variant_variadic.h

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/******************************************************************************
The MIT License(MIT)
 
Embedded Template Library.
https://github.com/ETLCPP/etl
https://www.etlcpp.com
 
Copyright(c) 2021 jwellbelove, Robin S�derholm
 
Permission is hereby granted, free of charge, to any person obtaining a copy
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in the Software without restriction, including without limitation the rights
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#include "../platform.h"
#include "../utility.h"
#include "../largest.h"
#include "../nth_type.h"
#include "../exception.h"
#include "../type_traits.h"
#include "../integral_limits.h"
#include "../static_assert.h"
#include "../alignment.h"
#include "../error_handler.h"
#include "../type_list.h"
#include "../placement_new.h"
#include "../visitor.h"
#include "../memory.h"
#include "../compare.h"
#include "../initializer_list.h"
#include "../monostate.h"
 
#include <stdint.h>
 
#if defined(ETL_COMPILER_KEIL)
  #pragma diag_suppress 940
  #pragma diag_suppress 111
#endif
 
#if ETL_CPP11_NOT_SUPPORTED
  #if !defined(ETL_IN_UNIT_TEST)
    #error NOT SUPPORTED FOR C++03 OR BELOW
  #endif
#else
//*****************************************************************************
//*****************************************************************************
 
namespace etl
{
  namespace private_variant
  {
    //*******************************************
    // The traits an object may have.
    //*******************************************
    static constexpr bool Copyable = true;
    static constexpr bool Non_Copyable = false;
    static constexpr bool Moveable = true;
    static constexpr bool Non_Moveable = false;
 
    //*******************************************
    // The types of operations we can perform.
    //*******************************************
    static constexpr int Copy = 0;
    static constexpr int Move = 1;
    static constexpr int Destroy = 2;
 
    //*******************************************
    // operation_type
    //*******************************************
    template <typename T, bool IsCopyable, bool IsMoveable>
    struct operation_type;
 
    //*******************************************
    // Specialisation for null operation.
    template <>
    struct operation_type<void, Non_Copyable, Non_Moveable>
    {
      static void do_operation(int, char*, const char*)
      {
        // This should never occur.
#if defined(ETL_IN_UNIT_TEST)
        assert(false);
#endif
      }
    };
 
    //*******************************************
    // Specialisation for no-copyable & non-moveable types.
    template <typename T>
    struct operation_type<T, Non_Copyable, Non_Moveable>
    {
      static void do_operation(int operation, char* pstorage, const char* /*pvalue*/)
      {
        switch (operation)
        {
          case Destroy:
          {
            reinterpret_cast<const T*>(pstorage)->~T();
            break;
          }
 
          default:
          {
            // This should never occur.
#if defined(ETL_IN_UNIT_TEST)
            assert(false);
#endif
            break;
          }
        }
      }
    };
 
    //*******************************************
    // Specialisation for no-copyable & moveable types.
    template <typename T>
    struct operation_type<T, Non_Copyable, Moveable>
    {
      static void do_operation(int operation, char* pstorage, const char* pvalue)
      {
        switch (operation)
        {
          case Move:
          {
            ::new (pstorage) T(etl::move(*reinterpret_cast<T*>(const_cast<char*>(pvalue))));
            break;
          }
 
          case Destroy:
          {
            reinterpret_cast<const T*>(pstorage)->~T();
            break;
          }
 
          default:
          {
            // This should never occur.
#if defined(ETL_IN_UNIT_TEST)
            assert(false);
#endif
            break;
          }
        }
      }
    };
 
    //*******************************************
    // Specialisation for copyable & non-moveable types.
    template <typename T>
    struct operation_type<T, Copyable, Non_Moveable>
    {
      static void do_operation(int operation, char* pstorage, const char* pvalue)
      {
        switch (operation)
        {
          case Copy:
          {
            ::new (pstorage) T(*reinterpret_cast<const T*>(pvalue));
            break;
          }
 
          case Destroy:
          {
            reinterpret_cast<const T*>(pstorage)->~T();
            break;
          }
 
          default:
          {
            // This should never occur.
#if defined(ETL_IN_UNIT_TEST)
            assert(false);
#endif
            break;
          }
        }
      }
    };
 
    //*******************************************
    // Specialisation for copyable & moveable types.
    template <typename T>
    struct operation_type<T, Copyable, Moveable>
    {
      static void do_operation(int operation, char* pstorage, const char* pvalue)
      {
        switch (operation)
        {
          case Copy:
          {
            ::new (pstorage) T(*reinterpret_cast<const T*>(pvalue));
            break;
          }
 
          case Move:
          {
            ::new (pstorage) T(etl::move(*reinterpret_cast<T*>(const_cast<char*>(pvalue))));
            break;
          }
 
          case Destroy:
          {
            reinterpret_cast<const T*>(pstorage)->~T();
            break;
          }
 
          default:
          {
            // This should never occur.
#if defined(ETL_IN_UNIT_TEST)
            assert(false);
#endif
            break;
          }
        }
      }
    };
  }

  constexpr size_t variant_npos = etl::integral_limits<size_t>::max;
 
  //***********************************
  // variant. Forward declaration
  template <typename... TTypes>
  class variant;
 
  //***************************************************************************
  //*************************************************************************** 
  template <size_t Index, typename T>
  struct variant_alternative;
 
  template <size_t Index, typename... TTypes>
  struct variant_alternative<Index, etl::variant<TTypes...>>
  {
    using type = etl::nth_type_t<Index, TTypes...>;
  };
 
  template <size_t Index, typename T>
  struct variant_alternative<Index, const T>
  {
    using type = typename variant_alternative<Index, T>::type;
  };
 
  template <size_t Index, typename T>
  using variant_alternative_t = typename variant_alternative<Index, T>::type;
 
  //***********************************
  // holds_alternative. Forward declaration
  template <typename T, typename... TTypes>
  ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) ETL_NOEXCEPT;
 
  //***********************************
  // get. Forward declarations
  template <size_t Index, typename... VTypes>
  ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<VTypes...>>&
    get(etl::variant<VTypes...>& v);
 
  template <size_t Index, typename... VTypes>
  ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<VTypes...>>&&
    get(etl::variant<VTypes...>&& v);
 
  template <size_t Index, typename... VTypes>
  ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<VTypes...>>&
    get(const etl::variant<VTypes...>& v);
 
  template <size_t Index, typename... VTypes>
  ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<VTypes...>>&&
    get(const etl::variant<VTypes...>&& v);
 
  template <typename T, typename... VTypes>
  ETL_CONSTEXPR14 T& get(etl::variant<VTypes...>& v);
 
  template <typename T, typename... VTypes>
  ETL_CONSTEXPR14 T&& get(etl::variant<VTypes...>&& v);
 
  template <typename T, typename... VTypes>
  ETL_CONSTEXPR14 const T& get(const etl::variant<VTypes...>& v);
 
  template <typename T, typename... VTypes>
  ETL_CONSTEXPR14 const T&& get(const etl::variant<VTypes...>&& v);
 
#if ETL_NOT_USING_CPP17
  #include "variant_select_do_visitor.h"
  #include "variant_select_do_operator.h"
#endif
 
  constexpr bool operator >(etl::monostate, etl::monostate) ETL_NOEXCEPT { return false; }
    constexpr bool operator <(etl::monostate, etl::monostate) ETL_NOEXCEPT { return false; }
    constexpr bool operator !=(etl::monostate, etl::monostate) ETL_NOEXCEPT { return false; }
    constexpr bool operator <=(etl::monostate, etl::monostate) ETL_NOEXCEPT { return true; }
    constexpr bool operator >=(etl::monostate, etl::monostate) ETL_NOEXCEPT { return true; }
    constexpr bool operator ==(etl::monostate, etl::monostate) ETL_NOEXCEPT { return true; }
#if ETL_USING_CPP20 && ETL_USING_STL && !(defined(ETL_DEVELOPMENT_OS_APPLE) && defined(ETL_COMPILER_CLANG))
  constexpr std::strong_ordering operator<=>(monostate, monostate) ETL_NOEXCEPT
  {
    return std::strong_ordering::equal;
  }
#endif
 
#if ETL_NOT_USING_STL && !defined(ETL_USE_TYPE_TRAITS_BUILTINS)
  template <>
  struct is_copy_constructible<etl::monostate> : public etl::true_type
  {
  };
 
  template <>
  struct is_move_constructible<etl::monostate> : public etl::true_type
  {
  };
#endif
 
  //***************************************************************************
  //***************************************************************************
  class variant_exception : public exception
  {
  public:
    variant_exception(string_type reason_, string_type file_name_, numeric_type line_number_)
      : exception(reason_, file_name_, line_number_)
    {
    }
  };
 
  //***************************************************************************
  //***************************************************************************
  class variant_incorrect_type_exception : public variant_exception
  {
  public:
    variant_incorrect_type_exception(string_type file_name_, numeric_type line_number_)
      : variant_exception(ETL_ERROR_TEXT("variant:unsupported type", ETL_VARIANT_FILE_ID"A"), file_name_, line_number_)
    {
    }
  };
 
  //***************************************************************************
  //***************************************************************************
  class bad_variant_access : public variant_exception
  {
  public:
    bad_variant_access(string_type file_name_, numeric_type line_number_)
    : variant_exception(ETL_ERROR_TEXT("variant:bad variant access", ETL_VARIANT_FILE_ID"B"), file_name_, line_number_)
    {}
  };
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  class variant
  {
  public:
 
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename... VTypes>
    friend ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<VTypes...>>&
      get(etl::variant<VTypes...>& v);
 
    template <size_t Index, typename... VTypes>
    friend ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<VTypes...>>&&
      get(etl::variant<VTypes...>&& v);
 
    template <size_t Index, typename... VTypes>
    friend ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<VTypes...>>&
      get(const etl::variant<VTypes...>& v);
 
    template <size_t Index, typename... VTypes>
    friend ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<VTypes...>>&&
      get(const etl::variant<VTypes...>&& v);
 
    template <typename T, typename... VTypes>
    friend ETL_CONSTEXPR14 T& get(etl::variant<VTypes...>& v);
 
    template <typename T, typename... VTypes>
    friend ETL_CONSTEXPR14 T&& get(etl::variant<VTypes...>&& v);
 
    template <typename T, typename... VTypes>
    friend ETL_CONSTEXPR14 const T& get(const etl::variant<VTypes...>& v);
 
    template <typename T, typename... VTypes>
    friend ETL_CONSTEXPR14 const T&& get(const etl::variant<VTypes...>&& v);
 
  private:
 
    // All types of variant are friends.
    template <typename... UTypes>
    friend class variant;
 
    //***************************************************************************
    //***************************************************************************
    using largest_t = typename largest_type<TTypes...>::type;
 
    //***************************************************************************
    //***************************************************************************
    static const size_t Size = sizeof(largest_t);
 
    //***************************************************************************
    //***************************************************************************
    static const size_t Alignment = etl::largest_alignment<TTypes...>::value;
 
    //***************************************************************************
    //***************************************************************************
    template <typename T, bool IsCopyable, bool IsMoveable>
    using operation_type = private_variant::operation_type<T, IsCopyable, IsMoveable>;
 
    //*******************************************
    // The types of operations we can perform.
    //*******************************************
    static constexpr int Copy    = private_variant::Copy;
    static constexpr int Move    = private_variant::Move;
    static constexpr int Destroy = private_variant::Destroy;
 
    //*******************************************
    // Get the index of a type.
    //*******************************************
    template <typename T>
    using index_of_type = etl::type_list_index_of_type<etl::type_list<TTypes...>, etl::remove_cvref_t<T>>;
 
    //*******************************************
    // Get the type from the index.
    //*******************************************
    template <size_t Index>
    using type_from_index = typename etl::type_list_type_at_index<etl::type_list<TTypes...>, Index>::type;
 
  public:
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    ETL_CONSTEXPR14 variant()
    {
      using type = type_from_index<0U>;
 
      default_construct_in_place<type>(data);
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
      type_id   = 0U;
    }
#include "diagnostic_pop.h"
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    template <typename T, etl::enable_if_t<!etl::is_same<etl::remove_cvref_t<T>, variant>::value, int> = 0>
    ETL_CONSTEXPR14 variant(T&& value)
      : operation(operation_type<etl::remove_cvref_t<T>, etl::is_copy_constructible<etl::remove_cvref_t<T>>::value, etl::is_move_constructible<etl::remove_cvref_t<T>>::value>::do_operation)
      , type_id(index_of_type<T>::value)
    {
      static_assert(etl::is_one_of<etl::remove_cvref_t<T>, TTypes...>::value, "Unsupported type");
 
      construct_in_place<etl::remove_cvref_t<T>>(data, etl::forward<T>(value));
    }
#include "diagnostic_pop.h"
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    template <typename T, typename... TArgs>
    ETL_CONSTEXPR14 explicit variant(etl::in_place_type_t<T>, TArgs&&... args)
      : operation(operation_type<etl::remove_cvref_t<T>, etl::is_copy_constructible<etl::remove_cvref_t<T>>::value, etl::is_move_constructible<etl::remove_cvref_t<T>>::value>::do_operation)
      , type_id(index_of_type<T>::value)
    {
      static_assert(etl::is_one_of<etl::remove_cvref_t<T>, TTypes...>::value, "Unsupported type");
 
      construct_in_place_args<etl::remove_cvref_t<T>>(data, etl::forward<TArgs>(args)...);
    }
#include "diagnostic_pop.h"
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    template <size_t Index, typename... TArgs>
    ETL_CONSTEXPR14 explicit variant(etl::in_place_index_t<Index>, TArgs&&... args)
      : type_id(Index)
    {
      using type = type_from_index<Index>;
      static_assert(etl::is_one_of<type, TTypes...> ::value, "Unsupported type");
 
      construct_in_place_args<type>(data, etl::forward<TArgs>(args)...);
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
    }
#include "diagnostic_pop.h"
 
#if ETL_HAS_INITIALIZER_LIST
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    template <typename T, typename U, typename... TArgs >
    ETL_CONSTEXPR14 explicit variant(etl::in_place_type_t<T>, std::initializer_list<U> init, TArgs&&... args)
      : operation(operation_type<etl::remove_cvref_t<T>, etl::is_copy_constructible<etl::remove_cvref_t<T>>::value, etl::is_move_constructible<etl::remove_cvref_t<T>>::value>::do_operation)
      , type_id(index_of_type<T>::value)
    {
      static_assert(etl::is_one_of<etl::remove_cvref_t<T>, TTypes...> ::value, "Unsupported type");
 
      construct_in_place_args<etl::remove_cvref_t<T>>(data, init, etl::forward<TArgs>(args)...);
    }
#include "diagnostic_pop.h"
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    template <size_t Index, typename U, typename... TArgs >
    ETL_CONSTEXPR14 explicit variant(etl::in_place_index_t<Index>, std::initializer_list<U> init, TArgs&&... args)
      : type_id(Index)
    {
      using type = type_from_index<Index>;
      static_assert(etl::is_one_of<type, TTypes...> ::value, "Unsupported type");
 
      construct_in_place_args<type>(data, init, etl::forward<TArgs>(args)...);
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
    }
#include "diagnostic_pop.h"
#endif
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    ETL_CONSTEXPR14 variant(const variant& other)
      : operation(other.operation)
      , type_id(other.type_id)
    {
      if (this != &other)
      {
        if (other.index() == variant_npos)
        {
          type_id = variant_npos;
        }
        else
        {
          operation(private_variant::Copy, data, other.data);
        }
      }
    }
#include "diagnostic_pop.h"
 
    //***************************************************************************
    //***************************************************************************
#include "diagnostic_uninitialized_push.h"
    ETL_CONSTEXPR14 variant(variant&& other)
      : operation(other.operation)
      , type_id(other.type_id)
    {
      if (this != &other)
      {
        if (other.index() == variant_npos)
        {
          type_id = variant_npos;
        }
        else
        {
          operation(private_variant::Move, data, other.data);
        }
      }
      else
      {
        type_id = variant_npos;
      }
    }
#include "diagnostic_pop.h"
 
    //***************************************************************************
    //***************************************************************************
    ~variant()
    {
      if (index() != variant_npos)
      {
        operation(private_variant::Destroy, data, nullptr);
      }
 
      operation = operation_type<void, false, false>::do_operation; // Null operation.
      type_id = variant_npos;
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename T, typename... TArgs>
    T& emplace(TArgs&&... args)
    {
      static_assert(etl::is_one_of<T, TTypes...>::value, "Unsupported type");
 
      using type = etl::remove_cvref_t<T>;
 
      operation(private_variant::Destroy, data, nullptr);
 
      construct_in_place_args<type>(data, etl::forward<TArgs>(args)...);
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
 
      type_id = index_of_type<T>::value;
 
      return *static_cast<T*>(data);
    }
 
#if ETL_HAS_INITIALIZER_LIST
    //***************************************************************************
    //***************************************************************************
    template <typename T, typename U, typename... TArgs>
    T& emplace(std::initializer_list<U> il, TArgs&&... args)
    {
      static_assert(etl::is_one_of<T, TTypes...>::value, "Unsupported type");
 
      using type = etl::remove_cvref_t<T>;
 
      operation(private_variant::Destroy, data, nullptr);
 
      construct_in_place_args<type>(data, il, etl::forward<TArgs>(args)...);
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
 
      type_id = index_of_type<T>::value;
 
      return *static_cast<T*>(data);
    }
#endif
 
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename... TArgs>
    typename etl::variant_alternative_t<Index, variant<TTypes...>>& emplace(TArgs&&... args)
    {
      static_assert(Index < sizeof...(TTypes), "Index out of range");
 
      using type = type_from_index<Index>;
 
      operation(private_variant::Destroy, data, nullptr);
 
      construct_in_place_args<type>(data, etl::forward<TArgs>(args)...);
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
 
      type_id = Index;
 
      return *static_cast<type*>(data);
    }
 
#if ETL_HAS_INITIALIZER_LIST
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename U, typename... TArgs>
    typename etl::variant_alternative_t<Index, variant<TTypes...>>& emplace(std::initializer_list<U> il, TArgs&&... args)
    {
      static_assert(Index < sizeof...(TTypes), "Index out of range");
 
      using type = type_from_index<Index>;
 
      operation(private_variant::Destroy, data, nullptr);
 
      construct_in_place_args<type>(data, il, etl::forward<TArgs>(args)...);
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
 
      type_id = Index;
 
      return *static_cast<type*>(data);
    }
#endif
 
    //***************************************************************************
    //***************************************************************************
    template <typename T, etl::enable_if_t<!etl::is_same<etl::remove_cvref_t<T>, variant>::value, int> = 0>
    variant& operator =(T&& value)
    {
      using type = etl::remove_cvref_t<T>;
 
      static_assert(etl::is_one_of<type, TTypes...>::value, "Unsupported type");
 
      operation(private_variant::Destroy, data, nullptr);
 
      construct_in_place<type>(data, etl::forward<T>(value));
 
      operation = operation_type<type, etl::is_copy_constructible<type>::value, etl::is_move_constructible<type>::value>::do_operation;
      type_id   = index_of_type<type>::value;
 
      return *this;
    }
 
    //***************************************************************************
    //***************************************************************************
    variant& operator =(const variant& other)
    {
      if (this != &other)
      {
        if (other.index() == variant_npos)
        {
          type_id = variant_npos;
        }
        else
        {
          operation(Destroy, data, nullptr);
 
          operation = other.operation;
          operation(Copy, data, other.data);
 
          type_id = other.type_id;
        }
      }
 
      return *this;
    }
 
    //***************************************************************************
    //***************************************************************************
    variant& operator =(variant&& other)
    {
      if (this != &other)
      {
        if (other.index() == variant_npos)
        {
          type_id = variant_npos;
        }
        else
        {
          operation(Destroy, data, nullptr);
 
          operation = other.operation;
          operation(Move, data, other.data);
 
          type_id = other.type_id;
        }
      }
 
      return *this;
    }
 
    //***************************************************************************
    //***************************************************************************
    constexpr bool valueless_by_exception() const ETL_NOEXCEPT
    {
      return type_id == variant_npos;
    }
 
    //***************************************************************************
    //***************************************************************************
    constexpr size_t index() const ETL_NOEXCEPT
    {
      return type_id;
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename T>
    static constexpr bool is_supported_type()
    {
      return etl::is_one_of<etl::remove_cvref_t<T>, TTypes...>::value;
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename T, etl::enable_if_t<is_supported_type<T>(), int> = 0>
    constexpr bool is_type() const ETL_NOEXCEPT
    {
      return (type_id == index_of_type<T>::value);
    }
 
    //***************************************************************************
    template <typename T, etl::enable_if_t<!is_supported_type<T>(), int> = 0>
    constexpr bool is_type() const ETL_NOEXCEPT
    {
      return false;
    }
 
    //***************************************************************************
    //***************************************************************************
    constexpr bool is_same_type(const variant& other) const
    {
      return type_id == other.type_id;
    }
 
    //***************************************************************************
    //***************************************************************************
    void swap(variant& rhs) ETL_NOEXCEPT
    {
      variant temp(etl::move(*this));
      *this = etl::move(rhs);
      rhs = etl::move(temp);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
    etl::enable_if_t<etl::is_visitor<TVisitor>::value, void>
      accept(TVisitor& v)
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_visitor(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_visitor<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
    etl::enable_if_t<etl::is_visitor<TVisitor>::value, void>
      accept(TVisitor& v) const
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_visitor(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_visitor<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
    etl::enable_if_t<!etl::is_visitor<TVisitor>::value, void>
      accept(TVisitor& v)
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_operator(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_operator<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
    etl::enable_if_t<!etl::is_visitor<TVisitor>::value, void>
      accept(TVisitor& v) const
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_operator(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_operator<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
#if !defined(ETL_IN_UNIT_TEST)
    ETL_DEPRECATED_REASON("Replace with accept()")
#endif
    void accept_visitor(TVisitor& v)
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_visitor(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_visitor<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
#if !defined(ETL_IN_UNIT_TEST)
    ETL_DEPRECATED_REASON("Replace with accept()")
#endif
    void accept_visitor(TVisitor& v) const
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_visitor(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_visitor<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
#if !defined(ETL_IN_UNIT_TEST)
    ETL_DEPRECATED_REASON("Replace with accept()")
#endif
    void accept_functor(TVisitor& v)
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_operator(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_operator<sizeof...(TTypes)>(v);
#endif
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor>
#if !defined(ETL_IN_UNIT_TEST)
    ETL_DEPRECATED_REASON("Replace with accept()")
#endif
    void accept_functor(TVisitor& v) const
    {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
      do_operator(v, etl::make_index_sequence<sizeof...(TTypes)>{});
#else
      do_operator<sizeof...(TTypes)>(v);
#endif
    }
 
  private:

    using operation_function = void(*)(int, char*, const char*);
 
    //***************************************************************************
    //***************************************************************************
    template <typename T>
    static void construct_in_place(char* pstorage, const T& value)
    {
      using type = etl::remove_cvref_t<T>;
 
      ::new (pstorage) type(value);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename T>
    static void construct_in_place(char* pstorage, T&& value)
    {
      using type = etl::remove_cvref_t<T>;
 
      ::new (pstorage) type(etl::move(value));
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename T, typename... TArgs>
    static void construct_in_place_args(char* pstorage, TArgs&&... args)
    {
      using type = etl::remove_cvref_t<T>;
 
      ::new (pstorage) type(etl::forward<TArgs>(args)...);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename T>
    static void default_construct_in_place(char* pstorage)
    {
      using type = etl::remove_cvref_t<T>;
 
      ::new (pstorage) type();
    }
 
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor, size_t... I>
    void do_visitor(TVisitor& visitor, etl::index_sequence<I...>)
    {
      (attempt_visitor<I>(visitor) || ...);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor, size_t... I>
    void do_visitor(TVisitor& visitor, etl::index_sequence<I...>) const
    {
      (attempt_visitor<I>(visitor) || ...);
    }
#else
    //***************************************************************************
    //***************************************************************************
    template <size_t NTypes, typename TVisitor>
    void do_visitor(TVisitor& visitor)
    {
      etl::private_variant::select_do_visitor<NTypes>::do_visitor(*this, visitor);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <size_t NTypes, typename TVisitor>
    void do_visitor(TVisitor& visitor) const
    {
      etl::private_variant::select_do_visitor<NTypes>::do_visitor(*this, visitor);
    }
#endif
 
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename TVisitor>
    bool attempt_visitor(TVisitor& visitor)
    {
      if (Index == index())
      {
        // Workaround for MSVC (2023/05/13)
        // It doesn't compile 'visitor.visit(etl::get<Index>(*this))' correctly for C++17 & C++20.
        // Changed all of the instances for consistency.
        auto& v = etl::get<Index>(*this);
        visitor.visit(v);
        return true;
      }
      else
      {
        return false;
      }
    }
 
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename TVisitor>
    bool attempt_visitor(TVisitor& visitor) const
    {
      if (Index == index())
      {
        // Workaround for MSVC (2023/05/13)
        // It doesn't compile 'visitor.visit(etl::get<Index>(*this))' correctly for C++17 & C++20.
        // Changed all of the instances for consistency.
        auto& v = etl::get<Index>(*this);
        visitor.visit(v);
        return true;
      }
      else
      {
        return false;
      }
    }
 
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor, size_t... I>
    void do_operator(TVisitor& visitor, etl::index_sequence<I...>)
    {
      (attempt_operator<I>(visitor) || ...);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVisitor, size_t... I>
    void do_operator(TVisitor& visitor, etl::index_sequence<I...>) const
    {
      (attempt_operator<I>(visitor) || ...);
    }
#else
    //***************************************************************************
    //***************************************************************************
    template <size_t NTypes, typename TVisitor>
    void do_operator(TVisitor& visitor)
    {
#if defined(ETL_VARIANT_CPP11_MAX_8_TYPES)
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 8U, "ETL_VARIANT_CPP11_MAX_8_TYPES - Only a maximum of 8 types are allowed in this variant");
#endif
 
#if defined(ETL_VARIANT_CPP11_MAX_16_TYPES)
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 16U, "ETL_VARIANT_CPP11_MAX_16_TYPES - Only a maximum of 16 types are allowed in this variant");
#endif
 
#if defined(ETL_VARIANT_CPP11_MAX_24_TYPES)
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 24U, "ETL_VARIANT_CPP11_MAX_24_TYPES - Only a maximum of 24 types are allowed in this variant");
#endif
 
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 32U, "A maximum of 32 types are allowed in this variant");
 
      etl::private_variant::select_do_operator<NTypes>::do_operator(*this, visitor);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <size_t NTypes, typename TVisitor>
    void do_operator(TVisitor& visitor) const
    {
#if defined(ETL_VARIANT_CPP11_MAX_8_TYPES)
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 8U, "ETL_VARIANT_CPP11_MAX_8_TYPES - Only a maximum of 8 types are allowed in this variant");
#endif
 
#if defined(ETL_VARIANT_CPP11_MAX_16_TYPES)
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 16U, "ETL_VARIANT_CPP11_MAX_16_TYPES - Only a maximum of 16 types are allowed in this variant");
#endif
 
#if defined(ETL_VARIANT_CPP11_MAX_24_TYPES)
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 24U, "ETL_VARIANT_CPP11_MAX_24_TYPES - Only a maximum of 24 types are allowed in this variant");
#endif
 
      ETL_STATIC_ASSERT(sizeof...(TTypes) <= 32U, "A maximum of 32 types are allowed in this variant");
 
      etl::private_variant::select_do_operator<NTypes>::do_operator(*this, visitor);
    }
#endif
 
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename TVisitor>
    bool attempt_operator(TVisitor& visitor)
    {
      if (Index == index())
      {
        auto& v = etl::get<Index>(*this);
        visitor(v);
        return true;
      }
      else
      {
        return false;
      }
    }
 
    //***************************************************************************
    //***************************************************************************
    template <size_t Index, typename TVisitor>
    bool attempt_operator(TVisitor& visitor) const
    {
      if (Index == index())
      {
        auto& v = etl::get<Index>(*this);
        visitor(v);
        return true;
      }
      else
      {
        return false;
      }
    }
 
    //***************************************************************************
    //***************************************************************************
    etl::uninitialized_buffer<Size, 1U, Alignment> data;
 
    //***************************************************************************
    //***************************************************************************
    operation_function operation;
 
    //***************************************************************************
    //***************************************************************************
    size_t type_id;
  };
 
  //***************************************************************************
  //***************************************************************************
    template <typename T, typename... TTypes>
    ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) ETL_NOEXCEPT
    {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    return (Index == variant_npos) ? false : (v.index() == Index);
    }
 
  //***************************************************************************
  //***************************************************************************
  template <size_t Index, typename... TTypes>
  ETL_CONSTEXPR14 bool holds_alternative(const etl::variant<TTypes...>& v) ETL_NOEXCEPT
  {
    return (Index == v.index());
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool holds_alternative(size_t index, const etl::variant<TTypes...>& v) ETL_NOEXCEPT
  {
    return (index == v.index());
  }
 
  //***************************************************************************
  //***************************************************************************
  template <size_t Index, typename... TTypes>
  ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<TTypes...>>&
    get(etl::variant<TTypes...>& v)
  {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
    static_assert(Index < sizeof...(TTypes), "Index out of range");
#endif
 
    ETL_ASSERT(Index == v.index(), ETL_ERROR(etl::variant_incorrect_type_exception));
 
        using type = etl::variant_alternative_t<Index, etl::variant<TTypes...>>;
 
    return *static_cast<type*>(v.data);
  }
 
  //***********************************
  template <size_t Index, typename... TTypes>
  ETL_CONSTEXPR14 etl::variant_alternative_t<Index, etl::variant<TTypes...>>&&
    get(etl::variant<TTypes...>&& v)
  {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
    static_assert(Index < sizeof...(TTypes), "Index out of range");
#endif
 
        using type = etl::variant_alternative_t<Index, etl::variant<TTypes...>>;
 
    return etl::move(*static_cast<type*>(v.data));
  }
 
  //***********************************
  template <size_t Index, typename... TTypes>
  ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<TTypes...>>&
    get(const etl::variant<TTypes...>& v)
  {
#if ETL_USING_CPP17 && !defined(ETL_VARIANT_FORCE_CPP11)
    static_assert(Index < sizeof...(TTypes), "Index out of range");
#endif
 
    ETL_ASSERT(Index == v.index(), ETL_ERROR(etl::variant_incorrect_type_exception));
 
        using type = etl::variant_alternative_t<Index, etl::variant<TTypes...>>;
 
    return *static_cast<const type*>(v.data);
  }
 
  //***********************************
  template <size_t Index, typename... TTypes>
  ETL_CONSTEXPR14 const etl::variant_alternative_t<Index, const etl::variant<TTypes...>>&&
    get(const etl::variant<TTypes...>&& v)
  {
#if ETL_USING_CPP17 & !defined(ETL_VARIANT_FORCE_CPP11)
    static_assert(Index < sizeof...(TTypes), "Index out of range");
#endif
 
    ETL_ASSERT(Index == v.index(), ETL_ERROR(etl::variant_incorrect_type_exception));
 
        using type = etl::variant_alternative_t<Index, etl::variant<TTypes...>>;
 
    return etl::move(*static_cast<const type*>(v.data));
  }
 
  //***********************************
  template <typename T, typename... TTypes>
  ETL_CONSTEXPR14 T& get(etl::variant<TTypes...>& v)
  {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    return get<Index>(v);
  }
 
  //***********************************
  template <typename T, typename... TTypes>
  ETL_CONSTEXPR14 T&& get(etl::variant<TTypes...>&& v)
  {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    return get<Index>(etl::move(v));
  }
 
  //***********************************
  template <typename T, typename... TTypes>
  ETL_CONSTEXPR14 const T& get(const etl::variant<TTypes...>& v)
  {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    return get<Index>(v);
  }
 
  //***********************************
  template <typename T, typename... TTypes>
  ETL_CONSTEXPR14 const T&& get(const etl::variant<TTypes...>&& v)
  {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    return get<Index>(etl::move(v));
  }
 
  //***************************************************************************
  //***************************************************************************
  template< size_t Index, typename... TTypes >
  ETL_CONSTEXPR14 etl::add_pointer_t<etl::variant_alternative_t<Index, etl::variant<TTypes...>>>
    get_if(etl::variant<TTypes...>* pv) ETL_NOEXCEPT
  {
    if ((pv != nullptr) && (pv->index() == Index))
    {
      return &etl::get<Index>(*pv);
    }
    else
    {
      return nullptr;
    }
  }
 
  //***********************************
  template< size_t Index, typename... TTypes >
  ETL_CONSTEXPR14 etl::add_pointer_t<const etl::variant_alternative_t<Index, etl::variant<TTypes...>>>
    get_if(const etl::variant<TTypes...>* pv) ETL_NOEXCEPT
  {
    if ((pv != nullptr) && (pv->index() == Index))
    {
      return &etl::get<Index>(*pv);
    }
    else
    {
      return nullptr;
    }
  }
 
  //***********************************
  template< class T, typename... TTypes >
  ETL_CONSTEXPR14 etl::add_pointer_t<T> get_if(etl::variant<TTypes...>* pv) ETL_NOEXCEPT
  {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    if ((pv != nullptr) && (pv->index() == Index))
    {
      return &etl::get<Index>(*pv);
    }
    else
    {
      return nullptr;
    }
  }
 
  //***********************************
  template< typename T, typename... TTypes >
  ETL_CONSTEXPR14 etl::add_pointer_t<const T> get_if(const etl::variant<TTypes...>* pv) ETL_NOEXCEPT
  {
    constexpr size_t Index = etl::type_list_index_of_type<etl::type_list<TTypes...>, T>::value;
 
    if ((pv != nullptr) && (pv->index() == Index))
    {
      return &etl::get<Index>(*pv);
    }
    else
    {
      return nullptr;
    }
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  void swap(etl::variant<TTypes...>& lhs, etl::variant<TTypes...>& rhs)
  {
    lhs.swap(rhs);
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename T>
  struct variant_size;
 
  template <typename... TTypes>
  struct variant_size<etl::variant<TTypes...>>
    : etl::integral_constant<size_t, sizeof...(TTypes)>
  {
  };
 
  template <typename T>
  struct variant_size<const T>
    : etl::integral_constant<size_t, variant_size<T>::value>
  {
  };
 
#if ETL_USING_CPP17
  template <typename... TTypes>
  inline constexpr size_t variant_size_v = variant_size<TTypes...>::value;
#endif
 
  //***************************************************************************
  //***************************************************************************
  namespace private_variant
  {
    template <typename TRet, typename TCallable, typename TVariant, size_t tIndex, typename TNext, typename... TVariants>
    static ETL_CONSTEXPR14 TRet do_visit_single(TCallable&& f, TVariant&& v, TNext&&, TVariants&&... vs);
 
    //***************************************************************************
    //***************************************************************************
    struct visit_auto_return
    {
    };
 
    //***************************************************************************
    //***************************************************************************
    template <typename TCallable, typename... Ts>
    struct single_visit_result_type
    {
      using type = decltype(declval<TCallable>()(declval<Ts>()...));
    };
 
    template <typename TCallable, typename... Ts>
    using single_visit_result_type_t = typename single_visit_result_type<TCallable, Ts...>::type;
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVar, typename T>
    using rlref_copy = conditional_t<is_reference<TVar>::value, T&, T&&>;
 
    //***************************************************************************
    //***************************************************************************
    template <template <typename...> class, typename...>
    struct visit_result_helper;
 
    template <template <typename...> class TToInject, size_t... tAltIndices, typename TCur>
    struct visit_result_helper<TToInject, index_sequence<tAltIndices...>, TCur>
    {
      template <size_t tIndex>
      using var_type = rlref_copy<TCur,
                                  variant_alternative_t<tIndex, remove_reference_t<TCur> > >;
 
      using type = common_type_t<TToInject<var_type<tAltIndices> >...>;
    };
 
    template <template <typename...> class TToInject, size_t... tAltIndices, typename TCur, typename TNext, typename... TVs>
    struct visit_result_helper<TToInject, index_sequence<tAltIndices...>, TCur, TNext, TVs...>
    {
      template <size_t tIndex>
      using var_type = rlref_copy<TCur, variant_alternative_t<tIndex, remove_reference_t<TCur> > >;
 
      template <size_t tIndex>
      struct next_inject_wrap
      {
        template <typename... TNextInj>
        using next_inject = TToInject<var_type<tIndex>, TNextInj...>;
        using recursive_result = typename visit_result_helper<next_inject, make_index_sequence<variant_size<remove_reference_t<TNext> >::value>, TNext, TVs...>::type;
      };
 
      using type = common_type_t<typename next_inject_wrap<tAltIndices>::recursive_result...>;
    };
 
    //***************************************************************************
    //***************************************************************************
    template <typename TRet, typename...>
    struct visit_result
    {
      using type = TRet;
    };
 
    template <typename TCallable, typename T1, typename... Ts>
    struct visit_result<visit_auto_return, TCallable, T1, Ts...>
    {
      // bind TCallable to the first argument in this variadic alias.
      template <typename... Ts2>
      using single_res = single_visit_result_type_t<TCallable, Ts2...>;
      using type = typename visit_result_helper<single_res, make_index_sequence<variant_size<remove_reference_t<T1> >::value>, T1, Ts...>::type;
    };
 
    template <typename... Ts>
    using visit_result_t = typename visit_result<Ts...>::type;
 
    //***************************************************************************
    //***************************************************************************
    template <typename TRet, typename TCallable, typename TVariant, size_t tIndex>
    constexpr TRet do_visit_single(TCallable&& f, TVariant&& v)
    {
      return static_cast<TCallable&&>(f)(etl::get<tIndex>(static_cast<TVariant&&>(v)));
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TRet, typename TCallable, typename TCurVariant, typename... TVarRest>
    struct do_visit_helper
    {
      using function_pointer = add_pointer_t<TRet(TCallable&&, TCurVariant&&, TVarRest&&...)>;
 
      template <size_t tIndex>
      static constexpr function_pointer fptr() ETL_NOEXCEPT
      {
        return &do_visit_single<TRet, TCallable, TCurVariant, tIndex, TVarRest...>;
      }
    };
 
    //***************************************************************************
    //***************************************************************************
    template <typename TRet, typename TCallable, typename TVariant, size_t... tIndices, typename... TVarRest>
    static ETL_CONSTEXPR14 TRet do_visit(TCallable&& f, TVariant&& v, index_sequence<tIndices...>, TVarRest&&... variants)
    {
      ETL_ASSERT(!v.valueless_by_exception(), ETL_ERROR(bad_variant_access));
 
      using helper_t = do_visit_helper<TRet, TCallable, TVariant, TVarRest...>;
      using func_ptr = typename helper_t::function_pointer;
 
      constexpr func_ptr jmp_table[]
      {
        helper_t::template fptr<tIndices>()...
      };
 
      return jmp_table[v.index()](static_cast<TCallable&&>(f), static_cast<TVariant&&>(v), static_cast<TVarRest&&>(variants)...);
    }
 
    template <typename TRet, typename TCallable, typename TVariant, typename... TVs>
    static ETL_CONSTEXPR14 TRet visit(TCallable&& f, TVariant&& v, TVs&&... vs)
    {
      constexpr size_t variants = etl::variant_size<typename remove_reference<TVariant>::type>::value;
      return private_variant::do_visit<TRet>(static_cast<TCallable&&>(f),
                                             static_cast<TVariant&&>(v),
                                             make_index_sequence<variants>{},
                                             static_cast<TVs&&>(vs)...);
    }
 
    //***************************************************************************
    //***************************************************************************
    template <typename TRet, typename TCallable, typename TVariant, size_t tIndex>
    class constexpr_visit_closure
    {
      add_pointer_t<TCallable> callable_;
      add_pointer_t<TVariant>  variant_;
 
    public:
      constexpr constexpr_visit_closure(TCallable&& c, TVariant&& v)
        : callable_(&c), variant_(&v)
      {
      }
 
      template <typename... Ts>
      ETL_CONSTEXPR14 TRet operator()(Ts&&... args) const
      {
        return static_cast<TCallable&&>(*callable_)(get<tIndex>(static_cast<TVariant&&>(*variant_)), static_cast<Ts&&>(args)...);
      }
    };
 
    template <typename TRet, typename TCallable, typename TVariant, size_t tIndex, typename TNext, typename... TVariants>
    static ETL_CONSTEXPR14 TRet do_visit_single(TCallable&& f, TVariant&& v, TNext&& next, TVariants&&... vs)
    {
      return private_variant::visit<TRet>(constexpr_visit_closure<TRet, TCallable, TVariant, tIndex>(static_cast<TCallable&&>(f), static_cast<TVariant&&>(v)),
                                          static_cast<TNext&&>(next), static_cast<TVariants&&>(vs)...);
    }
 
  }  // namespace private_variant
 
  //***************************************************************************
  //***************************************************************************
  template <typename TRet = private_variant::visit_auto_return, typename... TVariants, typename TCallable, typename TDeducedReturn = private_variant::visit_result_t<TRet, TCallable, TVariants...> >
  static ETL_CONSTEXPR14 TDeducedReturn visit(TCallable&& f, TVariants&&... vs)
  {
    return private_variant::visit<TDeducedReturn>(static_cast<TCallable&&>(f), static_cast<TVariants&&>(vs)...);
  }
 
  namespace private_variant
  {
    //***************************************************************************
    //***************************************************************************
    template <typename TVariant>
    struct equality_visitor
    {
      equality_visitor(const TVariant& rhs_)
        : rhs(rhs_)
      {
      }
 
      template <typename TValue>
      bool operator()(const TValue& lhs_downcasted)
      {
        return lhs_downcasted == etl::get<TValue>(rhs);
      }
 
      const TVariant& rhs;
    };
 
    //***************************************************************************
    //***************************************************************************
    template <typename TVariant>
    struct less_than_visitor
    {
      less_than_visitor(const TVariant& rhs_)
        : rhs(rhs_)
      {
      }
 
      template <typename TValue>
      bool operator()(const TValue& lhs_downcasted)
      {
        return lhs_downcasted < etl::get<TValue>(rhs);
      }
 
      const TVariant& rhs;
    };
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool operator ==(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    // If both variants are valueless, they are considered equal
    if (lhs.valueless_by_exception() && rhs.valueless_by_exception())
    {
      return true;
    }
 
    // If one variant is valueless and the other is not, they are not equal
    if (lhs.valueless_by_exception() || rhs.valueless_by_exception())
    {
      return false;
    }
 
    // If variants have different types, they are not equal
    if (lhs.index() != rhs.index())
    {
      return false;
    }
 
    // Variants have the same type, apply the equality operator for the contained values
    private_variant::equality_visitor<etl::variant<TTypes...>> visitor(rhs);
 
    return etl::visit(visitor, lhs);
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool operator !=(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    return !(lhs == rhs);
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool operator <(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    // If both variants are valueless, they are considered equal, so not less than
    if (lhs.valueless_by_exception() && rhs.valueless_by_exception())
    {
      return false;
    }
 
    // A valueless variant is always less than a variant with a value
    if (lhs.valueless_by_exception())
    {
      return true;
    }
 
    // A variant with a value is never less than a valueless variant
    if (rhs.valueless_by_exception())
    {
      return false;
    }
 
    // If variants have different types, compare the type index
    if (lhs.index() != rhs.index())
    {
      return lhs.index() < rhs.index();
    }
 
    // Variants have the same type, apply the less than operator for the contained values
    private_variant::less_than_visitor<etl::variant<TTypes...>> visitor(rhs);
 
    return etl::visit(visitor, lhs);
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool operator >(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    return (rhs < lhs);
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool operator <=(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    return  !(lhs > rhs);
  }
 
  //***************************************************************************
  //***************************************************************************
  template <typename... TTypes>
  ETL_CONSTEXPR14 bool operator >=(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    return !(lhs < rhs);
  }
 
  namespace private_variant
  {
#if ETL_USING_CPP20 && ETL_USING_STL && !(defined(ETL_DEVELOPMENT_OS_APPLE) && defined(ETL_COMPILER_CLANG))
    //***************************************************************************
    //***************************************************************************
    template <typename TVariant>
    struct compare_visitor
    {
      compare_visitor(const TVariant& rhs_)
        : rhs(rhs_)
      {
      }
 
      template <typename TValue>
      std::strong_ordering operator()(const TValue& lhs_downcasted)
      {
        return lhs_downcasted <=> etl::get<TValue>(rhs);
      }
 
      const TVariant& rhs;
    };
#endif
  }
 
  //***************************************************************************
  //***************************************************************************
#if ETL_USING_CPP20 && ETL_USING_STL && !(defined(ETL_DEVELOPMENT_OS_APPLE) && defined(ETL_COMPILER_CLANG))
  template <typename... TTypes>
  ETL_CONSTEXPR14 
  std::common_comparison_category_t<std::compare_three_way_result_t<TTypes>...>
    operator <=>(const etl::variant<TTypes...>& lhs, const etl::variant<TTypes...>& rhs)
  {
    if (lhs.valueless_by_exception() && rhs.valueless_by_exception())
    {
      return std::strong_ordering::equal;
    }
    else if (lhs.valueless_by_exception())
    {
      return std::strong_ordering::less;
    }
    else if (rhs.valueless_by_exception())
    {
      return std::strong_ordering::greater;
    }
    else if (lhs.index() != rhs.index())
    {
      return lhs.index() <=> rhs.index();
    }
    else
    {
      // Variants have the same type, apply the equality operator for the contained values
      private_variant::compare_visitor<etl::variant<TTypes...>> visitor(rhs);
 
      return etl::visit(visitor, lhs);
    }
  }
#endif
}
#endif