<h2>Editing File: /usr/include/c++/8/variant</h2><form method='post'><textarea name='content' class='file-edit'>// &lt;variant&gt; -*- C++ -*-

// Copyright (C) 2016-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// &lt;http://www.gnu.org/licenses/&gt;.

/** @file variant
 *  This is the &lt;variant&gt; C++ Library header.
 */

#ifndef _GLIBCXX_VARIANT
#define _GLIBCXX_VARIANT 1

#pragma GCC system_header

#if __cplusplus &gt;= 201703L

#include &lt;type_traits&gt;
#include &lt;utility&gt;
#include &lt;bits/enable_special_members.h&gt;
#include &lt;bits/functexcept.h&gt;
#include &lt;bits/move.h&gt;
#include &lt;bits/functional_hash.h&gt;
#include &lt;bits/invoke.h&gt;
#include &lt;ext/aligned_buffer.h&gt;
#include &lt;bits/parse_numbers.h&gt;
#include &lt;bits/stl_iterator_base_types.h&gt;
#include &lt;bits/stl_iterator_base_funcs.h&gt;
#include &lt;bits/stl_construct.h&gt;

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

namespace __detail
{
namespace __variant
{
  template&lt;size_t _Np, typename... _Types&gt;
    struct _Nth_type;

  template&lt;size_t _Np, typename _First, typename... _Rest&gt;
    struct _Nth_type&lt;_Np, _First, _Rest...&gt;
    : _Nth_type&lt;_Np-1, _Rest...&gt; { };

  template&lt;typename _First, typename... _Rest&gt;
    struct _Nth_type&lt;0, _First, _Rest...&gt;
    { using type = _First; };

} // namespace __variant
} // namespace __detail

#define __cpp_lib_variant 201606L

  template&lt;typename... _Types&gt; class tuple;
  template&lt;typename... _Types&gt; class variant;
  template &lt;typename&gt; struct hash;

  template&lt;typename _Variant&gt;
    struct variant_size;

  template&lt;typename _Variant&gt;
    struct variant_size&lt;const _Variant&gt; : variant_size&lt;_Variant&gt; {};

  template&lt;typename _Variant&gt;
    struct variant_size&lt;volatile _Variant&gt; : variant_size&lt;_Variant&gt; {};

  template&lt;typename _Variant&gt;
    struct variant_size&lt;const volatile _Variant&gt; : variant_size&lt;_Variant&gt; {};

  template&lt;typename... _Types&gt;
    struct variant_size&lt;variant&lt;_Types...&gt;&gt;
    : std::integral_constant&lt;size_t, sizeof...(_Types)&gt; {};

  template&lt;typename _Variant&gt;
    inline constexpr size_t variant_size_v = variant_size&lt;_Variant&gt;::value;

  template&lt;size_t _Np, typename _Variant&gt;
    struct variant_alternative;

  template&lt;size_t _Np, typename _First, typename... _Rest&gt;
    struct variant_alternative&lt;_Np, variant&lt;_First, _Rest...&gt;&gt;
    : variant_alternative&lt;_Np-1, variant&lt;_Rest...&gt;&gt; {};

  template&lt;typename _First, typename... _Rest&gt;
    struct variant_alternative&lt;0, variant&lt;_First, _Rest...&gt;&gt;
    { using type = _First; };

  template&lt;size_t _Np, typename _Variant&gt;
    using variant_alternative_t =
      typename variant_alternative&lt;_Np, _Variant&gt;::type;

  template&lt;size_t _Np, typename _Variant&gt;
    struct variant_alternative&lt;_Np, const _Variant&gt;
    { using type = add_const_t&lt;variant_alternative_t&lt;_Np, _Variant&gt;&gt;; };

  template&lt;size_t _Np, typename _Variant&gt;
    struct variant_alternative&lt;_Np, volatile _Variant&gt;
    { using type = add_volatile_t&lt;variant_alternative_t&lt;_Np, _Variant&gt;&gt;; };

  template&lt;size_t _Np, typename _Variant&gt;
    struct variant_alternative&lt;_Np, const volatile _Variant&gt;
    { using type = add_cv_t&lt;variant_alternative_t&lt;_Np, _Variant&gt;&gt;; };

  inline constexpr size_t variant_npos = -1;

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&amp;
    get(variant&lt;_Types...&gt;&amp;);

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&amp;&amp;
    get(variant&lt;_Types...&gt;&amp;&amp;);

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt; const&amp;
    get(const variant&lt;_Types...&gt;&amp;);

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt; const&amp;&amp;
    get(const variant&lt;_Types...&gt;&amp;&amp;);

namespace __detail
{
namespace __variant
{
  // Returns the first apparence of _Tp in _Types.
  // Returns sizeof...(_Types) if _Tp is not in _Types.
  template&lt;typename _Tp, typename... _Types&gt;
    struct __index_of : std::integral_constant&lt;size_t, 0&gt; {};

  template&lt;typename _Tp, typename... _Types&gt;
    inline constexpr size_t __index_of_v = __index_of&lt;_Tp, _Types...&gt;::value;

  template&lt;typename _Tp, typename _First, typename... _Rest&gt;
    struct __index_of&lt;_Tp, _First, _Rest...&gt; :
      std::integral_constant&lt;size_t, is_same_v&lt;_Tp, _First&gt;
	? 0 : __index_of_v&lt;_Tp, _Rest...&gt; + 1&gt; {};

  // _Uninitialized&lt;T&gt; is guaranteed to be a literal type, even if T is not.
  // We have to do this, because [basic.types]p10.5.3 (n4606) is not implemented
  // yet. When it's implemented, _Uninitialized&lt;T&gt; can be changed to the alias
  // to T, therefore equivalent to being removed entirely.
  //
  // Another reason we may not want to remove _Uninitialzied&lt;T&gt; may be that, we
  // want _Uninitialized&lt;T&gt; to be trivially destructible, no matter whether T
  // is; but we will see.
  template&lt;typename _Type, bool = std::is_literal_type_v&lt;_Type&gt;&gt;
    struct _Uninitialized;

  template&lt;typename _Type&gt;
    struct _Uninitialized&lt;_Type, true&gt;
    {
      template&lt;typename... _Args&gt;
      constexpr _Uninitialized(in_place_index_t&lt;0&gt;, _Args&amp;&amp;... __args)
      : _M_storage(std::forward&lt;_Args&gt;(__args)...)
      { }

      constexpr const _Type&amp; _M_get() const &amp;
      { return _M_storage; }

      constexpr _Type&amp; _M_get() &amp;
      { return _M_storage; }

      constexpr const _Type&amp;&amp; _M_get() const &amp;&amp;
      { return std::move(_M_storage); }

      constexpr _Type&amp;&amp; _M_get() &amp;&amp;
      { return std::move(_M_storage); }

      _Type _M_storage;
    };

  template&lt;typename _Type&gt;
    struct _Uninitialized&lt;_Type, false&gt;
    {
      template&lt;typename... _Args&gt;
      constexpr _Uninitialized(in_place_index_t&lt;0&gt;, _Args&amp;&amp;... __args)
      { ::new (&amp;_M_storage) _Type(std::forward&lt;_Args&gt;(__args)...); }

      const _Type&amp; _M_get() const &amp;
      { return *_M_storage._M_ptr(); }

      _Type&amp; _M_get() &amp;
      { return *_M_storage._M_ptr(); }

      const _Type&amp;&amp; _M_get() const &amp;&amp;
      { return std::move(*_M_storage._M_ptr()); }

      _Type&amp;&amp; _M_get() &amp;&amp;
      { return std::move(*_M_storage._M_ptr()); }

      __gnu_cxx::__aligned_membuf&lt;_Type&gt; _M_storage;
    };

  template&lt;typename _Ref&gt;
    _Ref __ref_cast(void* __ptr)
    {
      return static_cast&lt;_Ref&gt;(*static_cast&lt;remove_reference_t&lt;_Ref&gt;*&gt;(__ptr));
    }

  template&lt;typename _Union&gt;
    constexpr decltype(auto) __get(in_place_index_t&lt;0&gt;, _Union&amp;&amp; __u)
    { return std::forward&lt;_Union&gt;(__u)._M_first._M_get(); }

  template&lt;size_t _Np, typename _Union&gt;
    constexpr decltype(auto) __get(in_place_index_t&lt;_Np&gt;, _Union&amp;&amp; __u)
    {
      return __variant::__get(in_place_index&lt;_Np-1&gt;,
			      std::forward&lt;_Union&gt;(__u)._M_rest);
    }

  // Returns the typed storage for __v.
  template&lt;size_t _Np, typename _Variant&gt;
    constexpr decltype(auto) __get(_Variant&amp;&amp; __v)
    {
      return __variant::__get(std::in_place_index&lt;_Np&gt;,
			      std::forward&lt;_Variant&gt;(__v)._M_u);
    }

  // Various functions as &quot;vtable&quot; entries, where those vtables are used by
  // polymorphic operations.
  template&lt;typename _Lhs, typename _Rhs&gt;
    void
    __erased_ctor(void* __lhs, void* __rhs)
    {
      using _Type = remove_reference_t&lt;_Lhs&gt;;
      ::new (__lhs) _Type(__variant::__ref_cast&lt;_Rhs&gt;(__rhs));
    }

  template&lt;typename _Variant, size_t _Np&gt;
    void
    __erased_dtor(_Variant&amp;&amp; __v)
    { std::_Destroy(std::__addressof(__variant::__get&lt;_Np&gt;(__v))); }

  template&lt;typename _Lhs, typename _Rhs&gt;
    void
    __erased_assign(void* __lhs, void* __rhs)
    {
      __variant::__ref_cast&lt;_Lhs&gt;(__lhs) = __variant::__ref_cast&lt;_Rhs&gt;(__rhs);
    }

  template&lt;typename _Lhs, typename _Rhs&gt;
    void
    __erased_swap(void* __lhs, void* __rhs)
    {
      using std::swap;
      swap(__variant::__ref_cast&lt;_Lhs&gt;(__lhs),
	   __variant::__ref_cast&lt;_Rhs&gt;(__rhs));
    }

#define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
  template&lt;typename _Variant, size_t _Np&gt; \
    constexpr bool \
    __erased_##__NAME(const _Variant&amp; __lhs, const _Variant&amp; __rhs) \
    { \
      return __variant::__get&lt;_Np&gt;(std::forward&lt;_Variant&gt;(__lhs)) \
	  __OP __variant::__get&lt;_Np&gt;(std::forward&lt;_Variant&gt;(__rhs)); \
    }

  _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;, less)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;=, less_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;=, greater_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;, greater)

#undef _VARIANT_RELATION_FUNCTION_TEMPLATE

  template&lt;typename _Tp&gt;
    size_t
    __erased_hash(void* __t)
    {
      return std::hash&lt;remove_cv_t&lt;remove_reference_t&lt;_Tp&gt;&gt;&gt;{}(
	  __variant::__ref_cast&lt;_Tp&gt;(__t));
    }

  template&lt;typename... _Types&gt;
    struct _Traits
    {
      static constexpr bool _S_default_ctor =
	  is_default_constructible_v&lt;typename _Nth_type&lt;0, _Types...&gt;::type&gt;;
      static constexpr bool _S_copy_ctor =
	  (is_copy_constructible_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_move_ctor =
	  (is_move_constructible_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_copy_assign =
	  _S_copy_ctor &amp;&amp; _S_move_ctor
	  &amp;&amp; (is_copy_assignable_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_move_assign =
	  _S_move_ctor
	  &amp;&amp; (is_move_assignable_v&lt;_Types&gt; &amp;&amp; ...);

      static constexpr bool _S_trivial_dtor =
	  (is_trivially_destructible_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_trivial_copy_ctor =
	  (is_trivially_copy_constructible_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_trivial_move_ctor =
	  (is_trivially_move_constructible_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_trivial_copy_assign =
	  _S_trivial_dtor &amp;&amp; (is_trivially_copy_assignable_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_trivial_move_assign =
	  _S_trivial_dtor &amp;&amp; (is_trivially_move_assignable_v&lt;_Types&gt; &amp;&amp; ...);

      // The following nothrow traits are for non-trivial SMFs. Trivial SMFs
      // are always nothrow.
      static constexpr bool _S_nothrow_default_ctor =
	  is_nothrow_default_constructible_v&lt;
	      typename _Nth_type&lt;0, _Types...&gt;::type&gt;;
      static constexpr bool _S_nothrow_copy_ctor = false;
      static constexpr bool _S_nothrow_move_ctor =
	  (is_nothrow_move_constructible_v&lt;_Types&gt; &amp;&amp; ...);
      static constexpr bool _S_nothrow_copy_assign = false;
      static constexpr bool _S_nothrow_move_assign =
	  _S_nothrow_move_ctor &amp;&amp; (is_nothrow_move_assignable_v&lt;_Types&gt; &amp;&amp; ...);
    };

  // Defines members and ctors.
  template&lt;typename... _Types&gt;
    union _Variadic_union { };

  template&lt;typename _First, typename... _Rest&gt;
    union _Variadic_union&lt;_First, _Rest...&gt;
    {
      constexpr _Variadic_union() : _M_rest() { }

      template&lt;typename... _Args&gt;
	constexpr _Variadic_union(in_place_index_t&lt;0&gt;, _Args&amp;&amp;... __args)
	: _M_first(in_place_index&lt;0&gt;, std::forward&lt;_Args&gt;(__args)...)
	{ }

      template&lt;size_t _Np, typename... _Args&gt;
	constexpr _Variadic_union(in_place_index_t&lt;_Np&gt;, _Args&amp;&amp;... __args)
	: _M_rest(in_place_index&lt;_Np-1&gt;, std::forward&lt;_Args&gt;(__args)...)
	{ }

      _Uninitialized&lt;_First&gt; _M_first;
      _Variadic_union&lt;_Rest...&gt; _M_rest;
    };

  // Defines index and the dtor, possibly trivial.
  template&lt;bool __trivially_destructible, typename... _Types&gt;
    struct _Variant_storage;

  template &lt;typename... _Types&gt;
  using __select_index =
    typename __select_int::_Select_int_base&lt;sizeof...(_Types) + 1,
					    unsigned char,
					    unsigned short&gt;::type::value_type;

  template&lt;typename... _Types&gt;
    struct _Variant_storage&lt;false, _Types...&gt;
    {
      template&lt;size_t... __indices&gt;
	static constexpr void (*_S_vtable[])(const _Variant_storage&amp;) =
	    { &amp;__erased_dtor&lt;const _Variant_storage&amp;, __indices&gt;... };

      constexpr _Variant_storage() : _M_index(variant_npos) { }

      template&lt;size_t _Np, typename... _Args&gt;
	constexpr _Variant_storage(in_place_index_t&lt;_Np&gt;, _Args&amp;&amp;... __args)
	: _M_u(in_place_index&lt;_Np&gt;, std::forward&lt;_Args&gt;(__args)...),
	_M_index(_Np)
	{ }

      template&lt;size_t... __indices&gt;
	constexpr void _M_reset_impl(std::index_sequence&lt;__indices...&gt;)
	{
	  if (_M_index != __index_type(variant_npos))
	    _S_vtable&lt;__indices...&gt;[_M_index](*this);
	}

      void _M_reset()
      {
	_M_reset_impl(std::index_sequence_for&lt;_Types...&gt;{});
	_M_index = variant_npos;
      }

      ~_Variant_storage()
      { _M_reset(); }

      void*
      _M_storage() const
      {
	return const_cast&lt;void*&gt;(static_cast&lt;const void*&gt;(
	    std::addressof(_M_u)));
      }

      constexpr bool
      _M_valid() const noexcept
      {
	return this-&gt;_M_index != __index_type(variant_npos);
      }

      _Variadic_union&lt;_Types...&gt; _M_u;
      using __index_type = __select_index&lt;_Types...&gt;;
      __index_type _M_index;
    };

  template&lt;typename... _Types&gt;
    struct _Variant_storage&lt;true, _Types...&gt;
    {
      constexpr _Variant_storage() : _M_index(variant_npos) { }

      template&lt;size_t _Np, typename... _Args&gt;
	constexpr _Variant_storage(in_place_index_t&lt;_Np&gt;, _Args&amp;&amp;... __args)
	: _M_u(in_place_index&lt;_Np&gt;, std::forward&lt;_Args&gt;(__args)...),
	_M_index(_Np)
	{ }

      void _M_reset()
      { _M_index = variant_npos; }

      void*
      _M_storage() const
      {
	return const_cast&lt;void*&gt;(static_cast&lt;const void*&gt;(
	    std::addressof(_M_u)));
      }

      constexpr bool
      _M_valid() const noexcept
      {
	return this-&gt;_M_index != __index_type(variant_npos);
      }

      _Variadic_union&lt;_Types...&gt; _M_u;
      using __index_type = __select_index&lt;_Types...&gt;;
      __index_type _M_index;
    };

  template&lt;typename... _Types&gt;
    using _Variant_storage_alias =
	_Variant_storage&lt;_Traits&lt;_Types...&gt;::_S_trivial_dtor, _Types...&gt;;

  // The following are (Copy|Move) (ctor|assign) layers for forwarding
  // triviality and handling non-trivial SMF behaviors.

  template&lt;bool, typename... _Types&gt;
    struct _Copy_ctor_base : _Variant_storage_alias&lt;_Types...&gt;
    {
      using _Base = _Variant_storage_alias&lt;_Types...&gt;;
      using _Base::_Base;

      _Copy_ctor_base(const _Copy_ctor_base&amp; __rhs)
	  noexcept(_Traits&lt;_Types...&gt;::_S_nothrow_copy_ctor)
      {
	if (__rhs._M_valid())
	  {
	    static constexpr void (*_S_vtable[])(void*, void*) =
	      { &amp;__erased_ctor&lt;_Types&amp;, const _Types&amp;&gt;... };
	    _S_vtable[__rhs._M_index](this-&gt;_M_storage(), __rhs._M_storage());
	    this-&gt;_M_index = __rhs._M_index;
	  }
      }

      _Copy_ctor_base(_Copy_ctor_base&amp;&amp;) = default;
      _Copy_ctor_base&amp; operator=(const _Copy_ctor_base&amp;) = default;
      _Copy_ctor_base&amp; operator=(_Copy_ctor_base&amp;&amp;) = default;
    };

  template&lt;typename... _Types&gt;
    struct _Copy_ctor_base&lt;true, _Types...&gt; : _Variant_storage_alias&lt;_Types...&gt;
    {
      using _Base = _Variant_storage_alias&lt;_Types...&gt;;
      using _Base::_Base;
    };

  template&lt;typename... _Types&gt;
    using _Copy_ctor_alias =
	_Copy_ctor_base&lt;_Traits&lt;_Types...&gt;::_S_trivial_copy_ctor, _Types...&gt;;

  template&lt;bool, typename... _Types&gt;
    struct _Move_ctor_base : _Copy_ctor_alias&lt;_Types...&gt;
    {
      using _Base = _Copy_ctor_alias&lt;_Types...&gt;;
      using _Base::_Base;

      _Move_ctor_base(_Move_ctor_base&amp;&amp; __rhs)
	  noexcept(_Traits&lt;_Types...&gt;::_S_nothrow_move_ctor)
      {
	if (__rhs._M_valid())
	  {
	    static constexpr void (*_S_vtable[])(void*, void*) =
	      { &amp;__erased_ctor&lt;_Types&amp;, _Types&amp;&amp;&gt;... };
	    _S_vtable[__rhs._M_index](this-&gt;_M_storage(), __rhs._M_storage());
	    this-&gt;_M_index = __rhs._M_index;
	  }
      }

      void _M_destructive_move(_Move_ctor_base&amp;&amp; __rhs)
      {
	this-&gt;~_Move_ctor_base();
	__try
	  {
	    ::new (this) _Move_ctor_base(std::move(__rhs));
	  }
	__catch (...)
	  {
	    this-&gt;_M_index = variant_npos;
	    __throw_exception_again;
	  }
      }

      _Move_ctor_base(const _Move_ctor_base&amp;) = default;
      _Move_ctor_base&amp; operator=(const _Move_ctor_base&amp;) = default;
      _Move_ctor_base&amp; operator=(_Move_ctor_base&amp;&amp;) = default;
    };

  template&lt;typename... _Types&gt;
    struct _Move_ctor_base&lt;true, _Types...&gt; : _Copy_ctor_alias&lt;_Types...&gt;
    {
      using _Base = _Copy_ctor_alias&lt;_Types...&gt;;
      using _Base::_Base;

      void _M_destructive_move(_Move_ctor_base&amp;&amp; __rhs)
      {
	this-&gt;~_Move_ctor_base();
	::new (this) _Move_ctor_base(std::move(__rhs));
      }
    };

  template&lt;typename... _Types&gt;
    using _Move_ctor_alias =
	_Move_ctor_base&lt;_Traits&lt;_Types...&gt;::_S_trivial_move_ctor, _Types...&gt;;

  template&lt;bool, typename... _Types&gt;
    struct _Copy_assign_base : _Move_ctor_alias&lt;_Types...&gt;
    {
      using _Base = _Move_ctor_alias&lt;_Types...&gt;;
      using _Base::_Base;

      _Copy_assign_base&amp;
      operator=(const _Copy_assign_base&amp; __rhs)
	  noexcept(_Traits&lt;_Types...&gt;::_S_nothrow_copy_assign)
      {
	if (this-&gt;_M_index == __rhs._M_index)
	  {
	    if (__rhs._M_valid())
	      {
		static constexpr void (*_S_vtable[])(void*, void*) =
		  { &amp;__erased_assign&lt;_Types&amp;, const _Types&amp;&gt;... };
		_S_vtable[__rhs._M_index](this-&gt;_M_storage(),
					  __rhs._M_storage());
	      }
	  }
	else
	  {
	    _Copy_assign_base __tmp(__rhs);
	    this-&gt;_M_destructive_move(std::move(__tmp));
	  }
	__glibcxx_assert(this-&gt;_M_index == __rhs._M_index);
	return *this;
      }

      _Copy_assign_base(const _Copy_assign_base&amp;) = default;
      _Copy_assign_base(_Copy_assign_base&amp;&amp;) = default;
      _Copy_assign_base&amp; operator=(_Copy_assign_base&amp;&amp;) = default;
    };

  template&lt;typename... _Types&gt;
    struct _Copy_assign_base&lt;true, _Types...&gt; : _Move_ctor_alias&lt;_Types...&gt;
    {
      using _Base = _Move_ctor_alias&lt;_Types...&gt;;
      using _Base::_Base;
    };

  template&lt;typename... _Types&gt;
    using _Copy_assign_alias =
	_Copy_assign_base&lt;_Traits&lt;_Types...&gt;::_S_trivial_copy_assign,
			  _Types...&gt;;

  template&lt;bool, typename... _Types&gt;
    struct _Move_assign_base : _Copy_assign_alias&lt;_Types...&gt;
    {
      using _Base = _Copy_assign_alias&lt;_Types...&gt;;
      using _Base::_Base;

      _Move_assign_base&amp;
      operator=(_Move_assign_base&amp;&amp; __rhs)
	  noexcept(_Traits&lt;_Types...&gt;::_S_nothrow_move_assign)
      {
	if (this-&gt;_M_index == __rhs._M_index)
	  {
	    if (__rhs._M_valid())
	      {
		static constexpr void (*_S_vtable[])(void*, void*) =
		  { &amp;__erased_assign&lt;_Types&amp;, _Types&amp;&amp;&gt;... };
		_S_vtable[__rhs._M_index]
		  (this-&gt;_M_storage(), __rhs._M_storage());
	      }
	  }
	else
	  {
	    _Move_assign_base __tmp(std::move(__rhs));
	    this-&gt;_M_destructive_move(std::move(__tmp));
	  }
	__glibcxx_assert(this-&gt;_M_index == __rhs._M_index);
	return *this;
      }

      _Move_assign_base(const _Move_assign_base&amp;) = default;
      _Move_assign_base(_Move_assign_base&amp;&amp;) = default;
      _Move_assign_base&amp; operator=(const _Move_assign_base&amp;) = default;
    };

  template&lt;typename... _Types&gt;
    struct _Move_assign_base&lt;true, _Types...&gt; : _Copy_assign_alias&lt;_Types...&gt;
    {
      using _Base = _Copy_assign_alias&lt;_Types...&gt;;
      using _Base::_Base;
    };

  template&lt;typename... _Types&gt;
    using _Move_assign_alias =
	_Move_assign_base&lt;_Traits&lt;_Types...&gt;::_S_trivial_move_assign,
			  _Types...&gt;;

  template&lt;typename... _Types&gt;
    struct _Variant_base : _Move_assign_alias&lt;_Types...&gt;
    {
      using _Base = _Move_assign_alias&lt;_Types...&gt;;

      constexpr
      _Variant_base()
	  noexcept(_Traits&lt;_Types...&gt;::_S_nothrow_default_ctor)
      : _Variant_base(in_place_index&lt;0&gt;) { }

      template&lt;size_t _Np, typename... _Args&gt;
	constexpr explicit
	_Variant_base(in_place_index_t&lt;_Np&gt; __i, _Args&amp;&amp;... __args)
	: _Base(__i, std::forward&lt;_Args&gt;(__args)...)
	{ }

      _Variant_base(const _Variant_base&amp;) = default;
      _Variant_base(_Variant_base&amp;&amp;) = default;
      _Variant_base&amp; operator=(const _Variant_base&amp;) = default;
      _Variant_base&amp; operator=(_Variant_base&amp;&amp;) = default;
    };

  // For how many times does _Tp appear in _Tuple?
  template&lt;typename _Tp, typename _Tuple&gt;
    struct __tuple_count;

  template&lt;typename _Tp, typename _Tuple&gt;
    inline constexpr size_t __tuple_count_v =
      __tuple_count&lt;_Tp, _Tuple&gt;::value;

  template&lt;typename _Tp, typename... _Types&gt;
    struct __tuple_count&lt;_Tp, tuple&lt;_Types...&gt;&gt;
    : integral_constant&lt;size_t, 0&gt; { };

  template&lt;typename _Tp, typename _First, typename... _Rest&gt;
    struct __tuple_count&lt;_Tp, tuple&lt;_First, _Rest...&gt;&gt;
    : integral_constant&lt;
	size_t,
	__tuple_count_v&lt;_Tp, tuple&lt;_Rest...&gt;&gt; + is_same_v&lt;_Tp, _First&gt;&gt; { };

  // TODO: Reuse this in &lt;tuple&gt; ?
  template&lt;typename _Tp, typename... _Types&gt;
    inline constexpr bool __exactly_once =
      __tuple_count_v&lt;_Tp, tuple&lt;_Types...&gt;&gt; == 1;

  // Takes _Types and create an overloaded _S_fun for each type.
  // If a type appears more than once in _Types, create only one overload.
  template&lt;typename... _Types&gt;
    struct __overload_set
    { static void _S_fun(); };

  template&lt;typename _First, typename... _Rest&gt;
    struct __overload_set&lt;_First, _Rest...&gt; : __overload_set&lt;_Rest...&gt;
    {
      using __overload_set&lt;_Rest...&gt;::_S_fun;
      static integral_constant&lt;size_t, sizeof...(_Rest)&gt; _S_fun(_First);
    };

  template&lt;typename... _Rest&gt;
    struct __overload_set&lt;void, _Rest...&gt; : __overload_set&lt;_Rest...&gt;
    {
      using __overload_set&lt;_Rest...&gt;::_S_fun;
    };

  // Helper for variant(_Tp&amp;&amp;) and variant::operator=(_Tp&amp;&amp;).
  // __accepted_index maps the arbitrary _Tp to an alternative type in _Variant.
  template&lt;typename _Tp, typename _Variant, typename = void&gt;
    struct __accepted_index
    { static constexpr size_t value = variant_npos; };

  template&lt;typename _Tp, typename... _Types&gt;
    struct __accepted_index&lt;
      _Tp, variant&lt;_Types...&gt;,
      decltype(__overload_set&lt;_Types...&gt;::_S_fun(std::declval&lt;_Tp&gt;()),
	       std::declval&lt;void&gt;())&gt;
    {
      static constexpr size_t value = sizeof...(_Types) - 1
	- decltype(__overload_set&lt;_Types...&gt;::
		   _S_fun(std::declval&lt;_Tp&gt;()))::value;
    };

  // Returns the raw storage for __v.
  template&lt;typename _Variant&gt;
    void* __get_storage(_Variant&amp;&amp; __v)
    { return __v._M_storage(); }

  // Used for storing multi-dimensional vtable.
  template&lt;typename _Tp, size_t... _Dimensions&gt;
    struct _Multi_array
    {
      constexpr const _Tp&amp;
      _M_access() const
      { return _M_data; }

      _Tp _M_data;
    };

  template&lt;typename _Tp, size_t __first, size_t... __rest&gt;
    struct _Multi_array&lt;_Tp, __first, __rest...&gt;
    {
      template&lt;typename... _Args&gt;
	constexpr const _Tp&amp;
	_M_access(size_t __first_index, _Args... __rest_indices) const
	{ return _M_arr[__first_index]._M_access(__rest_indices...); }

      _Multi_array&lt;_Tp, __rest...&gt; _M_arr[__first];
    };

  // Creates a multi-dimensional vtable recursively.
  //
  // For example,
  // visit([](auto, auto){},
  //       variant&lt;int, char&gt;(),  // typedef'ed as V1
  //       variant&lt;float, double, long double&gt;())  // typedef'ed as V2
  // will trigger instantiations of:
  // __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;), 2, 3&gt;,
  //                   tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;&gt;&gt;
  //   __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;), 3&gt;,
  //                     tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;0&gt;&gt;
  //     __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;)&gt;,
  //                       tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;0, 0&gt;&gt;
  //     __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;)&gt;,
  //                       tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;0, 1&gt;&gt;
  //     __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;)&gt;,
  //                       tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;0, 2&gt;&gt;
  //   __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;), 3&gt;,
  //                     tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;1&gt;&gt;
  //     __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;)&gt;,
  //                       tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;1, 0&gt;&gt;
  //     __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;)&gt;,
  //                       tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;1, 1&gt;&gt;
  //     __gen_vtable_impl&lt;_Multi_array&lt;void(*)(V1&amp;&amp;, V2&amp;&amp;)&gt;,
  //                       tuple&lt;V1&amp;&amp;, V2&amp;&amp;&gt;, std::index_sequence&lt;1, 2&gt;&gt;
  // The returned multi-dimensional vtable can be fast accessed by the visitor
  // using index calculation.
  template&lt;typename _Array_type, typename _Variant_tuple, typename _Index_seq&gt;
    struct __gen_vtable_impl;

  template&lt;typename _Result_type, typename _Visitor, size_t... __dimensions,
	   typename... _Variants, size_t... __indices&gt;
    struct __gen_vtable_impl&lt;
	_Multi_array&lt;_Result_type (*)(_Visitor, _Variants...), __dimensions...&gt;,
	tuple&lt;_Variants...&gt;, std::index_sequence&lt;__indices...&gt;&gt;
    {
      using _Next =
	  remove_reference_t&lt;typename _Nth_type&lt;sizeof...(__indices),
			     _Variants...&gt;::type&gt;;
      using _Array_type =
	  _Multi_array&lt;_Result_type (*)(_Visitor, _Variants...),
		       __dimensions...&gt;;

      static constexpr _Array_type
      _S_apply()
      {
	_Array_type __vtable{};
	_S_apply_all_alts(
	  __vtable, make_index_sequence&lt;variant_size_v&lt;_Next&gt;&gt;());
	return __vtable;
      }

      template&lt;size_t... __var_indices&gt;
	static constexpr void
	_S_apply_all_alts(_Array_type&amp; __vtable,
			  std::index_sequence&lt;__var_indices...&gt;)
	{
	  (_S_apply_single_alt&lt;__var_indices&gt;(
	     __vtable._M_arr[__var_indices]), ...);
	}

      template&lt;size_t __index, typename _Tp&gt;
	static constexpr void
	_S_apply_single_alt(_Tp&amp; __element)
	{
	  using _Alternative = variant_alternative_t&lt;__index, _Next&gt;;
	  __element = __gen_vtable_impl&lt;
	    remove_reference_t&lt;decltype(__element)&gt;, tuple&lt;_Variants...&gt;,
	    std::index_sequence&lt;__indices..., __index&gt;&gt;::_S_apply();
	}
    };

  template&lt;typename _Result_type, typename _Visitor, typename... _Variants,
	   size_t... __indices&gt;
    struct __gen_vtable_impl&lt;
      _Multi_array&lt;_Result_type (*)(_Visitor, _Variants...)&gt;,
		   tuple&lt;_Variants...&gt;, std::index_sequence&lt;__indices...&gt;&gt;
    {
      using _Array_type =
	  _Multi_array&lt;_Result_type (*)(_Visitor&amp;&amp;, _Variants...)&gt;;

      static constexpr decltype(auto)
      __visit_invoke(_Visitor&amp;&amp; __visitor, _Variants... __vars)
      {
	return std::__invoke(std::forward&lt;_Visitor&gt;(__visitor),
	    __variant::__get&lt;__indices&gt;(std::forward&lt;_Variants&gt;(__vars))...);
      }

      static constexpr auto
      _S_apply()
      { return _Array_type{&amp;__visit_invoke}; }
    };

  template&lt;typename _Result_type, typename _Visitor, typename... _Variants&gt;
    struct __gen_vtable
    {
      using _Func_ptr = _Result_type (*)(_Visitor&amp;&amp;, _Variants...);
      using _Array_type =
	  _Multi_array&lt;_Func_ptr,
		       variant_size_v&lt;remove_reference_t&lt;_Variants&gt;&gt;...&gt;;

      static constexpr _Array_type
      _S_apply()
      {
	return __gen_vtable_impl&lt;_Array_type, tuple&lt;_Variants...&gt;,
				 std::index_sequence&lt;&gt;&gt;::_S_apply();
      }

      static constexpr auto _S_vtable = _S_apply();
    };

  template&lt;size_t _Np, typename _Tp&gt;
    struct _Base_dedup : public _Tp { };

  template&lt;typename _Variant, typename __indices&gt;
    struct _Variant_hash_base;

  template&lt;typename... _Types, size_t... __indices&gt;
    struct _Variant_hash_base&lt;variant&lt;_Types...&gt;,
			      std::index_sequence&lt;__indices...&gt;&gt;
    : _Base_dedup&lt;__indices, __poison_hash&lt;remove_const_t&lt;_Types&gt;&gt;&gt;... { };

} // namespace __variant
} // namespace __detail

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr bool
    holds_alternative(const variant&lt;_Types...&gt;&amp; __v) noexcept
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      return __v.index() == __detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;;
    }

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr _Tp&amp; get(variant&lt;_Types...&gt;&amp; __v)
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      static_assert(!is_void_v&lt;_Tp&gt;, &quot;_Tp should not be void&quot;);
      return std::get&lt;__detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;&gt;(__v);
    }

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr _Tp&amp;&amp; get(variant&lt;_Types...&gt;&amp;&amp; __v)
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      static_assert(!is_void_v&lt;_Tp&gt;, &quot;_Tp should not be void&quot;);
      return std::get&lt;__detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;&gt;(
	std::move(__v));
    }

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr const _Tp&amp; get(const variant&lt;_Types...&gt;&amp; __v)
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      static_assert(!is_void_v&lt;_Tp&gt;, &quot;_Tp should not be void&quot;);
      return std::get&lt;__detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;&gt;(__v);
    }

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr const _Tp&amp;&amp; get(const variant&lt;_Types...&gt;&amp;&amp; __v)
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      static_assert(!is_void_v&lt;_Tp&gt;, &quot;_Tp should not be void&quot;);
      return std::get&lt;__detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;&gt;(
	std::move(__v));
    }

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr add_pointer_t&lt;variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&gt;
    get_if(variant&lt;_Types...&gt;* __ptr) noexcept
    {
      using _Alternative_type = variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;;
      static_assert(_Np &lt; sizeof...(_Types),
		    &quot;The index should be in [0, number of alternatives)&quot;);
      static_assert(!is_void_v&lt;_Alternative_type&gt;, &quot;_Tp should not be void&quot;);
      if (__ptr &amp;&amp; __ptr-&gt;index() == _Np)
	return &amp;__detail::__variant::__get&lt;_Np&gt;(*__ptr);
      return nullptr;
    }

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr
    add_pointer_t&lt;const variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&gt;
    get_if(const variant&lt;_Types...&gt;* __ptr) noexcept
    {
      using _Alternative_type = variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;;
      static_assert(_Np &lt; sizeof...(_Types),
		    &quot;The index should be in [0, number of alternatives)&quot;);
      static_assert(!is_void_v&lt;_Alternative_type&gt;, &quot;_Tp should not be void&quot;);
      if (__ptr &amp;&amp; __ptr-&gt;index() == _Np)
	return &amp;__detail::__variant::__get&lt;_Np&gt;(*__ptr);
      return nullptr;
    }

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr add_pointer_t&lt;_Tp&gt;
    get_if(variant&lt;_Types...&gt;* __ptr) noexcept
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      static_assert(!is_void_v&lt;_Tp&gt;, &quot;_Tp should not be void&quot;);
      return std::get_if&lt;__detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;&gt;(
	  __ptr);
    }

  template&lt;typename _Tp, typename... _Types&gt;
    constexpr add_pointer_t&lt;const _Tp&gt;
    get_if(const variant&lt;_Types...&gt;* __ptr)
    noexcept
    {
      static_assert(__detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;,
		    &quot;T should occur for exactly once in alternatives&quot;);
      static_assert(!is_void_v&lt;_Tp&gt;, &quot;_Tp should not be void&quot;);
      return std::get_if&lt;__detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;&gt;(
	  __ptr);
    }

  struct monostate { };

#define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
  template&lt;typename... _Types&gt; \
    constexpr bool operator __OP(const variant&lt;_Types...&gt;&amp; __lhs, \
				 const variant&lt;_Types...&gt;&amp; __rhs) \
    { \
      return __lhs._M_##__NAME(__rhs, std::index_sequence_for&lt;_Types...&gt;{}); \
    } \
\
  constexpr bool operator __OP(monostate, monostate) noexcept \
  { return 0 __OP 0; }

  _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;, less)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;=, less_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;=, greater_equal)
  _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;, greater)

#undef _VARIANT_RELATION_FUNCTION_TEMPLATE

  template&lt;typename _Visitor, typename... _Variants&gt;
    constexpr decltype(auto) visit(_Visitor&amp;&amp;, _Variants&amp;&amp;...);

  template&lt;typename... _Types&gt;
    inline enable_if_t&lt;(is_move_constructible_v&lt;_Types&gt; &amp;&amp; ...)
			&amp;&amp; (is_swappable_v&lt;_Types&gt; &amp;&amp; ...)&gt;
    swap(variant&lt;_Types...&gt;&amp; __lhs, variant&lt;_Types...&gt;&amp; __rhs)
    noexcept(noexcept(__lhs.swap(__rhs)))
    { __lhs.swap(__rhs); }

  template&lt;typename... _Types&gt;
    enable_if_t&lt;!((is_move_constructible_v&lt;_Types&gt; &amp;&amp; ...)
		   &amp;&amp; (is_swappable_v&lt;_Types&gt; &amp;&amp; ...))&gt;
    swap(variant&lt;_Types...&gt;&amp;, variant&lt;_Types...&gt;&amp;) = delete;

  class bad_variant_access : public exception
  {
  public:
    bad_variant_access() noexcept : _M_reason(&quot;Unknown reason&quot;) { }
    const char* what() const noexcept override
    { return _M_reason; }

  private:
    bad_variant_access(const char* __reason) : _M_reason(__reason) { }

    const char* _M_reason;

    friend void __throw_bad_variant_access(const char* __what);
  };

  inline void
  __throw_bad_variant_access(const char* __what)
  { _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__what)); }

  template&lt;typename... _Types&gt;
    class variant
    : private __detail::__variant::_Variant_base&lt;_Types...&gt;,
      private _Enable_default_constructor&lt;
	__detail::__variant::_Traits&lt;_Types...&gt;::_S_default_ctor,
	  variant&lt;_Types...&gt;&gt;,
      private _Enable_copy_move&lt;
	__detail::__variant::_Traits&lt;_Types...&gt;::_S_copy_ctor,
	__detail::__variant::_Traits&lt;_Types...&gt;::_S_copy_assign,
	__detail::__variant::_Traits&lt;_Types...&gt;::_S_move_ctor,
	__detail::__variant::_Traits&lt;_Types...&gt;::_S_move_assign,
	variant&lt;_Types...&gt;&gt;
    {
    private:
      static_assert(sizeof...(_Types) &gt; 0,
		    &quot;variant must have at least one alternative&quot;);
      static_assert(!(std::is_reference_v&lt;_Types&gt; || ...),
		    &quot;variant must have no reference alternative&quot;);
      static_assert(!(std::is_void_v&lt;_Types&gt; || ...),
		    &quot;variant must have no void alternative&quot;);

      using _Base = __detail::__variant::_Variant_base&lt;_Types...&gt;;
      using _Default_ctor_enabler =
	_Enable_default_constructor&lt;
	  __detail::__variant::_Traits&lt;_Types...&gt;::_S_default_ctor,
	    variant&lt;_Types...&gt;&gt;;

      template&lt;typename _Tp&gt;
	static constexpr bool
	__exactly_once = __detail::__variant::__exactly_once&lt;_Tp, _Types...&gt;;

      template&lt;typename _Tp&gt;
	static constexpr size_t __accepted_index =
	  __detail::__variant::__accepted_index&lt;_Tp&amp;&amp;, variant&gt;::value;

      template&lt;size_t _Np, bool = _Np &lt; sizeof...(_Types)&gt;
	struct __to_type_impl;

      template&lt;size_t _Np&gt;
	struct __to_type_impl&lt;_Np, true&gt;
	{ using type = variant_alternative_t&lt;_Np, variant&gt;; };

      template&lt;size_t _Np&gt;
	using __to_type = typename __to_type_impl&lt;_Np&gt;::type;

      template&lt;typename _Tp&gt;
	using __accepted_type = __to_type&lt;__accepted_index&lt;_Tp&gt;&gt;;

      template&lt;typename _Tp&gt;
	static constexpr size_t __index_of =
	  __detail::__variant::__index_of_v&lt;_Tp, _Types...&gt;;

      using _Traits = __detail::__variant::_Traits&lt;_Types...&gt;;

      template&lt;typename _Tp&gt;
	struct __is_in_place_tag : false_type { };
      template&lt;typename _Tp&gt;
	struct __is_in_place_tag&lt;in_place_type_t&lt;_Tp&gt;&gt; : true_type { };
      template&lt;size_t _Np&gt;
	struct __is_in_place_tag&lt;in_place_index_t&lt;_Np&gt;&gt; : true_type { };

      template&lt;typename _Tp&gt;
	static constexpr bool __not_in_place_tag
	  = !__is_in_place_tag&lt;decay_t&lt;_Tp&gt;&gt;::value;

    public:
      variant() = default;
      variant(const variant&amp; __rhs) = default;
      variant(variant&amp;&amp;) = default;
      variant&amp; operator=(const variant&amp;) = default;
      variant&amp; operator=(variant&amp;&amp;) = default;
      ~variant() = default;

      template&lt;typename _Tp,
	       typename = enable_if_t&lt;!is_same_v&lt;decay_t&lt;_Tp&gt;, variant&gt;&gt;,
	       typename = enable_if_t&lt;(sizeof...(_Types)&gt;0)&gt;,
	       typename = enable_if_t&lt;__not_in_place_tag&lt;_Tp&gt;&gt;,
	       typename = enable_if_t&lt;__exactly_once&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;&gt;
			  &amp;&amp; is_constructible_v&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;, _Tp&amp;&amp;&gt;&gt;&gt;
	constexpr
	variant(_Tp&amp;&amp; __t)
	noexcept(is_nothrow_constructible_v&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;, _Tp&amp;&amp;&gt;)
	: variant(in_place_index&lt;__accepted_index&lt;_Tp&amp;&amp;&gt;&gt;,
		  std::forward&lt;_Tp&gt;(__t))
	{ __glibcxx_assert(holds_alternative&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;&gt;(*this)); }

      template&lt;typename _Tp, typename... _Args,
	       typename = enable_if_t&lt;__exactly_once&lt;_Tp&gt;
			  &amp;&amp; is_constructible_v&lt;_Tp, _Args&amp;&amp;...&gt;&gt;&gt;
	constexpr explicit
	variant(in_place_type_t&lt;_Tp&gt;, _Args&amp;&amp;... __args)
	: variant(in_place_index&lt;__index_of&lt;_Tp&gt;&gt;,
		  std::forward&lt;_Args&gt;(__args)...)
	{ __glibcxx_assert(holds_alternative&lt;_Tp&gt;(*this)); }

      template&lt;typename _Tp, typename _Up, typename... _Args,
	       typename = enable_if_t&lt;__exactly_once&lt;_Tp&gt;
			  &amp;&amp; is_constructible_v&lt;
			    _Tp, initializer_list&lt;_Up&gt;&amp;, _Args&amp;&amp;...&gt;&gt;&gt;
	constexpr explicit
	variant(in_place_type_t&lt;_Tp&gt;, initializer_list&lt;_Up&gt; __il,
		_Args&amp;&amp;... __args)
	: variant(in_place_index&lt;__index_of&lt;_Tp&gt;&gt;, __il,
		  std::forward&lt;_Args&gt;(__args)...)
	{ __glibcxx_assert(holds_alternative&lt;_Tp&gt;(*this)); }

      template&lt;size_t _Np, typename... _Args,
	       typename = enable_if_t&lt;
		 is_constructible_v&lt;__to_type&lt;_Np&gt;, _Args&amp;&amp;...&gt;&gt;&gt;
	constexpr explicit
	variant(in_place_index_t&lt;_Np&gt;, _Args&amp;&amp;... __args)
	: _Base(in_place_index&lt;_Np&gt;, std::forward&lt;_Args&gt;(__args)...),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ __glibcxx_assert(index() == _Np); }

      template&lt;size_t _Np, typename _Up, typename... _Args,
	       typename = enable_if_t&lt;is_constructible_v&lt;__to_type&lt;_Np&gt;,
				      initializer_list&lt;_Up&gt;&amp;, _Args&amp;&amp;...&gt;&gt;&gt;
	constexpr explicit
	variant(in_place_index_t&lt;_Np&gt;, initializer_list&lt;_Up&gt; __il,
		_Args&amp;&amp;... __args)
	: _Base(in_place_index&lt;_Np&gt;, __il, std::forward&lt;_Args&gt;(__args)...),
	_Default_ctor_enabler(_Enable_default_constructor_tag{})
	{ __glibcxx_assert(index() == _Np); }

      template&lt;typename _Tp&gt;
	enable_if_t&lt;__exactly_once&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;&gt;
		    &amp;&amp; is_constructible_v&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;, _Tp&amp;&amp;&gt;
		    &amp;&amp; is_assignable_v&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;&amp;, _Tp&amp;&amp;&gt;
		    &amp;&amp; !is_same_v&lt;decay_t&lt;_Tp&gt;, variant&gt;, variant&amp;&gt;
	operator=(_Tp&amp;&amp; __rhs)
	noexcept(is_nothrow_assignable_v&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;&amp;, _Tp&amp;&amp;&gt;
		 &amp;&amp; is_nothrow_constructible_v&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;, _Tp&amp;&amp;&gt;)
	{
	  constexpr auto __index = __accepted_index&lt;_Tp&amp;&amp;&gt;;
	  if (index() == __index)
	    std::get&lt;__index&gt;(*this) = std::forward&lt;_Tp&gt;(__rhs);
	  else
	    this-&gt;emplace&lt;__index&gt;(std::forward&lt;_Tp&gt;(__rhs));
	  __glibcxx_assert(holds_alternative&lt;__accepted_type&lt;_Tp&amp;&amp;&gt;&gt;(*this));
	  return *this;
	}

      template&lt;typename _Tp, typename... _Args&gt;
	enable_if_t&lt;is_constructible_v&lt;_Tp, _Args...&gt; &amp;&amp; __exactly_once&lt;_Tp&gt;,
		    _Tp&amp;&gt;
	emplace(_Args&amp;&amp;... __args)
	{
	  auto&amp; ret =
	    this-&gt;emplace&lt;__index_of&lt;_Tp&gt;&gt;(std::forward&lt;_Args&gt;(__args)...);
	  __glibcxx_assert(holds_alternative&lt;_Tp&gt;(*this));
	  return ret;
	}

      template&lt;typename _Tp, typename _Up, typename... _Args&gt;
	enable_if_t&lt;is_constructible_v&lt;_Tp, initializer_list&lt;_Up&gt;&amp;, _Args...&gt;
		    &amp;&amp; __exactly_once&lt;_Tp&gt;,
		    _Tp&amp;&gt;
	emplace(initializer_list&lt;_Up&gt; __il, _Args&amp;&amp;... __args)
	{
	  auto&amp; ret =
	    this-&gt;emplace&lt;__index_of&lt;_Tp&gt;&gt;(__il,
					   std::forward&lt;_Args&gt;(__args)...);
	  __glibcxx_assert(holds_alternative&lt;_Tp&gt;(*this));
	  return ret;
	}

      template&lt;size_t _Np, typename... _Args&gt;
	enable_if_t&lt;is_constructible_v&lt;variant_alternative_t&lt;_Np, variant&gt;,
				       _Args...&gt;,
		    variant_alternative_t&lt;_Np, variant&gt;&amp;&gt;
	emplace(_Args&amp;&amp;... __args)
	{
	  static_assert(_Np &lt; sizeof...(_Types),
			&quot;The index should be in [0, number of alternatives)&quot;);
	  this-&gt;~variant();
	  __try
	    {
	      ::new (this) variant(in_place_index&lt;_Np&gt;,
				   std::forward&lt;_Args&gt;(__args)...);
	    }
	  __catch (...)
	    {
	      this-&gt;_M_index = variant_npos;
	      __throw_exception_again;
	    }
	  __glibcxx_assert(index() == _Np);
	  return std::get&lt;_Np&gt;(*this);
	}

      template&lt;size_t _Np, typename _Up, typename... _Args&gt;
	enable_if_t&lt;is_constructible_v&lt;variant_alternative_t&lt;_Np, variant&gt;,
				       initializer_list&lt;_Up&gt;&amp;, _Args...&gt;,
		    variant_alternative_t&lt;_Np, variant&gt;&amp;&gt;
	emplace(initializer_list&lt;_Up&gt; __il, _Args&amp;&amp;... __args)
	{
	  static_assert(_Np &lt; sizeof...(_Types),
			&quot;The index should be in [0, number of alternatives)&quot;);
	  this-&gt;~variant();
	  __try
	    {
	      ::new (this) variant(in_place_index&lt;_Np&gt;, __il,
				   std::forward&lt;_Args&gt;(__args)...);
	    }
	  __catch (...)
	    {
	      this-&gt;_M_index = variant_npos;
	      __throw_exception_again;
	    }
	  __glibcxx_assert(index() == _Np);
	  return std::get&lt;_Np&gt;(*this);
	}

      constexpr bool valueless_by_exception() const noexcept
      { return !this-&gt;_M_valid(); }

      constexpr size_t index() const noexcept
      {
	if (this-&gt;_M_index ==
	    typename _Base::__index_type(variant_npos))
	  return variant_npos;
	return this-&gt;_M_index;
      }

      void
      swap(variant&amp; __rhs)
      noexcept((__is_nothrow_swappable&lt;_Types&gt;::value &amp;&amp; ...)
	       &amp;&amp; is_nothrow_move_constructible_v&lt;variant&gt;)
      {
	if (this-&gt;index() == __rhs.index())
	  {
	    if (this-&gt;_M_valid())
	      {
		static constexpr void (*_S_vtable[])(void*, void*) =
		  { &amp;__detail::__variant::__erased_swap&lt;_Types&amp;, _Types&amp;&gt;... };
		_S_vtable[__rhs._M_index](this-&gt;_M_storage(),
					  __rhs._M_storage());
	      }
	  }
	else if (!this-&gt;_M_valid())
	  {
	    this-&gt;_M_destructive_move(std::move(__rhs));
	    __rhs._M_reset();
	  }
	else if (!__rhs._M_valid())
	  {
	    __rhs._M_destructive_move(std::move(*this));
	    this-&gt;_M_reset();
	  }
	else
	  {
	    auto __tmp = std::move(__rhs);
	    __rhs._M_destructive_move(std::move(*this));
	    this-&gt;_M_destructive_move(std::move(__tmp));
	  }
      }

    private:
#define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
      template&lt;size_t... __indices&gt; \
	static constexpr bool \
	(*_S_erased_##__NAME[])(const variant&amp;, const variant&amp;) = \
	  { &amp;__detail::__variant::__erased_##__NAME&lt; \
		const variant&amp;, __indices&gt;... }; \
      template&lt;size_t... __indices&gt; \
	constexpr bool \
	_M_##__NAME(const variant&amp; __rhs, \
		    std::index_sequence&lt;__indices...&gt;) const \
	{ \
	  auto __lhs_index = this-&gt;index(); \
	  auto __rhs_index = __rhs.index(); \
	  if (__lhs_index != __rhs_index || valueless_by_exception()) \
	    /* Modulo addition. */ \
	    return __lhs_index + 1 __OP __rhs_index + 1; \
	  return _S_erased_##__NAME&lt;__indices...&gt;[__lhs_index](*this, __rhs); \
	}

      _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;, less)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;=, less_equal)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;=, greater_equal)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;, greater)

#undef _VARIANT_RELATION_FUNCTION_TEMPLATE

#ifdef __clang__
    public:
      using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
    private:
#endif

      template&lt;size_t _Np, typename _Vp&gt;
	friend constexpr decltype(auto) __detail::__variant::__get(_Vp&amp;&amp; __v);

      template&lt;typename _Vp&gt;
	friend void* __detail::__variant::__get_storage(_Vp&amp;&amp; __v);

#define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP) \
      template&lt;typename... _Tp&gt; \
	friend constexpr bool \
	operator __OP(const variant&lt;_Tp...&gt;&amp; __lhs, \
		      const variant&lt;_Tp...&gt;&amp; __rhs);

      _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(&lt;=)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(==)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(!=)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;=)
      _VARIANT_RELATION_FUNCTION_TEMPLATE(&gt;)

#undef _VARIANT_RELATION_FUNCTION_TEMPLATE
    };

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&amp;
    get(variant&lt;_Types...&gt;&amp; __v)
    {
      static_assert(_Np &lt; sizeof...(_Types),
		    &quot;The index should be in [0, number of alternatives)&quot;);
      if (__v.index() != _Np)
	__throw_bad_variant_access(&quot;Unexpected index&quot;);
      return __detail::__variant::__get&lt;_Np&gt;(__v);
    }

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&amp;&amp;
    get(variant&lt;_Types...&gt;&amp;&amp; __v)
    {
      static_assert(_Np &lt; sizeof...(_Types),
		    &quot;The index should be in [0, number of alternatives)&quot;);
      if (__v.index() != _Np)
	__throw_bad_variant_access(&quot;Unexpected index&quot;);
      return __detail::__variant::__get&lt;_Np&gt;(std::move(__v));
    }

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr const variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&amp;
    get(const variant&lt;_Types...&gt;&amp; __v)
    {
      static_assert(_Np &lt; sizeof...(_Types),
		    &quot;The index should be in [0, number of alternatives)&quot;);
      if (__v.index() != _Np)
	__throw_bad_variant_access(&quot;Unexpected index&quot;);
      return __detail::__variant::__get&lt;_Np&gt;(__v);
    }

  template&lt;size_t _Np, typename... _Types&gt;
    constexpr const variant_alternative_t&lt;_Np, variant&lt;_Types...&gt;&gt;&amp;&amp;
    get(const variant&lt;_Types...&gt;&amp;&amp; __v)
    {
      static_assert(_Np &lt; sizeof...(_Types),
		    &quot;The index should be in [0, number of alternatives)&quot;);
      if (__v.index() != _Np)
	__throw_bad_variant_access(&quot;Unexpected index&quot;);
      return __detail::__variant::__get&lt;_Np&gt;(std::move(__v));
    }

  template&lt;typename _Visitor, typename... _Variants&gt;
    constexpr decltype(auto)
    visit(_Visitor&amp;&amp; __visitor, _Variants&amp;&amp;... __variants)
    {
      if ((__variants.valueless_by_exception() || ...))
	__throw_bad_variant_access(&quot;Unexpected index&quot;);

      using _Result_type =
	decltype(std::forward&lt;_Visitor&gt;(__visitor)(
	    std::get&lt;0&gt;(std::forward&lt;_Variants&gt;(__variants))...));

      constexpr auto&amp; __vtable = __detail::__variant::__gen_vtable&lt;
	_Result_type, _Visitor&amp;&amp;, _Variants&amp;&amp;...&gt;::_S_vtable;

      auto __func_ptr = __vtable._M_access(__variants.index()...);
      return (*__func_ptr)(std::forward&lt;_Visitor&gt;(__visitor),
			   std::forward&lt;_Variants&gt;(__variants)...);
    }

  template&lt;bool, typename... _Types&gt;
    struct __variant_hash_call_base_impl
    {
      size_t
      operator()(const variant&lt;_Types...&gt;&amp; __t) const
      noexcept((is_nothrow_invocable_v&lt;hash&lt;decay_t&lt;_Types&gt;&gt;, _Types&gt; &amp;&amp; ...))
      {
	if (!__t.valueless_by_exception())
	  {
	    namespace __edv = __detail::__variant;
	    static constexpr size_t (*_S_vtable[])(void*) =
	      { &amp;__edv::__erased_hash&lt;const _Types&amp;&gt;... };
	    return hash&lt;size_t&gt;{}(__t.index())
	      + _S_vtable[__t.index()](__edv::__get_storage(__t));
	  }
	return hash&lt;size_t&gt;{}(__t.index());
      }
    };

  template&lt;typename... _Types&gt;
    struct __variant_hash_call_base_impl&lt;false, _Types...&gt; {};

  template&lt;typename... _Types&gt;
    using __variant_hash_call_base =
    __variant_hash_call_base_impl&lt;(__poison_hash&lt;remove_const_t&lt;_Types&gt;&gt;::
				   __enable_hash_call &amp;&amp;...), _Types...&gt;;

  template&lt;typename... _Types&gt;
    struct hash&lt;variant&lt;_Types...&gt;&gt;
    : private __detail::__variant::_Variant_hash_base&lt;
	variant&lt;_Types...&gt;, std::index_sequence_for&lt;_Types...&gt;&gt;,
      public __variant_hash_call_base&lt;_Types...&gt;
    {
      using result_type [[__deprecated__]] = size_t;
      using argument_type [[__deprecated__]] = variant&lt;_Types...&gt;;
    };

  template&lt;&gt;
    struct hash&lt;monostate&gt;
    {
      using result_type [[__deprecated__]] = size_t;
      using argument_type [[__deprecated__]] = monostate;

      size_t
      operator()(const monostate&amp; __t) const noexcept
      {
	constexpr size_t __magic_monostate_hash = -7777;
	return __magic_monostate_hash;
      }
    };

  template&lt;typename... _Types&gt;
    struct __is_fast_hash&lt;hash&lt;variant&lt;_Types...&gt;&gt;&gt;
    : bool_constant&lt;(__is_fast_hash&lt;_Types&gt;::value &amp;&amp; ...)&gt;
    { };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++17

#endif // _GLIBCXX_VARIANT
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