abseil-cpp/absl/types/internal/variant.h

// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Implementation details of absl/types/variant.h, pulled into a
// separate file to avoid cluttering the top of the API header with
// implementation details.

#ifndef ABSL_TYPES_variant_internal_H_
#define ABSL_TYPES_variant_internal_H_

#include <cassert>
#include <cstddef>
#include <cstdlib>
#include <memory>
#include <stdexcept>
#include <tuple>
#include <type_traits>

#include "absl/base/config.h"
#include "absl/base/internal/identity.h"
#include "absl/base/internal/inline_variable.h"
#include "absl/base/internal/invoke.h"
#include "absl/base/macros.h"
#include "absl/base/optimization.h"
#include "absl/meta/type_traits.h"
#include "absl/types/bad_variant_access.h"
#include "absl/utility/utility.h"

#if !defined(ABSL_USES_STD_VARIANT)

namespace absl {
ABSL_NAMESPACE_BEGIN

template <class... Types>
class variant;

ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);

template <class T>
struct variant_size;

template <std::size_t I, class T>
struct variant_alternative;

namespace variant_internal {

// NOTE: See specializations below for details.
template <std::size_t I, class T>
struct VariantAlternativeSfinae {};

// Requires: I < variant_size_v<T>.
//
// Value: The Ith type of Types...
template <std::size_t I, class T0, class... Tn>
struct VariantAlternativeSfinae<I, variant<T0, Tn...>>
    : VariantAlternativeSfinae<I - 1, variant<Tn...>> {};

// Value: T0
template <class T0, class... Ts>
struct VariantAlternativeSfinae<0, variant<T0, Ts...>> {
  using type = T0;
};

template <std::size_t I, class T>
using VariantAlternativeSfinaeT = typename VariantAlternativeSfinae<I, T>::type;

// NOTE: Requires T to be a reference type.
template <class T, class U>
struct GiveQualsTo;

template <class T, class U>
struct GiveQualsTo<T&, U> {
  using type = U&;
};

template <class T, class U>
struct GiveQualsTo<T&&, U> {
  using type = U&&;
};

template <class T, class U>
struct GiveQualsTo<const T&, U> {
  using type = const U&;
};

template <class T, class U>
struct GiveQualsTo<const T&&, U> {
  using type = const U&&;
};

template <class T, class U>
struct GiveQualsTo<volatile T&, U> {
  using type = volatile U&;
};

template <class T, class U>
struct GiveQualsTo<volatile T&&, U> {
  using type = volatile U&&;
};

template <class T, class U>
struct GiveQualsTo<volatile const T&, U> {
  using type = volatile const U&;
};

template <class T, class U>
struct GiveQualsTo<volatile const T&&, U> {
  using type = volatile const U&&;
};

template <class T, class U>
using GiveQualsToT = typename GiveQualsTo<T, U>::type;

// Convenience alias, since size_t integral_constant is used a lot in this file.
template <std::size_t I>
using SizeT = std::integral_constant<std::size_t, I>;

using NPos = SizeT<variant_npos>;

template <class Variant, class T, class = void>
struct IndexOfConstructedType {};

template <std::size_t I, class Variant>
struct VariantAccessResultImpl;

template <std::size_t I, template <class...> class Variantemplate, class... T>
struct VariantAccessResultImpl<I, Variantemplate<T...>&> {
  using type = typename absl::variant_alternative<I, variant<T...>>::type&;
};

template <std::size_t I, template <class...> class Variantemplate, class... T>
struct VariantAccessResultImpl<I, const Variantemplate<T...>&> {
  using type =
      const typename absl::variant_alternative<I, variant<T...>>::type&;
};

template <std::size_t I, template <class...> class Variantemplate, class... T>
struct VariantAccessResultImpl<I, Variantemplate<T...>&&> {
  using type = typename absl::variant_alternative<I, variant<T...>>::type&&;
};

template <std::size_t I, template <class...> class Variantemplate, class... T>
struct VariantAccessResultImpl<I, const Variantemplate<T...>&&> {
  using type =
      const typename absl::variant_alternative<I, variant<T...>>::type&&;
};

template <std::size_t I, class Variant>
using VariantAccessResult =
    typename VariantAccessResultImpl<I, Variant&&>::type;

// NOTE: This is used instead of std::array to reduce instantiation overhead.
template <class T, std::size_t Size>
struct SimpleArray {
  static_assert(Size != 0, "");
  T value[Size];
};

template <class T>
struct AccessedType {
  using type = T;
};

template <class T>
using AccessedTypeT = typename AccessedType<T>::type;

template <class T, std::size_t Size>
struct AccessedType<SimpleArray<T, Size>> {
  using type = AccessedTypeT<T>;
};

template <class T>
constexpr T AccessSimpleArray(const T& value) {
  return value;
}

template <class T, std::size_t Size, class... SizeT>
constexpr AccessedTypeT<T> AccessSimpleArray(const SimpleArray<T, Size>& table,
                                             std::size_t head_index,
                                             SizeT... tail_indices) {
  return AccessSimpleArray(table.value[head_index], tail_indices...);
}

// Note: Intentionally is an alias.
template <class T>
using AlwaysZero = SizeT<0>;

template <class Op, class... Vs>
struct VisitIndicesResultImpl {
  using type = absl::result_of_t<Op(AlwaysZero<Vs>...)>;
};

template <class Op, class... Vs>
using VisitIndicesResultT = typename VisitIndicesResultImpl<Op, Vs...>::type;

template <class ReturnType, class FunctionObject, class EndIndices,
          class BoundIndices>
struct MakeVisitationMatrix;

template <class ReturnType, class FunctionObject, std::size_t... Indices>
constexpr ReturnType call_with_indices(FunctionObject&& function) {
  static_assert(
      std::is_same<ReturnType, decltype(std::declval<FunctionObject>()(
                                   SizeT<Indices>()...))>::value,
      "Not all visitation overloads have the same return type.");
  return absl::forward<FunctionObject>(function)(SizeT<Indices>()...);
}

template <class ReturnType, class FunctionObject, std::size_t... BoundIndices>
struct MakeVisitationMatrix<ReturnType, FunctionObject, index_sequence<>,
                            index_sequence<BoundIndices...>> {
  using ResultType = ReturnType (*)(FunctionObject&&);
  static constexpr ResultType Run() {
    return &call_with_indices<ReturnType, FunctionObject,
                              (BoundIndices - 1)...>;
  }
};

template <typename Is, std::size_t J>
struct AppendToIndexSequence;

template <typename Is, std::size_t J>
using AppendToIndexSequenceT = typename AppendToIndexSequence<Is, J>::type;

template <std::size_t... Is, std::size_t J>
struct AppendToIndexSequence<index_sequence<Is...>, J> {
  using type = index_sequence<Is..., J>;
};

template <class ReturnType, class FunctionObject, class EndIndices,
          class CurrIndices, class BoundIndices>
struct MakeVisitationMatrixImpl;

template <class ReturnType, class FunctionObject, class EndIndices,
          std::size_t... CurrIndices, class BoundIndices>
struct MakeVisitationMatrixImpl<ReturnType, FunctionObject, EndIndices,
                                index_sequence<CurrIndices...>, BoundIndices> {
  using ResultType = SimpleArray<
      typename MakeVisitationMatrix<ReturnType, FunctionObject, EndIndices,
                                    index_sequence<>>::ResultType,
      sizeof...(CurrIndices)>;

  static constexpr ResultType Run() {
    return {{MakeVisitationMatrix<
        ReturnType, FunctionObject, EndIndices,
        AppendToIndexSequenceT<BoundIndices, CurrIndices>>::Run()...}};
  }
};

template <class ReturnType, class FunctionObject, std::size_t HeadEndIndex,
          std::size_t... TailEndIndices, std::size_t... BoundIndices>
struct MakeVisitationMatrix<ReturnType, FunctionObject,
                            index_sequence<HeadEndIndex, TailEndIndices...>,
                            index_sequence<BoundIndices...>>
    : MakeVisitationMatrixImpl<ReturnType, FunctionObject,
                               index_sequence<TailEndIndices...>,
                               absl::make_index_sequence<HeadEndIndex>,
                               index_sequence<BoundIndices...>> {};

struct UnreachableSwitchCase {
  template <class Op>
  [[noreturn]] static VisitIndicesResultT<Op, std::size_t> Run(
      Op&& /*ignored*/) {
#if ABSL_HAVE_BUILTIN(__builtin_unreachable) || \
    (defined(__GNUC__) && !defined(__clang__))
    __builtin_unreachable();
#elif defined(_MSC_VER)
    __assume(false);
#else
    // Try to use assert of false being identified as an unreachable intrinsic.
    // NOTE: We use assert directly to increase chances of exploiting an assume
    //       intrinsic.
    assert(false);  // NOLINT

    // Hack to silence potential no return warning -- cause an infinite loop.
    return Run(absl::forward<Op>(op));
#endif  // Checks for __builtin_unreachable
  }
};

template <class Op, std::size_t I>
struct ReachableSwitchCase {
  static VisitIndicesResultT<Op, std::size_t> Run(Op&& op) {
    return absl::base_internal::Invoke(absl::forward<Op>(op), SizeT<I>());
  }
};

// The number 33 is just a guess at a reasonable maximum to our switch. It is
// not based on any analysis. The reason it is a power of 2 plus 1 instead of a
// power of 2 is because the number was picked to correspond to a power of 2
// amount of "normal" alternatives, plus one for the possibility of the user
// providing "monostate" in addition to the more natural alternatives.
ABSL_INTERNAL_INLINE_CONSTEXPR(std::size_t, MaxUnrolledVisitCases, 33);

// Note: The default-definition is for unreachable cases.
template <bool IsReachable>
struct PickCaseImpl {
  template <class Op, std::size_t I>
  using Apply = UnreachableSwitchCase;
};

template <>
struct PickCaseImpl</*IsReachable =*/true> {
  template <class Op, std::size_t I>
  using Apply = ReachableSwitchCase<Op, I>;
};

// Note: This form of dance with template aliases is to make sure that we
//       instantiate a number of templates proportional to the number of variant
//       alternatives rather than a number of templates proportional to our
//       maximum unrolled amount of visitation cases (aliases are effectively
//       "free" whereas other template instantiations are costly).
template <class Op, std::size_t I, std::size_t EndIndex>
using PickCase = typename PickCaseImpl<(I < EndIndex)>::template Apply<Op, I>;

template <class ReturnType>
[[noreturn]] ReturnType TypedThrowBadVariantAccess() {
  absl::variant_internal::ThrowBadVariantAccess();
}

// Given N variant sizes, determine the number of cases there would need to be
// in a single switch-statement that would cover every possibility in the
// corresponding N-ary visit operation.
template <std::size_t... NumAlternatives>
struct NumCasesOfSwitch;

template <std::size_t HeadNumAlternatives, std::size_t... TailNumAlternatives>
struct NumCasesOfSwitch<HeadNumAlternatives, TailNumAlternatives...> {
  static constexpr std::size_t value =
      (HeadNumAlternatives + 1) *
      NumCasesOfSwitch<TailNumAlternatives...>::value;
};

template <>
struct NumCasesOfSwitch<> {
  static constexpr std::size_t value = 1;
};

// A switch statement optimizes better than the table of function pointers.
template <std::size_t EndIndex>
struct VisitIndicesSwitch {
  static_assert(EndIndex <= MaxUnrolledVisitCases,
                "Maximum unrolled switch size exceeded.");

  template <class Op>
  static VisitIndicesResultT<Op, std::size_t> Run(Op&& op, std::size_t i) {
    switch (i) {
      case 0:
        return PickCase<Op, 0, EndIndex>::Run(absl::forward<Op>(op));
      case 1:
        return PickCase<Op, 1, EndIndex>::Run(absl::forward<Op>(op));
      case 2:
        return PickCase<Op, 2, EndIndex>::Run(absl::forward<Op>(op));
      case 3:
        return PickCase<Op, 3, EndIndex>::Run(absl::forward<Op>(op));
      case 4:
        return PickCase<Op, 4, EndIndex>::Run(absl::forward<Op>(op));
      case 5:
        return PickCase<Op, 5, EndIndex>::Run(absl::forward<Op>(op));
      case 6:
        return PickCase<Op, 6, EndIndex>::Run(absl::forward<Op>(op));
      case 7:
        return PickCase<Op, 7, EndIndex>::Run(absl::forward<Op>(op));
      case 8:
        return PickCase<Op, 8, EndIndex>::Run(absl::forward<Op>(op));
      case 9:
        return PickCase<Op, 9, EndIndex>::Run(absl::forward<Op>(op));
      case 10:
        return PickCase<Op, 10, EndIndex>::Run(absl::forward<Op>(op));
      case 11:
        return PickCase<Op, 11, EndIndex>::Run(absl::forward<Op>(op));
      case 12:
        return PickCase<Op, 12, EndIndex>::Run(absl::forward<Op>(op));
      case 13:
        return PickCase<Op, 13, EndIndex>::Run(absl::forward<Op>(op));
      case 14:
        return PickCase<Op, 14, EndIndex>::Run(absl::forward<Op>(op));
      case 15:
        return PickCase<Op, 15, EndIndex>::Run(absl::forward<Op>(op));
      case 16:
        return PickCase<Op, 16, EndIndex>::Run(absl::forward<Op>(op));
      case 17:
        return PickCase<Op, 17, EndIndex>::Run(absl::forward<Op>(op));
      case 18:
        return PickCase<Op, 18, EndIndex>::Run(absl::forward<Op>(op));
      case 19:
        return PickCase<Op, 19, EndIndex>::Run(absl::forward<Op>(op));
      case 20:
        return PickCase<Op, 20, EndIndex>::Run(absl::forward<Op>(op));
      case 21:
        return PickCase<Op, 21, EndIndex>::Run(absl::forward<Op>(op));
      case 22:
        return PickCase<Op, 22, EndIndex>::Run(absl::forward<Op>(op));
      case 23:
        return PickCase<Op, 23, EndIndex>::Run(absl::forward<Op>(op));
      case 24:
        return PickCase<Op, 24, EndIndex>::Run(absl::forward<Op>(op));
      case 25:
        return PickCase<Op, 25, EndIndex>::Run(absl::forward<Op>(op));
      case 26:
        return PickCase<Op, 26, EndIndex>::Run(absl::forward<Op>(op));
      case 27:
        return PickCase<Op, 27, EndIndex>::Run(absl::forward<Op>(op));
      case 28:
        return PickCase<Op, 28, EndIndex>::Run(absl::forward<Op>(op));
      case 29:
        return PickCase<Op, 29, EndIndex>::Run(absl::forward<Op>(op));
      case 30:
        return PickCase<Op, 30, EndIndex>::Run(absl::forward<Op>(op));
      case 31:
        return PickCase<Op, 31, EndIndex>::Run(absl::forward<Op>(op));
      case 32:
        return PickCase<Op, 32, EndIndex>::Run(absl::forward<Op>(op));
      default:
        ABSL_ASSERT(i == variant_npos);
        return absl::base_internal::Invoke(absl::forward<Op>(op), NPos());
    }
  }
};

template <std::size_t... EndIndices>
struct VisitIndicesFallback {
  template <class Op, class... SizeT>
  static VisitIndicesResultT<Op, SizeT...> Run(Op&& op, SizeT... indices) {
    return AccessSimpleArray(
        MakeVisitationMatrix<VisitIndicesResultT<Op, SizeT...>, Op,
                             index_sequence<(EndIndices + 1)...>,
                             index_sequence<>>::Run(),
        (indices + 1)...)(absl::forward<Op>(op));
  }
};

// Take an N-dimensional series of indices and convert them into a single index
// without loss of information. The purpose of this is to be able to convert an
// N-ary visit operation into a single switch statement.
template <std::size_t...>
struct FlattenIndices;

template <std::size_t HeadSize, std::size_t... TailSize>
struct FlattenIndices<HeadSize, TailSize...> {
  template<class... SizeType>
  static constexpr std::size_t Run(std::size_t head, SizeType... tail) {
    return head + HeadSize * FlattenIndices<TailSize...>::Run(tail...);
  }
};

template <>
struct FlattenIndices<> {
  static constexpr std::size_t Run() { return 0; }
};

// Take a single "flattened" index (flattened by FlattenIndices) and determine
// the value of the index of one of the logically represented dimensions.
template <std::size_t I, std::size_t IndexToGet, std::size_t HeadSize,
          std::size_t... TailSize>
struct UnflattenIndex {
  static constexpr std::size_t value =
      UnflattenIndex<I / HeadSize, IndexToGet - 1, TailSize...>::value;
};

template <std::size_t I, std::size_t HeadSize, std::size_t... TailSize>
struct UnflattenIndex<I, 0, HeadSize, TailSize...> {
  static constexpr std::size_t value = (I % HeadSize);
};

// The backend for converting an N-ary visit operation into a unary visit.
template <class IndexSequence, std::size_t... EndIndices>
struct VisitIndicesVariadicImpl;

template <std::size_t... N, std::size_t... EndIndices>
struct VisitIndicesVariadicImpl<absl::index_sequence<N...>, EndIndices...> {
  // A type that can take an N-ary function object and converts it to a unary
  // function object that takes a single, flattened index, and "unflattens" it
  // into its individual dimensions when forwarding to the wrapped object.
  template <class Op>
  struct FlattenedOp {
    template <std::size_t I>
    VisitIndicesResultT<Op, decltype(EndIndices)...> operator()(
        SizeT<I> /*index*/) && {
      return base_internal::Invoke(
          absl::forward<Op>(op),
          SizeT<UnflattenIndex<I, N, (EndIndices + 1)...>::value -
                std::size_t{1}>()...);
    }

    Op&& op;
  };

  template <class Op, class... SizeType>
  static VisitIndicesResultT<Op, decltype(EndIndices)...> Run(
      Op&& op, SizeType... i) {
    return VisitIndicesSwitch<NumCasesOfSwitch<EndIndices...>::value>::Run(
        FlattenedOp<Op>{absl::forward<Op>(op)},
        FlattenIndices<(EndIndices + std::size_t{1})...>::Run(
            (i + std::size_t{1})...));
  }
};

template <std::size_t... EndIndices>
struct VisitIndicesVariadic
    : VisitIndicesVariadicImpl<absl::make_index_sequence<sizeof...(EndIndices)>,
                               EndIndices...> {};

// This implementation will flatten N-ary visit operations into a single switch
// statement when the number of cases would be less than our maximum specified
// switch-statement size.
// TODO(calabrese)
//   Based on benchmarks, determine whether the function table approach actually
//   does optimize better than a chain of switch statements and possibly update
//   the implementation accordingly. Also consider increasing the maximum switch
//   size.
template <std::size_t... EndIndices>
struct VisitIndices
    : absl::conditional_t<(NumCasesOfSwitch<EndIndices...>::value <=
                           MaxUnrolledVisitCases),
                          VisitIndicesVariadic<EndIndices...>,
                          VisitIndicesFallback<EndIndices...>> {};

template <std::size_t EndIndex>
struct VisitIndices<EndIndex>
    : absl::conditional_t<(EndIndex <= MaxUnrolledVisitCases),
                          VisitIndicesSwitch<EndIndex>,
                          VisitIndicesFallback<EndIndex>> {};

// Suppress bogus warning on MSVC: MSVC complains that the `reinterpret_cast`
// below is returning the address of a temporary or local object.
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4172)
#endif  // _MSC_VER

// TODO(calabrese) std::launder
// TODO(calabrese) constexpr
// NOTE: DO NOT REMOVE the `inline` keyword as it is necessary to work around a
// MSVC bug. See https://github.com/abseil/abseil-cpp/issues/129 for details.
template <class Self, std::size_t I>
inline VariantAccessResult<I, Self> AccessUnion(Self&& self, SizeT<I> /*i*/) {
  return reinterpret_cast<VariantAccessResult<I, Self>>(self);
}

#ifdef _MSC_VER
#pragma warning(pop)
#endif  // _MSC_VER

template <class T>
void DeducedDestroy(T& self) {  // NOLINT
  self.~T();
}

// NOTE: This type exists as a single entity for variant and its bases to
// befriend. It contains helper functionality that manipulates the state of the
// variant, such as the implementation of things like assignment and emplace
// operations.
struct VariantCoreAccess {
  template <class VariantType>
  static typename VariantType::Variant& Derived(VariantType& self) {  // NOLINT
    return static_cast<typename VariantType::Variant&>(self);
  }

  template <class VariantType>
  static const typename VariantType::Variant& Derived(
      const VariantType& self) {  // NOLINT
    return static_cast<const typename VariantType::Variant&>(self);
  }

  template <class VariantType>
  static void Destroy(VariantType& self) {  // NOLINT
    Derived(self).destroy();
    self.index_ = absl::variant_npos;
  }

  template <class Variant>
  static void SetIndex(Variant& self, std::size_t i) {  // NOLINT
    self.index_ = i;
  }

  template <class Variant>
  static void InitFrom(Variant& self, Variant&& other) {  // NOLINT
    VisitIndices<absl::variant_size<Variant>::value>::Run(
        InitFromVisitor<Variant, Variant&&>{&self,
                                            std::forward<Variant>(other)},
        other.index());
    self.index_ = other.index();
  }

  // Access a variant alternative, assuming the index is correct.
  template <std::size_t I, class Variant>
  static VariantAccessResult<I, Variant> Access(Variant&& self) {
    // This cast instead of invocation of AccessUnion with an rvalue is a
    // workaround for msvc. Without this there is a runtime failure when dealing
    // with rvalues.
    // TODO(calabrese) Reduce test case and find a simpler workaround.
    return static_cast<VariantAccessResult<I, Variant>>(
        variant_internal::AccessUnion(self.state_, SizeT<I>()));
  }

  // Access a variant alternative, throwing if the index is incorrect.
  template <std::size_t I, class Variant>
  static VariantAccessResult<I, Variant> CheckedAccess(Variant&& self) {
    if (ABSL_PREDICT_FALSE(self.index_ != I)) {
      TypedThrowBadVariantAccess<VariantAccessResult<I, Variant>>();
    }

    return Access<I>(absl::forward<Variant>(self));
  }

  // The implementation of the move-assignment operation for a variant.
  template <class VType>
  struct MoveAssignVisitor {
    using DerivedType = typename VType::Variant;
    template <std::size_t NewIndex>
    void operator()(SizeT<NewIndex> /*new_i*/) const {
      if (left->index_ == NewIndex) {
        Access<NewIndex>(*left) = std::move(Access<NewIndex>(*right));
      } else {
        Derived(*left).template emplace<NewIndex>(
            std::move(Access<NewIndex>(*right)));
      }
    }

    void operator()(SizeT<absl::variant_npos> /*new_i*/) const {
      Destroy(*left);
    }

    VType* left;
    VType* right;
  };

  template <class VType>
  static MoveAssignVisitor<VType> MakeMoveAssignVisitor(VType* left,
                                                        VType* other) {
    return {left, other};
  }

  // The implementation of the assignment operation for a variant.
  template <class VType>
  struct CopyAssignVisitor {
    using DerivedType = typename VType::Variant;
    template <std::size_t NewIndex>
    void operator()(SizeT<NewIndex> /*new_i*/) const {
      using New =
          typename absl::variant_alternative<NewIndex, DerivedType>::type;

      if (left->index_ == NewIndex) {
        Access<NewIndex>(*left) = Access<NewIndex>(*right);
      } else if (std::is_nothrow_copy_constructible<New>::value ||
                 !std::is_nothrow_move_constructible<New>::value) {
        Derived(*left).template emplace<NewIndex>(Access<NewIndex>(*right));
      } else {
        Derived(*left) = DerivedType(Derived(*right));
      }
    }

    void operator()(SizeT<absl::variant_npos> /*new_i*/) const {
      Destroy(*left);
    }

    VType* left;
    const VType* right;
  };

  template <class VType>
  static CopyAssignVisitor<VType> MakeCopyAssignVisitor(VType* left,
                                                        const VType& other) {
    return {left, &other};
  }

  // The implementation of conversion-assignment operations for variant.
  template <class Left, class QualifiedNew>
  struct ConversionAssignVisitor {
    using NewIndex =
        variant_internal::IndexOfConstructedType<Left, QualifiedNew>;

    void operator()(SizeT<NewIndex::value> /*old_i*/
                    ) const {
      Access<NewIndex::value>(*left) = absl::forward<QualifiedNew>(other);
    }

    template <std::size_t OldIndex>
    void operator()(SizeT<OldIndex> /*old_i*/
                    ) const {
      using New =
          typename absl::variant_alternative<NewIndex::value, Left>::type;
      if (std::is_nothrow_constructible<New, QualifiedNew>::value ||
          !std::is_nothrow_move_constructible<New>::value) {
        left->template emplace<NewIndex::value>(
            absl::forward<QualifiedNew>(other));
      } else {
        // the standard says "equivalent to
        // operator=(variant(std::forward<T>(t)))", but we use `emplace` here
        // because the variant's move assignment operator could be deleted.
        left->template emplace<NewIndex::value>(
            New(absl::forward<QualifiedNew>(other)));
      }
    }

    Left* left;
    QualifiedNew&& other;
  };

  template <class Left, class QualifiedNew>
  static ConversionAssignVisitor<Left, QualifiedNew>
  MakeConversionAssignVisitor(Left* left, QualifiedNew&& qual) {
    return {left, absl::forward<QualifiedNew>(qual)};
  }

  // Backend for operations for `emplace()` which destructs `*self` then
  // construct a new alternative with `Args...`.
  template <std::size_t NewIndex, class Self, class... Args>
  static typename absl::variant_alternative<NewIndex, Self>::type& Replace(
      Self* self, Args&&... args) {
    Destroy(*self);
    using New = typename absl::variant_alternative<NewIndex, Self>::type;
    New* const result = ::new (static_cast<void*>(&self->state_))
        New(absl::forward<Args>(args)...);
    self->index_ = NewIndex;
    return *result;
  }

  template <class LeftVariant, class QualifiedRightVariant>
  struct InitFromVisitor {
    template <std::size_t NewIndex>
    void operator()(SizeT<NewIndex> /*new_i*/) const {
      using Alternative =
          typename variant_alternative<NewIndex, LeftVariant>::type;
      ::new (static_cast<void*>(&left->state_)) Alternative(
          Access<NewIndex>(std::forward<QualifiedRightVariant>(right)));
    }

    void operator()(SizeT<absl::variant_npos> /*new_i*/) const {
      // This space intentionally left blank.
    }
    LeftVariant* left;
    QualifiedRightVariant&& right;
  };
};

template <class Expected, class... T>
struct IndexOfImpl;

template <class Expected>
struct IndexOfImpl<Expected> {
  using IndexFromEnd = SizeT<0>;
  using MatchedIndexFromEnd = IndexFromEnd;
  using MultipleMatches = std::false_type;
};

template <class Expected, class Head, class... Tail>
struct IndexOfImpl<Expected, Head, Tail...> : IndexOfImpl<Expected, Tail...> {
  using IndexFromEnd =
      SizeT<IndexOfImpl<Expected, Tail...>::IndexFromEnd::value + 1>;
};

template <class Expected, class... Tail>
struct IndexOfImpl<Expected, Expected, Tail...>
    : IndexOfImpl<Expected, Tail...> {
  using IndexFromEnd =
      SizeT<IndexOfImpl<Expected, Tail...>::IndexFromEnd::value + 1>;
  using MatchedIndexFromEnd = IndexFromEnd;
  using MultipleMatches = std::integral_constant<
      bool, IndexOfImpl<Expected, Tail...>::MatchedIndexFromEnd::value != 0>;
};

template <class Expected, class... Types>
struct IndexOfMeta {
  using Results = IndexOfImpl<Expected, Types...>;
  static_assert(!Results::MultipleMatches::value,
                "Attempted to access a variant by specifying a type that "
                "matches more than one alternative.");
  static_assert(Results::MatchedIndexFromEnd::value != 0,
                "Attempted to access a variant by specifying a type that does "
                "not match any alternative.");
  using type = SizeT<sizeof...(Types) - Results::MatchedIndexFromEnd::value>;
};

template <class Expected, class... Types>
using IndexOf = typename IndexOfMeta<Expected, Types...>::type;

template <class Variant, class T, std::size_t CurrIndex>
struct UnambiguousIndexOfImpl;

// Terminating case encountered once we've checked all of the alternatives
template <class T, std::size_t CurrIndex>
struct UnambiguousIndexOfImpl<variant<>, T, CurrIndex> : SizeT<CurrIndex> {};

// Case where T is not Head
template <class Head, class... Tail, class T, std::size_t CurrIndex>
struct UnambiguousIndexOfImpl<variant<Head, Tail...>, T, CurrIndex>
    : UnambiguousIndexOfImpl<variant<Tail...>, T, CurrIndex + 1>::type {};

// Case where T is Head
template <class Head, class... Tail, std::size_t CurrIndex>
struct UnambiguousIndexOfImpl<variant<Head, Tail...>, Head, CurrIndex>
    : SizeT<UnambiguousIndexOfImpl<variant<Tail...>, Head, 0>::value ==
                    sizeof...(Tail)
                ? CurrIndex
                : CurrIndex + sizeof...(Tail) + 1> {};

template <class Variant, class T>
struct UnambiguousIndexOf;

struct NoMatch {
  struct type {};
};

template <class... Alts, class T>
struct UnambiguousIndexOf<variant<Alts...>, T>
    : std::conditional<UnambiguousIndexOfImpl<variant<Alts...>, T, 0>::value !=
                           sizeof...(Alts),
                       UnambiguousIndexOfImpl<variant<Alts...>, T, 0>,
                       NoMatch>::type::type {};

template <class T, std::size_t /*Dummy*/>
using UnambiguousTypeOfImpl = T;

template <class Variant, class T>
using UnambiguousTypeOfT =
    UnambiguousTypeOfImpl<T, UnambiguousIndexOf<Variant, T>::value>;

template <class H, class... T>
class VariantStateBase;

// This is an implementation of the "imaginary function" that is described in
// [variant.ctor]
// It is used in order to determine which alternative to construct during
// initialization from some type T.
template <class Variant, std::size_t I = 0>
struct ImaginaryFun;

template <std::size_t I>
struct ImaginaryFun<variant<>, I> {
  static void Run() = delete;
};

template <class H, class... T, std::size_t I>
struct ImaginaryFun<variant<H, T...>, I> : ImaginaryFun<variant<T...>, I + 1> {
  using ImaginaryFun<variant<T...>, I + 1>::Run;

  // NOTE: const& and && are used instead of by-value due to lack of guaranteed
  // move elision of C++17. This may have other minor differences, but tests
  // pass.
  static SizeT<I> Run(const H&, SizeT<I>);
  static SizeT<I> Run(H&&, SizeT<I>);
};

// The following metafunctions are used in constructor and assignment
// constraints.
template <class Self, class T>
struct IsNeitherSelfNorInPlace : std::true_type {};

template <class Self>
struct IsNeitherSelfNorInPlace<Self, Self> : std::false_type {};

template <class Self, class T>
struct IsNeitherSelfNorInPlace<Self, in_place_type_t<T>> : std::false_type {};

template <class Self, std::size_t I>
struct IsNeitherSelfNorInPlace<Self, in_place_index_t<I>> : std::false_type {};

template <class Variant, class T, class = void>
struct ConversionIsPossibleImpl : std::false_type {};

template <class Variant, class T>
struct ConversionIsPossibleImpl<
    Variant, T,
    void_t<decltype(ImaginaryFun<Variant>::Run(std::declval<T>(), {}))>>
    : std::true_type {};

template <class Variant, class T>
struct ConversionIsPossible : ConversionIsPossibleImpl<Variant, T>::type {};

template <class Variant, class T>
struct IndexOfConstructedType<
    Variant, T,
    void_t<decltype(ImaginaryFun<Variant>::Run(std::declval<T>(), {}))>>
    : decltype(ImaginaryFun<Variant>::Run(std::declval<T>(), {})) {};

template <std::size_t... Is>
struct ContainsVariantNPos
    : absl::negation<std::is_same<  // NOLINT
          absl::integer_sequence<bool, 0 <= Is...>,
          absl::integer_sequence<bool, Is != absl::variant_npos...>>> {};

template <class Op, class... QualifiedVariants>
using RawVisitResult =
    absl::result_of_t<Op(VariantAccessResult<0, QualifiedVariants>...)>;

// NOTE: The spec requires that all return-paths yield the same type and is not
// SFINAE-friendly, so we can deduce the return type by examining the first
// result. If it's not callable, then we get an error, but are compliant and
// fast to compile.
// TODO(calabrese) Possibly rewrite in a way that yields better compile errors
// at the cost of longer compile-times.
template <class Op, class... QualifiedVariants>
struct VisitResultImpl {
  using type =
      absl::result_of_t<Op(VariantAccessResult<0, QualifiedVariants>...)>;
};

// Done in two steps intentionally so that we don't cause substitution to fail.
template <class Op, class... QualifiedVariants>
using VisitResult = typename VisitResultImpl<Op, QualifiedVariants...>::type;

template <class Op, class... QualifiedVariants>
struct PerformVisitation {
  using ReturnType = VisitResult<Op, QualifiedVariants...>;

  template <std::size_t... Is>
  constexpr ReturnType operator()(SizeT<Is>... indices) const {
    return Run(typename ContainsVariantNPos<Is...>::type{},
               absl::index_sequence_for<QualifiedVariants...>(), indices...);
  }

  template <std::size_t... TupIs, std::size_t... Is>
  constexpr ReturnType Run(std::false_type /*has_valueless*/,
                           index_sequence<TupIs...>, SizeT<Is>...) const {
    static_assert(
        std::is_same<ReturnType,
                     absl::result_of_t<Op(VariantAccessResult<
                                          Is, QualifiedVariants>...)>>::value,
        "All visitation overloads must have the same return type.");
    return absl::base_internal::Invoke(
        absl::forward<Op>(op),
        VariantCoreAccess::Access<Is>(
            absl::forward<QualifiedVariants>(std::get<TupIs>(variant_tup)))...);
  }

  template <std::size_t... TupIs, std::size_t... Is>
  [[noreturn]] ReturnType Run(std::true_type /*has_valueless*/,
                              index_sequence<TupIs...>, SizeT<Is>...) const {
    absl::variant_internal::ThrowBadVariantAccess();
  }

  // TODO(calabrese) Avoid using a tuple, which causes lots of instantiations
  // Attempts using lambda variadic captures fail on current GCC.
  std::tuple<QualifiedVariants&&...> variant_tup;
  Op&& op;
};

template <class... T>
union Union;

// We want to allow for variant<> to be trivial. For that, we need the default
// constructor to be trivial, which means we can't define it ourselves.
// Instead, we use a non-default constructor that takes NoopConstructorTag
// that doesn't affect the triviality of the types.
struct NoopConstructorTag {};

template <std::size_t I>
struct EmplaceTag {};

template <>
union Union<> {
  constexpr explicit Union(NoopConstructorTag) noexcept {}
};

// Suppress bogus warning on MSVC: MSVC complains that Union<T...> has a defined
// deleted destructor from the `std::is_destructible` check below.
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4624)
#endif  // _MSC_VER

template <class Head, class... Tail>
union Union<Head, Tail...> {
  using TailUnion = Union<Tail...>;

  explicit constexpr Union(NoopConstructorTag /*tag*/) noexcept
      : tail(NoopConstructorTag()) {}

  template <class... P>
  explicit constexpr Union(EmplaceTag<0>, P&&... args)
      : head(absl::forward<P>(args)...) {}

  template <std::size_t I, class... P>
  explicit constexpr Union(EmplaceTag<I>, P&&... args)
      : tail(EmplaceTag<I - 1>{}, absl::forward<P>(args)...) {}

  Head head;
  TailUnion tail;
};

#ifdef _MSC_VER
#pragma warning(pop)
#endif  // _MSC_VER

// TODO(calabrese) Just contain a Union in this union (certain configs fail).
template <class... T>
union DestructibleUnionImpl;

template <>
union DestructibleUnionImpl<> {
  constexpr explicit DestructibleUnionImpl(NoopConstructorTag) noexcept {}
};

template <class Head, class... Tail>
union DestructibleUnionImpl<Head, Tail...> {
  using TailUnion = DestructibleUnionImpl<Tail...>;

  explicit constexpr DestructibleUnionImpl(NoopConstructorTag /*tag*/) noexcept
      : tail(NoopConstructorTag()) {}

  template <class... P>
  explicit constexpr DestructibleUnionImpl(EmplaceTag<0>, P&&... args)
      : head(absl::forward<P>(args)...) {}

  template <std::size_t I, class... P>
  explicit constexpr DestructibleUnionImpl(EmplaceTag<I>, P&&... args)
      : tail(EmplaceTag<I - 1>{}, absl::forward<P>(args)...) {}

  ~DestructibleUnionImpl() {}

  Head head;
  TailUnion tail;
};

// This union type is destructible even if one or more T are not trivially
// destructible. In the case that all T are trivially destructible, then so is
// this resultant type.
template <class... T>
using DestructibleUnion =
    absl::conditional_t<std::is_destructible<Union<T...>>::value, Union<T...>,
                        DestructibleUnionImpl<T...>>;

// Deepest base, containing the actual union and the discriminator
template <class H, class... T>
class VariantStateBase {
 protected:
  using Variant = variant<H, T...>;

  template <class LazyH = H,
            class ConstructibleH = absl::enable_if_t<
                std::is_default_constructible<LazyH>::value, LazyH>>
  constexpr VariantStateBase() noexcept(
      std::is_nothrow_default_constructible<ConstructibleH>::value)
      : state_(EmplaceTag<0>()), index_(0) {}

  template <std::size_t I, class... P>
  explicit constexpr VariantStateBase(EmplaceTag<I> tag, P&&... args)
      : state_(tag, absl::forward<P>(args)...), index_(I) {}

  explicit constexpr VariantStateBase(NoopConstructorTag)
      : state_(NoopConstructorTag()), index_(variant_npos) {}

  void destroy() {}  // Does nothing (shadowed in child if non-trivial)

  DestructibleUnion<H, T...> state_;
  std::size_t index_;
};

using absl::internal::identity;

// OverloadSet::Overload() is a unary function which is overloaded to
// take any of the element types of the variant, by reference-to-const.
// The return type of the overload on T is identity<T>, so that you
// can statically determine which overload was called.
//
// Overload() is not defined, so it can only be called in unevaluated
// contexts.
template <typename... Ts>
struct OverloadSet;

template <typename T, typename... Ts>
struct OverloadSet<T, Ts...> : OverloadSet<Ts...> {
  using Base = OverloadSet<Ts...>;
  static identity<T> Overload(const T&);
  using Base::Overload;
};

template <>
struct OverloadSet<> {
  // For any case not handled above.
  static void Overload(...);
};

template <class T>
using LessThanResult = decltype(std::declval<T>() < std::declval<T>());

template <class T>
using GreaterThanResult = decltype(std::declval<T>() > std::declval<T>());

template <class T>
using LessThanOrEqualResult = decltype(std::declval<T>() <= std::declval<T>());

template <class T>
using GreaterThanOrEqualResult =
    decltype(std::declval<T>() >= std::declval<T>());

template <class T>
using EqualResult = decltype(std::declval<T>() == std::declval<T>());

template <class T>
using NotEqualResult = decltype(std::declval<T>() != std::declval<T>());

using type_traits_internal::is_detected_convertible;

template <class... T>
using RequireAllHaveEqualT = absl::enable_if_t<
    absl::conjunction<is_detected_convertible<bool, EqualResult, T>...>::value,
    bool>;

template <class... T>
using RequireAllHaveNotEqualT =
    absl::enable_if_t<absl::conjunction<is_detected_convertible<
                          bool, NotEqualResult, T>...>::value,
                      bool>;

template <class... T>
using RequireAllHaveLessThanT =
    absl::enable_if_t<absl::conjunction<is_detected_convertible<
                          bool, LessThanResult, T>...>::value,
                      bool>;

template <class... T>
using RequireAllHaveLessThanOrEqualT =
    absl::enable_if_t<absl::conjunction<is_detected_convertible<
                          bool, LessThanOrEqualResult, T>...>::value,
                      bool>;

template <class... T>
using RequireAllHaveGreaterThanOrEqualT =
    absl::enable_if_t<absl::conjunction<is_detected_convertible<
                          bool, GreaterThanOrEqualResult, T>...>::value,
                      bool>;

template <class... T>
using RequireAllHaveGreaterThanT =
    absl::enable_if_t<absl::conjunction<is_detected_convertible<
                          bool, GreaterThanResult, T>...>::value,
                      bool>;

// Helper template containing implementations details of variant that can't go
// in the private section. For convenience, this takes the variant type as a
// single template parameter.
template <typename T>
struct VariantHelper;

template <typename... Ts>
struct VariantHelper<variant<Ts...>> {
  // Type metafunction which returns the element type selected if
  // OverloadSet::Overload() is well-formed when called with argument type U.
  template <typename U>
  using BestMatch = decltype(
      variant_internal::OverloadSet<Ts...>::Overload(std::declval<U>()));

  // Type metafunction which returns true if OverloadSet::Overload() is
  // well-formed when called with argument type U.
  // CanAccept can't be just an alias because there is a MSVC bug on parameter
  // pack expansion involving decltype.
  template <typename U>
  struct CanAccept :
      std::integral_constant<bool, !std::is_void<BestMatch<U>>::value> {};

  // Type metafunction which returns true if Other is an instantiation of
  // variant, and variants's converting constructor from Other will be
  // well-formed. We will use this to remove constructors that would be
  // ill-formed from the overload set.
  template <typename Other>
  struct CanConvertFrom;

  template <typename... Us>
  struct CanConvertFrom<variant<Us...>>
      : public absl::conjunction<CanAccept<Us>...> {};
};

// A type with nontrivial copy ctor and trivial move ctor.
struct TrivialMoveOnly {
  TrivialMoveOnly(TrivialMoveOnly&&) = default;
};

// Trait class to detect whether a type is trivially move constructible.
// A union's defaulted copy/move constructor is deleted if any variant member's
// copy/move constructor is nontrivial.
template <typename T>
struct IsTriviallyMoveConstructible:
  std::is_move_constructible<Union<T, TrivialMoveOnly>> {};

// To guarantee triviality of all special-member functions that can be trivial,
// we use a chain of conditional bases for each one.
// The order of inheritance of bases from child to base are logically:
//
// variant
// VariantCopyAssignBase
// VariantMoveAssignBase
// VariantCopyBase
// VariantMoveBase
// VariantStateBaseDestructor
// VariantStateBase
//
// Note that there is a separate branch at each base that is dependent on
// whether or not that corresponding special-member-function can be trivial in
// the resultant variant type.

template <class... T>
class VariantStateBaseDestructorNontrivial;

template <class... T>
class VariantMoveBaseNontrivial;

template <class... T>
class VariantCopyBaseNontrivial;

template <class... T>
class VariantMoveAssignBaseNontrivial;

template <class... T>
class VariantCopyAssignBaseNontrivial;

// Base that is dependent on whether or not the destructor can be trivial.
template <class... T>
using VariantStateBaseDestructor =
    absl::conditional_t<std::is_destructible<Union<T...>>::value,
                        VariantStateBase<T...>,
                        VariantStateBaseDestructorNontrivial<T...>>;

// Base that is dependent on whether or not the move-constructor can be
// implicitly generated by the compiler (trivial or deleted).
// Previously we were using `std::is_move_constructible<Union<T...>>` to check
// whether all Ts have trivial move constructor, but it ran into a GCC bug:
// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84866
// So we have to use a different approach (i.e. `HasTrivialMoveConstructor`) to
// work around the bug.
template <class... T>
using VariantMoveBase = absl::conditional_t<
    absl::disjunction<
        absl::negation<absl::conjunction<std::is_move_constructible<T>...>>,
        absl::conjunction<IsTriviallyMoveConstructible<T>...>>::value,
    VariantStateBaseDestructor<T...>, VariantMoveBaseNontrivial<T...>>;

// Base that is dependent on whether or not the copy-constructor can be trivial.
template <class... T>
using VariantCopyBase = absl::conditional_t<
    absl::disjunction<
        absl::negation<absl::conjunction<std::is_copy_constructible<T>...>>,
        std::is_copy_constructible<Union<T...>>>::value,
    VariantMoveBase<T...>, VariantCopyBaseNontrivial<T...>>;

// Base that is dependent on whether or not the move-assign can be trivial.
template <class... T>
using VariantMoveAssignBase = absl::conditional_t<
    absl::disjunction<
        absl::conjunction<absl::is_move_assignable<Union<T...>>,
                          std::is_move_constructible<Union<T...>>,
                          std::is_destructible<Union<T...>>>,
        absl::negation<absl::conjunction<std::is_move_constructible<T>...,
                                         // Note: We're not qualifying this with
                                         // absl:: because it doesn't compile
                                         // under MSVC.
                                         is_move_assignable<T>...>>>::value,
    VariantCopyBase<T...>, VariantMoveAssignBaseNontrivial<T...>>;

// Base that is dependent on whether or not the copy-assign can be trivial.
template <class... T>
using VariantCopyAssignBase = absl::conditional_t<
    absl::disjunction<
        absl::conjunction<absl::is_copy_assignable<Union<T...>>,
                          std::is_copy_constructible<Union<T...>>,
                          std::is_destructible<Union<T...>>>,
        absl::negation<absl::conjunction<std::is_copy_constructible<T>...,
                                         // Note: We're not qualifying this with
                                         // absl:: because it doesn't compile
                                         // under MSVC.
                                         is_copy_assignable<T>...>>>::value,
    VariantMoveAssignBase<T...>, VariantCopyAssignBaseNontrivial<T...>>;

template <class... T>
using VariantBase = VariantCopyAssignBase<T...>;

template <class... T>
class VariantStateBaseDestructorNontrivial : protected VariantStateBase<T...> {
 private:
  using Base = VariantStateBase<T...>;

 protected:
  using Base::Base;

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

  struct Destroyer {
    template <std::size_t I>
    void operator()(SizeT<I> i) const {
      using Alternative =
          typename absl::variant_alternative<I, variant<T...>>::type;
      variant_internal::AccessUnion(self->state_, i).~Alternative();
    }

    void operator()(SizeT<absl::variant_npos> /*i*/) const {
      // This space intentionally left blank
    }

    VariantStateBaseDestructorNontrivial* self;
  };

  void destroy() { VisitIndices<sizeof...(T)>::Run(Destroyer{this}, index_); }

  ~VariantStateBaseDestructorNontrivial() { destroy(); }

 protected:
  using Base::index_;
  using Base::state_;
};

template <class... T>
class VariantMoveBaseNontrivial : protected VariantStateBaseDestructor<T...> {
 private:
  using Base = VariantStateBaseDestructor<T...>;

 protected:
  using Base::Base;

  struct Construct {
    template <std::size_t I>
    void operator()(SizeT<I> i) const {
      using Alternative =
          typename absl::variant_alternative<I, variant<T...>>::type;
      ::new (static_cast<void*>(&self->state_)) Alternative(
          variant_internal::AccessUnion(absl::move(other->state_), i));
    }

    void operator()(SizeT<absl::variant_npos> /*i*/) const {}

    VariantMoveBaseNontrivial* self;
    VariantMoveBaseNontrivial* other;
  };

  VariantMoveBaseNontrivial() = default;
  VariantMoveBaseNontrivial(VariantMoveBaseNontrivial&& other) noexcept(
      absl::conjunction<std::is_nothrow_move_constructible<T>...>::value)
      : Base(NoopConstructorTag()) {
    VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
    index_ = other.index_;
  }

  VariantMoveBaseNontrivial(VariantMoveBaseNontrivial const&) = default;

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

 protected:
  using Base::index_;
  using Base::state_;
};

template <class... T>
class VariantCopyBaseNontrivial : protected VariantMoveBase<T...> {
 private:
  using Base = VariantMoveBase<T...>;

 protected:
  using Base::Base;

  VariantCopyBaseNontrivial() = default;
  VariantCopyBaseNontrivial(VariantCopyBaseNontrivial&&) = default;

  struct Construct {
    template <std::size_t I>
    void operator()(SizeT<I> i) const {
      using Alternative =
          typename absl::variant_alternative<I, variant<T...>>::type;
      ::new (static_cast<void*>(&self->state_))
          Alternative(variant_internal::AccessUnion(other->state_, i));
    }

    void operator()(SizeT<absl::variant_npos> /*i*/) const {}

    VariantCopyBaseNontrivial* self;
    const VariantCopyBaseNontrivial* other;
  };

  VariantCopyBaseNontrivial(VariantCopyBaseNontrivial const& other)
      : Base(NoopConstructorTag()) {
    VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
    index_ = other.index_;
  }

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

 protected:
  using Base::index_;
  using Base::state_;
};

template <class... T>
class VariantMoveAssignBaseNontrivial : protected VariantCopyBase<T...> {
  friend struct VariantCoreAccess;

 private:
  using Base = VariantCopyBase<T...>;

 protected:
  using Base::Base;

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

    VariantMoveAssignBaseNontrivial&
    operator=(VariantMoveAssignBaseNontrivial&& other) noexcept(
        absl::conjunction<std::is_nothrow_move_constructible<T>...,
                          std::is_nothrow_move_assignable<T>...>::value) {
      VisitIndices<sizeof...(T)>::Run(
          VariantCoreAccess::MakeMoveAssignVisitor(this, &other), other.index_);
      return *this;
    }

 protected:
  using Base::index_;
  using Base::state_;
};

template <class... T>
class VariantCopyAssignBaseNontrivial : protected VariantMoveAssignBase<T...> {
  friend struct VariantCoreAccess;

 private:
  using Base = VariantMoveAssignBase<T...>;

 protected:
  using Base::Base;

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

    VariantCopyAssignBaseNontrivial& operator=(
        const VariantCopyAssignBaseNontrivial& other) {
      VisitIndices<sizeof...(T)>::Run(
          VariantCoreAccess::MakeCopyAssignVisitor(this, other), other.index_);
      return *this;
    }

 protected:
  using Base::index_;
  using Base::state_;
};

////////////////////////////////////////
// Visitors for Comparison Operations //
////////////////////////////////////////

template <class... Types>
struct EqualsOp {
  const variant<Types...>* v;
  const variant<Types...>* w;

  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
    return true;
  }

  template <std::size_t I>
  constexpr bool operator()(SizeT<I> /*v_i*/) const {
    return VariantCoreAccess::Access<I>(*v) == VariantCoreAccess::Access<I>(*w);
  }
};

template <class... Types>
struct NotEqualsOp {
  const variant<Types...>* v;
  const variant<Types...>* w;

  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
    return false;
  }

  template <std::size_t I>
  constexpr bool operator()(SizeT<I> /*v_i*/) const {
    return VariantCoreAccess::Access<I>(*v) != VariantCoreAccess::Access<I>(*w);
  }
};

template <class... Types>
struct LessThanOp {
  const variant<Types...>* v;
  const variant<Types...>* w;

  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
    return false;
  }

  template <std::size_t I>
  constexpr bool operator()(SizeT<I> /*v_i*/) const {
    return VariantCoreAccess::Access<I>(*v) < VariantCoreAccess::Access<I>(*w);
  }
};

template <class... Types>
struct GreaterThanOp {
  const variant<Types...>* v;
  const variant<Types...>* w;

  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
    return false;
  }

  template <std::size_t I>
  constexpr bool operator()(SizeT<I> /*v_i*/) const {
    return VariantCoreAccess::Access<I>(*v) > VariantCoreAccess::Access<I>(*w);
  }
};

template <class... Types>
struct LessThanOrEqualsOp {
  const variant<Types...>* v;
  const variant<Types...>* w;

  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
    return true;
  }

  template <std::size_t I>
  constexpr bool operator()(SizeT<I> /*v_i*/) const {
    return VariantCoreAccess::Access<I>(*v) <= VariantCoreAccess::Access<I>(*w);
  }
};

template <class... Types>
struct GreaterThanOrEqualsOp {
  const variant<Types...>* v;
  const variant<Types...>* w;

  constexpr bool operator()(SizeT<absl::variant_npos> /*v_i*/) const {
    return true;
  }

  template <std::size_t I>
  constexpr bool operator()(SizeT<I> /*v_i*/) const {
    return VariantCoreAccess::Access<I>(*v) >= VariantCoreAccess::Access<I>(*w);
  }
};

// Precondition: v.index() == w.index();
template <class... Types>
struct SwapSameIndex {
  variant<Types...>* v;
  variant<Types...>* w;
  template <std::size_t I>
  void operator()(SizeT<I>) const {
    type_traits_internal::Swap(VariantCoreAccess::Access<I>(*v),
                               VariantCoreAccess::Access<I>(*w));
  }

  void operator()(SizeT<variant_npos>) const {}
};

// TODO(calabrese) do this from a different namespace for proper adl usage
template <class... Types>
struct Swap {
  variant<Types...>* v;
  variant<Types...>* w;

  void generic_swap() const {
    variant<Types...> tmp(std::move(*w));
    VariantCoreAccess::Destroy(*w);
    VariantCoreAccess::InitFrom(*w, std::move(*v));
    VariantCoreAccess::Destroy(*v);
    VariantCoreAccess::InitFrom(*v, std::move(tmp));
  }

  void operator()(SizeT<absl::variant_npos> /*w_i*/) const {
    if (!v->valueless_by_exception()) {
      generic_swap();
    }
  }

  template <std::size_t Wi>
  void operator()(SizeT<Wi> /*w_i*/) {
    if (v->index() == Wi) {
      VisitIndices<sizeof...(Types)>::Run(SwapSameIndex<Types...>{v, w}, Wi);
    } else {
      generic_swap();
    }
  }
};

template <typename Variant, typename = void, typename... Ts>
struct VariantHashBase {
  VariantHashBase() = delete;
  VariantHashBase(const VariantHashBase&) = delete;
  VariantHashBase(VariantHashBase&&) = delete;
  VariantHashBase& operator=(const VariantHashBase&) = delete;
  VariantHashBase& operator=(VariantHashBase&&) = delete;
};

struct VariantHashVisitor {
  template <typename T>
  size_t operator()(const T& t) {
    return std::hash<T>{}(t);
  }
};

template <typename Variant, typename... Ts>
struct VariantHashBase<Variant,
                       absl::enable_if_t<absl::conjunction<
                           type_traits_internal::IsHashable<Ts>...>::value>,
                       Ts...> {
  using argument_type = Variant;
  using result_type = size_t;
  size_t operator()(const Variant& var) const {
    type_traits_internal::AssertHashEnabled<Ts...>();
    if (var.valueless_by_exception()) {
      return 239799884;
    }
    size_t result = VisitIndices<variant_size<Variant>::value>::Run(
        PerformVisitation<VariantHashVisitor, const Variant&>{
            std::forward_as_tuple(var), VariantHashVisitor{}},
        var.index());
    // Combine the index and the hash result in order to distinguish
    // std::variant<int, int> holding the same value as different alternative.
    return result ^ var.index();
  }
};

}  // namespace variant_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // !defined(ABSL_USES_STD_VARIANT)
#endif  // ABSL_TYPES_variant_internal_H_