
  template <typename T>
  static constexpr bool is_applicable_property_v =
    is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

  /// The top-level relationship_t property cannot be required.
  static constexpr bool is_requirable = false;

  /// The top-level relationship_t property cannot be preferred.
  static constexpr bool is_preferable = false;

  /// The type returned by queries against an @c any_executor.
  typedef relationship_t polymorphic_query_result_type;

  /// A sub-property that indicates that the executor does not represent a
  /// continuation of the calling context.
  struct fork_t
  {
    /// The relationship_t::fork_t property applies to executors, senders, and
    /// schedulers.
    template <typename T>
    static constexpr bool is_applicable_property_v =
      is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

    /// The relationship_t::fork_t property can be required.
    static constexpr bool is_requirable = true;

    /// The relationship_t::fork_t property can be preferred.
    static constexpr bool is_preferable = true;

    /// The type returned by queries against an @c any_executor.
    typedef relationship_t polymorphic_query_result_type;

    /// Default constructor.
    constexpr fork_t();

    /// Get the value associated with a property object.
    /**
     * @returns fork_t();
     */
    static constexpr relationship_t value();
  };

  /// A sub-property that indicates that the executor represents a continuation
  /// of the calling context.
  struct continuation_t
  {
    /// The relationship_t::continuation_t property applies to executors,
    /// senders, and schedulers.
    template <typename T>
    static constexpr bool is_applicable_property_v =
      is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

    /// The relationship_t::continuation_t property can be required.
    static constexpr bool is_requirable = true;

    /// The relationship_t::continuation_t property can be preferred.
    static constexpr bool is_preferable = true;

    /// The type returned by queries against an @c any_executor.
    typedef relationship_t polymorphic_query_result_type;

    /// Default constructor.
    constexpr continuation_t();

    /// Get the value associated with a property object.
    /**
     * @returns continuation_t();
     */
    static constexpr relationship_t value();
  };

  /// A special value used for accessing the relationship_t::fork_t property.
  static constexpr fork_t fork;

  /// A special value used for accessing the relationship_t::continuation_t
  /// property.
  static constexpr continuation_t continuation;

  /// Default constructor.
  constexpr relationship_t();

  /// Construct from a sub-property value.
  constexpr relationship_t(fork_t);

  /// Construct from a sub-property value.
  constexpr relationship_t(continuation_t);

  /// Compare property values for equality.
  friend constexpr bool operator==(
      const relationship_t& a, const relationship_t& b) noexcept;

  /// Compare property values for inequality.
  friend constexpr bool operator!=(
      const relationship_t& a, const relationship_t& b) noexcept;
};

/// A special value used for accessing the relationship_t property.
constexpr relationship_t relationship;

} // namespace execution

#else // defined(GENERATING_DOCUMENTATION)

namespace execution {
namespace detail {
namespace relationship {

template <int I> struct fork_t;
template <int I> struct continuation_t;

} // namespace relationship

template <int I = 0>
struct relationship_t
{
#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
  template <typename T>
  ASIO_STATIC_CONSTEXPR(bool,
    is_applicable_property_v = is_executor<T>::value
      || is_sender<T>::value || is_scheduler<T>::value);
#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

  ASIO_STATIC_CONSTEXPR(bool, is_requirable = false);
  ASIO_STATIC_CONSTEXPR(bool, is_preferable = false);
  typedef relationship_t polymorphic_query_result_type;

  typedef detail::relationship::fork_t<I> fork_t;
  typedef detail::relationship::continuation_t<I> continuation_t;

  ASIO_CONSTEXPR relationship_t()
    : value_(-1)
  {
  }

  ASIO_CONSTEXPR relationship_t(fork_t)
    : value_(0)
  {
  }

  ASIO_CONSTEXPR relationship_t(continuation_t)
    : value_(1)
  {
  }

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
  template <typename T>
  static ASIO_CONSTEXPR
  typename traits::query_static_constexpr_member<
      T, relationship_t>::result_type
  static_query()
    ASIO_NOEXCEPT_IF((
      traits::query_static_constexpr_member<
        T, relationship_t
      >::is_noexcept))
  {
    return traits::query_static_constexpr_member<
        T, relationship_t>::value();
  }

  template <typename T>
  static ASIO_CONSTEXPR
  typename traits::static_query<T, fork_t>::result_type
  static_query(
      typename enable_if<
        !traits::query_static_constexpr_member<
            T, relationship_t>::is_valid
          && !traits::query_member<T, relationship_t>::is_valid
          && traits::static_query<T, fork_t>::is_valid
      >::type* = 0) ASIO_NOEXCEPT
  {
    return traits::static_query<T, fork_t>::value();
  }

  template <typename T>
  static ASIO_CONSTEXPR
  typename traits::static_query<T, continuation_t>::result_type
  static_query(
      typename enable_if<
        !traits::query_static_constexpr_member<
            T, relationship_t>::is_valid
          && !traits::query_member<T, relationship_t>::is_valid
          && !traits::static_query<T, fork_t>::is_valid
          && traits::static_query<T, continuation_t>::is_valid
      >::type* = 0) ASIO_NOEXCEPT
  {
    return traits::static_query<T, continuation_t>::value();
  }

  template <typename E,
      typename T = decltype(relationship_t::static_query<E>())>
  static ASIO_CONSTEXPR const T static_query_v
    = relationship_t::static_query<E>();
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

  friend ASIO_CONSTEXPR bool operator==(
      const relationship_t& a, const relationship_t& b)
  {
    return a.value_ == b.value_;
  }

  friend ASIO_CONSTEXPR bool operator!=(
      const relationship_t& a, const relationship_t& b)
  {
    return a.value_ != b.value_;
  }

  struct convertible_from_relationship_t
  {
    ASIO_CONSTEXPR convertible_from_relationship_t(relationship_t)
    {
    }
  };

  template <typename Executor>
  friend ASIO_CONSTEXPR relationship_t query(
      const Executor& ex, convertible_from_relationship_t,
      typename enable_if<
        can_query<const Executor&, fork_t>::value
      >::type* = 0)
#if !defined(__clang__) // Clang crashes if noexcept is used here.
#if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
    ASIO_NOEXCEPT_IF((
      is_nothrow_query<const Executor&, relationship_t<>::fork_t>::value))
#else // defined(ASIO_MSVC)
    ASIO_NOEXCEPT_IF((
      is_nothrow_query<const Executor&, fork_t>::value))
#endif // defined(ASIO_MSVC)
#endif // !defined(__clang__)
  {
    return asio::query(ex, fork_t());
  }

  template <typename Executor>
  friend ASIO_CONSTEXPR relationship_t query(
      const Executor& ex, convertible_from_relationship_t,
      typename enable_if<
        !can_query<const Executor&, fork_t>::value
          && can_query<const Executor&, continuation_t>::value
      >::type* = 0)
#if !defined(__clang__) // Clang crashes if noexcept is used here.
#if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
    ASIO_NOEXCEPT_IF((
      is_nothrow_query<const Executor&,
        relationship_t<>::continuation_t>::value))
#else // defined(ASIO_MSVC)
    ASIO_NOEXCEPT_IF((
      is_nothrow_query<const Executor&, continuation_t>::value))
#endif // defined(ASIO_MSVC)
#endif // !defined(__clang__)
  {
    return asio::query(ex, continuation_t());
  }

  ASIO_STATIC_CONSTEXPR_DEFAULT_INIT(fork_t, fork);
  ASIO_STATIC_CONSTEXPR_DEFAULT_INIT(continuation_t, continuation);

#if !defined(ASIO_HAS_CONSTEXPR)
  static const relationship_t instance;
#endif // !defined(ASIO_HAS_CONSTEXPR)

private:
  int value_;
};

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
template <int I> template <typename E, typename T>
const T relationship_t<I>::static_query_v;
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

#if !defined(ASIO_HAS_CONSTEXPR)
template <int I>
const relationship_t<I> relationship_t<I>::instance;
#endif

template <int I>
const typename relationship_t<I>::fork_t
relationship_t<I>::fork;

template <int I>
const typename relationship_t<I>::continuation_t
relationship_t<I>::continuation;

namespace relationship {

template <int I = 0>
struct fork_t
{
#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
  template <typename T>
  ASIO_STATIC_CONSTEXPR(bool,
    is_applicable_property_v = is_executor<T>::value
      || is_sender<T>::value || is_scheduler<T>::value);
#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

  ASIO_STATIC_CONSTEXPR(bool, is_requirable = true);
  ASIO_STATIC_CONSTEXPR(bool, is_preferable = true);
  typedef relationship_t<I> polymorphic_query_result_type;

  ASIO_CONSTEXPR fork_t()
  {
  }

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
  template <typename T>
  static ASIO_CONSTEXPR
  typename traits::query_static_constexpr_member<T, fork_t>::result_type
  static_query()
    ASIO_NOEXCEPT_IF((
      traits::query_static_constexpr_member<T, fork_t>::is_noexcept))
  {
    return traits::query_static_constexpr_member<T, fork_t>::value();
  }

  template <typename T>
  static ASIO_CONSTEXPR fork_t static_query(
      typename enable_if<
        !traits::query_static_constexpr_member<T, fork_t>::is_valid
          && !traits::query_member<T, fork_t>::is_valid
          && !traits::query_free<T, fork_t>::is_valid
          && !can_query<T, continuation_t<I> >::value
      >::type* = 0) ASIO_NOEXCEPT
  {
    return fork_t();
  }

  template <typename E, typename T = decltype(fork_t::static_query<E>())>
  static ASIO_CONSTEXPR const T static_query_v
    = fork_t::static_query<E>();
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

  static ASIO_CONSTEXPR relationship_t<I> value()
  {
    return fork_t();
  }

  friend ASIO_CONSTEXPR bool operator==(
      const fork_t&, const fork_t&)
  {
    return true;
  }

  friend ASIO_CONSTEXPR bool operator!=(
      const fork_t&, const fork_t&)
  {
    return false;
  }
};

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
template <int I> template <typename E, typename T>
const T fork_t<I>::static_query_v;
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

template <int I = 0>
struct continuation_t
{
#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
  template <typename T>
  ASIO_STATIC_CONSTEXPR(bool,
    is_applicable_property_v = is_executor<T>::value
      || is_sender<T>::value || is_scheduler<T>::value);
#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)

  ASIO_STATIC_CONSTEXPR(bool, is_requirable = true);
  ASIO_STATIC_CONSTEXPR(bool, is_preferable = true);
  typedef relationship_t<I> polymorphic_query_result_type;

  ASIO_CONSTEXPR continuation_t()
  {
  }

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
  template <typename T>
  static ASIO_CONSTEXPR
  typename traits::query_static_constexpr_member<T, continuation_t>::result_type
  static_query()
    ASIO_NOEXCEPT_IF((
      traits::query_static_constexpr_member<T, continuation_t>::is_noexcept))
  {
    return traits::query_static_constexpr_member<T, continuation_t>::value();
  }

  template <typename E,
      typename T = decltype(continuation_t::static_query<E>())>
  static ASIO_CONSTEXPR const T static_query_v
    = continuation_t::static_query<E>();
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

  static ASIO_CONSTEXPR relationship_t<I> value()
  {
    return continuation_t();
  }

  friend ASIO_CONSTEXPR bool operator==(
      const continuation_t&, const continuation_t&)
  {
    return true;
  }

  friend ASIO_CONSTEXPR bool operator!=(
      const continuation_t&, const continuation_t&)
  {
    return false;
  }
};

#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
template <int I> template <typename E, typename T>
const T continuation_t<I>::static_query_v;
#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

} // namespace relationship
} // namespace detail

typedef detail::relationship_t<> relationship_t;

#if defined(ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
constexpr relationship_t relationship;
#else // defined(ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
namespace { static const relationship_t&
  relationship = relationship_t::instance; }
#endif

} // namespace execution

#if !defined(ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T>
struct is_applicable_property<T, execution::relationship_t>
  : integral_constant<bool,
      execution::is_executor<T>::value
        || execution::is_sender<T>::value
        || execution::is_scheduler<T>::value>
{
};

template <typename T>
struct is_applicable_property<T, execution::relationship_t::fork_t>
  : integral_constant<bool,
      execution::is_executor<T>::value
        || execution::is_sender<T>::value
        || execution::is_scheduler<T>::value>
{
};

template <typename T>
struct is_applicable_property<T, execution::relationship_t::continuation_t>
  : integral_constant<bool,
      execution::is_executor<T>::value
        || execution::is_sender<T>::value
        || execution::is_scheduler<T>::value>
{
};

#endif // !defined(ASIO_HAS_VARIABLE_TEMPLATES)

namespace traits {

#if !defined(ASIO_HAS_DEDUCED_QUERY_FREE_TRAIT)

template <typename T>
struct query_free_default<T, execution::relationship_t,
  typename enable_if<
    can_query<T, execution::relationship_t::fork_t>::value
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
    (is_nothrow_query<T, execution::relationship_t::fork_t>::value));

  typedef execution::relationship_t result_type;
};

template <typename T>
struct query_free_default<T, execution::relationship_t,
  typename enable_if<
    !can_query<T, execution::relationship_t::fork_t>::value
      && can_query<T, execution::relationship_t::continuation_t>::value
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
    (is_nothrow_query<T, execution::relationship_t::continuation_t>::value));

  typedef execution::relationship_t result_type;
};

#endif // !defined(ASIO_HAS_DEDUCED_QUERY_FREE_TRAIT)

#if !defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  || !defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

template <typename T>
struct static_query<T, execution::relationship_t,
  typename enable_if<
    traits::query_static_constexpr_member<T,
      execution::relationship_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

  typedef typename traits::query_static_constexpr_member<T,
    execution::relationship_t>::result_type result_type;

  static ASIO_CONSTEXPR result_type value()
  {
    return traits::query_static_constexpr_member<T,
      execution::relationship_t>::value();
  }
};

template <typename T>
struct static_query<T, execution::relationship_t,
  typename enable_if<
    !traits::query_static_constexpr_member<T,
        execution::relationship_t>::is_valid
      && !traits::query_member<T,
        execution::relationship_t>::is_valid
      && traits::static_query<T,
        execution::relationship_t::fork_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

  typedef typename traits::static_query<T,
    execution::relationship_t::fork_t>::result_type result_type;

  static ASIO_CONSTEXPR result_type value()
  {
    return traits::static_query<T,
        execution::relationship_t::fork_t>::value();
  }
};

template <typename T>
struct static_query<T, execution::relationship_t,
  typename enable_if<
    !traits::query_static_constexpr_member<T,
        execution::relationship_t>::is_valid
      && !traits::query_member<T,
        execution::relationship_t>::is_valid
      && !traits::static_query<T,
        execution::relationship_t::fork_t>::is_valid
      && traits::static_query<T,
        execution::relationship_t::continuation_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

  typedef typename traits::static_query<T,
    execution::relationship_t::continuation_t>::result_type result_type;

  static ASIO_CONSTEXPR result_type value()
  {
    return traits::static_query<T,
        execution::relationship_t::continuation_t>::value();
  }
};

template <typename T>
struct static_query<T, execution::relationship_t::fork_t,
  typename enable_if<
    traits::query_static_constexpr_member<T,
      execution::relationship_t::fork_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

  typedef typename traits::query_static_constexpr_member<T,
    execution::relationship_t::fork_t>::result_type result_type;

  static ASIO_CONSTEXPR result_type value()
  {
    return traits::query_static_constexpr_member<T,
      execution::relationship_t::fork_t>::value();
  }
};

template <typename T>
struct static_query<T, execution::relationship_t::fork_t,
  typename enable_if<
    !traits::query_static_constexpr_member<T,
        execution::relationship_t::fork_t>::is_valid
      && !traits::query_member<T,
        execution::relationship_t::fork_t>::is_valid
      && !traits::query_free<T,
        execution::relationship_t::fork_t>::is_valid
      && !can_query<T, execution::relationship_t::continuation_t>::value
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

  typedef execution::relationship_t::fork_t result_type;

  static ASIO_CONSTEXPR result_type value()
  {
    return result_type();
  }
};

template <typename T>
struct static_query<T, execution::relationship_t::continuation_t,
  typename enable_if<
    traits::query_static_constexpr_member<T,
      execution::relationship_t::continuation_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);

  typedef typename traits::query_static_constexpr_member<T,
    execution::relationship_t::continuation_t>::result_type result_type;

  static ASIO_CONSTEXPR result_type value()
  {
    return traits::query_static_constexpr_member<T,
      execution::relationship_t::continuation_t>::value();
  }
};

#endif // !defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   || !defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

#if !defined(ASIO_HAS_DEDUCED_STATIC_REQUIRE_TRAIT)

template <typename T>
struct static_require<T, execution::relationship_t::fork_t,
  typename enable_if<
    static_query<T, execution::relationship_t::fork_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid =
    (is_same<typename static_query<T,
      execution::relationship_t::fork_t>::result_type,
        execution::relationship_t::fork_t>::value));
};

template <typename T>
struct static_require<T, execution::relationship_t::continuation_t,
  typename enable_if<
    static_query<T, execution::relationship_t::continuation_t>::is_valid
  >::type>
{
  ASIO_STATIC_CONSTEXPR(bool, is_valid =
    (is_same<typename static_query<T,
      execution::relationship_t::continuation_t>::result_type,
        execution::relationship_t::continuation_t>::value));
};

#endif // !defined(ASIO_HAS_DEDUCED_STATIC_REQUIRE_TRAIT)

} // namespace traits

#endif // defined(GENERATING_DOCUMENTATION)

} // namespace asio

#include "asio/detail/pop_options.hpp"

#endif // ASIO_EXECUTION_RELATIONSHIP_HPP