Program Listing for File aggregate.hpp

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  SEQUANT_DESIGNATED_INIT_ONLY;
  bool unitary = false;
  bool use_connected_form = false;
};
//
// Created by Ajay Melekamburath on 8/4/26.
//

#ifndef SEQUANT_CORE_UTILITY_AGGREGATE_HPP
#define SEQUANT_CORE_UTILITY_AGGREGATE_HPP

#include <compare>
#include <type_traits>

namespace sequant::detail {

class designated_init_only {
 public:
  constexpr designated_init_only(const designated_init_only&) noexcept =
      default;
  constexpr designated_init_only& operator=(
      const designated_init_only&) noexcept = default;
  static constexpr designated_init_only make() noexcept { return {}; }

  friend constexpr bool operator==(designated_init_only,
                                   designated_init_only) noexcept {
    return true;
  }
  friend constexpr std::strong_ordering operator<=>(
      designated_init_only, designated_init_only) noexcept {
    return std::strong_ordering::equal;
  }

 private:
  constexpr designated_init_only() noexcept = default;
};

}  // namespace sequant::detail

// clang-format off
// clang-format on
#define SEQUANT_DESIGNATED_INIT_ONLY                            \
  [[no_unique_address]] ::sequant::detail::designated_init_only \
      sequant_designated_init_only_ =                           \
          ::sequant::detail::designated_init_only::make()

namespace sequant::detail {

struct designated_init_only_probe {
  SEQUANT_DESIGNATED_INIT_ONLY;
  bool a = false;
  bool b = false;
};
static_assert(std::is_aggregate_v<designated_init_only_probe>);
static_assert(std::is_trivially_copyable_v<designated_init_only_probe>);
// [[no_unique_address]] elision is permitted, not required, and MSVC ignores
// the standard spelling outright (it has [[msvc::no_unique_address]], since
// honoring this one would change its ABI). A toolchain that does not elide
// still gets a working guard -- the tag merely costs a byte -- so check the
// size only where elision is guaranteed rather than failing the build over it.
#if !defined(_MSC_VER) || defined(__clang__)
static_assert(sizeof(designated_init_only_probe) == 2 * sizeof(bool),
              "[[no_unique_address]] must elide the tag member");
#endif
static_assert([] {
  designated_init_only_probe p{};
  p = designated_init_only_probe{.a = true};
  return p.a;
}());

}  // namespace sequant::detail

#endif  // SEQUANT_CORE_UTILITY_AGGREGATE_HPP