// Implementation of std::function -*- C++ -*-
	
	// Copyright (C) 2004-2019 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
	// <http://www.gnu.org/licenses/>.
	
	/** @file include/bits/std_function.h
	 *  This is an internal header file, included by other library headers.
	 *  Do not attempt to use it directly. @headername{functional}
	 */
	
	#ifndef _GLIBCXX_STD_FUNCTION_H
	#define _GLIBCXX_STD_FUNCTION_H 1
	
	#pragma GCC system_header
	
	#if __cplusplus < 201103L
	# include <bits/c++0x_warning.h>
	#else
	
	#if __cpp_rtti
	# include <typeinfo>
	#endif
	#include <bits/stl_function.h>
	#include <bits/invoke.h>
	#include <bits/refwrap.h>
	#include <bits/functexcept.h>
	
	namespace std _GLIBCXX_VISIBILITY(default)
	{
	_GLIBCXX_BEGIN_NAMESPACE_VERSION
	
	  /**
	   *  @brief Exception class thrown when class template function's
	   *  operator() is called with an empty target.
	   *  @ingroup exceptions
	   */
	  class bad_function_call : public std::exception
	  {
	  public:
	    virtual ~bad_function_call() noexcept;
	
	    const char* what() const noexcept;
	  };
	
	  /**
	   *  Trait identifying "location-invariant" types, meaning that the
	   *  address of the object (or any of its members) will not escape.
	   *  Trivially copyable types are location-invariant and users can
	   *  specialize this trait for other types.
	   */
	  template<typename _Tp>
	    struct __is_location_invariant
	    : is_trivially_copyable<_Tp>::type
	    { };
	
	  class _Undefined_class;
	
	  union _Nocopy_types
	  {
	    void*       _M_object;
	    const void* _M_const_object;
	    void (*_M_function_pointer)();
	    void (_Undefined_class::*_M_member_pointer)();
	  };
	
	  union [[gnu::may_alias]] _Any_data
	  {
	    void*       _M_access()       { return &_M_pod_data[0]; }
	    const void* _M_access() const { return &_M_pod_data[0]; }
	
	    template<typename _Tp>
	      _Tp&
	      _M_access()
	      { return *static_cast<_Tp*>(_M_access()); }
	
	    template<typename _Tp>
	      const _Tp&
	      _M_access() const
	      { return *static_cast<const _Tp*>(_M_access()); }
	
	    _Nocopy_types _M_unused;
	    char _M_pod_data[sizeof(_Nocopy_types)];
	  };
	
	  enum _Manager_operation
	  {
	    __get_type_info,
	    __get_functor_ptr,
	    __clone_functor,
	    __destroy_functor
	  };
	
	  // Simple type wrapper that helps avoid annoying const problems
	  // when casting between void pointers and pointers-to-pointers.
	  template<typename _Tp>
	    struct _Simple_type_wrapper
	    {
	      _Simple_type_wrapper(_Tp __value) : __value(__value) { }
	
	      _Tp __value;
	    };
	
	  template<typename _Tp>
	    struct __is_location_invariant<_Simple_type_wrapper<_Tp> >
	    : __is_location_invariant<_Tp>
	    { };
	
	  template<typename _Signature>
	    class function;
	
	  /// Base class of all polymorphic function object wrappers.
	  class _Function_base
	  {
	  public:
	    static const size_t _M_max_size = sizeof(_Nocopy_types);
	    static const size_t _M_max_align = __alignof__(_Nocopy_types);
	
	    template<typename _Functor>
	      class _Base_manager
	      {
	      protected:
		static const bool __stored_locally =
		(__is_location_invariant<_Functor>::value
		 && sizeof(_Functor) <= _M_max_size
		 && __alignof__(_Functor) <= _M_max_align
		 && (_M_max_align % __alignof__(_Functor) == 0));
	
		typedef integral_constant<bool, __stored_locally> _Local_storage;
	
		// Retrieve a pointer to the function object
		static _Functor*
		_M_get_pointer(const _Any_data& __source)
		{
		  if _GLIBCXX17_CONSTEXPR (__stored_locally)
		    {
		      const _Functor& __f = __source._M_access<_Functor>();
		      return const_cast<_Functor*>(std::__addressof(__f));
		    }
		  else // have stored a pointer
		    return __source._M_access<_Functor*>();
		}
	
		// Clone a location-invariant function object that fits within
		// an _Any_data structure.
		static void
		_M_clone(_Any_data& __dest, const _Any_data& __source, true_type)
		{
		  ::new (__dest._M_access()) _Functor(__source._M_access<_Functor>());
		}
	
		// Clone a function object that is not location-invariant or
		// that cannot fit into an _Any_data structure.
		static void
		_M_clone(_Any_data& __dest, const _Any_data& __source, false_type)
		{
		  __dest._M_access<_Functor*>() =
		    new _Functor(*__source._M_access<const _Functor*>());
		}
	
		// Destroying a location-invariant object may still require
		// destruction.
		static void
		_M_destroy(_Any_data& __victim, true_type)
		{
		  __victim._M_access<_Functor>().~_Functor();
		}
	
		// Destroying an object located on the heap.
		static void
		_M_destroy(_Any_data& __victim, false_type)
		{
		  delete __victim._M_access<_Functor*>();
		}
	
	      public:
		static bool
		_M_manager(_Any_data& __dest, const _Any_data& __source,
			   _Manager_operation __op)
		{
		  switch (__op)
		    {
	#if __cpp_rtti
		    case __get_type_info:
		      __dest._M_access<const type_info*>() = &typeid(_Functor);
		      break;
	#endif
		    case __get_functor_ptr:
		      __dest._M_access<_Functor*>() = _M_get_pointer(__source);
		      break;
	
		    case __clone_functor:
		      _M_clone(__dest, __source, _Local_storage());
		      break;
	
		    case __destroy_functor:
		      _M_destroy(__dest, _Local_storage());
		      break;
		    }
		  return false;
		}
	
		static void
		_M_init_functor(_Any_data& __functor, _Functor&& __f)
		{ _M_init_functor(__functor, std::move(__f), _Local_storage()); }
	
		template<typename _Signature>
		  static bool
		  _M_not_empty_function(const function<_Signature>& __f)
		  { return static_cast<bool>(__f); }
	
		template<typename _Tp>
		  static bool
		  _M_not_empty_function(_Tp* __fp)
		  { return __fp != nullptr; }
	
		template<typename _Class, typename _Tp>
		  static bool
		  _M_not_empty_function(_Tp _Class::* __mp)
		  { return __mp != nullptr; }
	
		template<typename _Tp>
		  static bool
		  _M_not_empty_function(const _Tp&)
		  { return true; }
	
	      private:
		static void
		_M_init_functor(_Any_data& __functor, _Functor&& __f, true_type)
		{ ::new (__functor._M_access()) _Functor(std::move(__f)); }
	
		static void
		_M_init_functor(_Any_data& __functor, _Functor&& __f, false_type)
		{ __functor._M_access<_Functor*>() = new _Functor(std::move(__f)); }
	      };
	
	    _Function_base() : _M_manager(nullptr) { }
	
	    ~_Function_base()
	    {
	      if (_M_manager)
		_M_manager(_M_functor, _M_functor, __destroy_functor);
	    }
	
	    bool _M_empty() const { return !_M_manager; }
	
	    typedef bool (*_Manager_type)(_Any_data&, const _Any_data&,
					  _Manager_operation);
	
	    _Any_data     _M_functor;
	    _Manager_type _M_manager;
	  };
	
	  template<typename _Signature, typename _Functor>
	    class _Function_handler;
	
	  template<typename _Res, typename _Functor, typename... _ArgTypes>
	    class _Function_handler<_Res(_ArgTypes...), _Functor>
	    : public _Function_base::_Base_manager<_Functor>
	    {
	      typedef _Function_base::_Base_manager<_Functor> _Base;
	
	    public:
	      static _Res
	      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
	      {
		return (*_Base::_M_get_pointer(__functor))(
		    std::forward<_ArgTypes>(__args)...);
	      }
	    };
	
	  template<typename _Functor, typename... _ArgTypes>
	    class _Function_handler<void(_ArgTypes...), _Functor>
	    : public _Function_base::_Base_manager<_Functor>
	    {
	      typedef _Function_base::_Base_manager<_Functor> _Base;
	
	     public:
	      static void
	      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
	      {
		(*_Base::_M_get_pointer(__functor))(
		    std::forward<_ArgTypes>(__args)...);
	      }
	    };
	
	  template<typename _Class, typename _Member, typename _Res,
		   typename... _ArgTypes>
	    class _Function_handler<_Res(_ArgTypes...), _Member _Class::*>
	    : public _Function_handler<void(_ArgTypes...), _Member _Class::*>
	    {
	      typedef _Function_handler<void(_ArgTypes...), _Member _Class::*>
		_Base;
	
	     public:
	      static _Res
	      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
	      {
		return std::__invoke(_Base::_M_get_pointer(__functor)->__value,
				     std::forward<_ArgTypes>(__args)...);
	      }
	    };
	
	  template<typename _Class, typename _Member, typename... _ArgTypes>
	    class _Function_handler<void(_ArgTypes...), _Member _Class::*>
	    : public _Function_base::_Base_manager<
			 _Simple_type_wrapper< _Member _Class::* > >
	    {
	      typedef _Member _Class::* _Functor;
	      typedef _Simple_type_wrapper<_Functor> _Wrapper;
	      typedef _Function_base::_Base_manager<_Wrapper> _Base;
	
	    public:
	      static bool
	      _M_manager(_Any_data& __dest, const _Any_data& __source,
			 _Manager_operation __op)
	      {
		switch (__op)
		  {
	#if __cpp_rtti
		  case __get_type_info:
		    __dest._M_access<const type_info*>() = &typeid(_Functor);
		    break;
	#endif
		  case __get_functor_ptr:
		    __dest._M_access<_Functor*>() =
		      &_Base::_M_get_pointer(__source)->__value;
		    break;
	
		  default:
		    _Base::_M_manager(__dest, __source, __op);
		  }
		return false;
	      }
	
	      static void
	      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
	      {
		std::__invoke(_Base::_M_get_pointer(__functor)->__value,
			      std::forward<_ArgTypes>(__args)...);
	      }
	    };
	
	  template<typename _From, typename _To>
	    using __check_func_return_type
	      = __or_<is_void<_To>, is_same<_From, _To>, is_convertible<_From, _To>>;
	
	  /**
	   *  @brief Primary class template for std::function.
	   *  @ingroup functors
	   *
	   *  Polymorphic function wrapper.
	   */
	  template<typename _Res, typename... _ArgTypes>
	    class function<_Res(_ArgTypes...)>
	    : public _Maybe_unary_or_binary_function<_Res, _ArgTypes...>,
	      private _Function_base
	    {
	      template<typename _Func,
		       typename _Res2 = typename result_of<_Func&(_ArgTypes...)>::type>
		struct _Callable : __check_func_return_type<_Res2, _Res> { };
	
	      // Used so the return type convertibility checks aren't done when
	      // performing overload resolution for copy construction/assignment.
	      template<typename _Tp>
		struct _Callable<function, _Tp> : false_type { };
	
	      template<typename _Cond, typename _Tp>
		using _Requires = typename enable_if<_Cond::value, _Tp>::type;
	
	    public:
	      typedef _Res result_type;
	
	      // [3.7.2.1] construct/copy/destroy
	
	      /**
	       *  @brief Default construct creates an empty function call wrapper.
	       *  @post @c !(bool)*this
	       */
	      function() noexcept
	      : _Function_base() { }
	
	      /**
	       *  @brief Creates an empty function call wrapper.
	       *  @post @c !(bool)*this
	       */
	      function(nullptr_t) noexcept
	      : _Function_base() { }
	
	      /**
	       *  @brief %Function copy constructor.
	       *  @param __x A %function object with identical call signature.
	       *  @post @c bool(*this) == bool(__x)
	       *
	       *  The newly-created %function contains a copy of the target of @a
	       *  __x (if it has one).
	       */
	      function(const function& __x);
	
	      /**
	       *  @brief %Function move constructor.
	       *  @param __x A %function object rvalue with identical call signature.
	       *
	       *  The newly-created %function contains the target of @a __x
	       *  (if it has one).
	       */
	      function(function&& __x) noexcept : _Function_base()
	      {
		__x.swap(*this);
	      }
	
	      /**
	       *  @brief Builds a %function that targets a copy of the incoming
	       *  function object.
	       *  @param __f A %function object that is callable with parameters of
	       *  type @c T1, @c T2, ..., @c TN and returns a value convertible
	       *  to @c Res.
	       *
	       *  The newly-created %function object will target a copy of
	       *  @a __f. If @a __f is @c reference_wrapper<F>, then this function
	       *  object will contain a reference to the function object @c
	       *  __f.get(). If @a __f is a NULL function pointer or NULL
	       *  pointer-to-member, the newly-created object will be empty.
	       *
	       *  If @a __f is a non-NULL function pointer or an object of type @c
	       *  reference_wrapper<F>, this function will not throw.
	       */
	      template<typename _Functor,
		       typename = _Requires<__not_<is_same<_Functor, function>>, void>,
		       typename = _Requires<_Callable<_Functor>, void>>
		function(_Functor);
	
	      /**
	       *  @brief %Function assignment operator.
	       *  @param __x A %function with identical call signature.
	       *  @post @c (bool)*this == (bool)x
	       *  @returns @c *this
	       *
	       *  The target of @a __x is copied to @c *this. If @a __x has no
	       *  target, then @c *this will be empty.
	       *
	       *  If @a __x targets a function pointer or a reference to a function
	       *  object, then this operation will not throw an %exception.
	       */
	      function&
	      operator=(const function& __x)
	      {
		function(__x).swap(*this);
		return *this;
	      }
	
	      /**
	       *  @brief %Function move-assignment operator.
	       *  @param __x A %function rvalue with identical call signature.
	       *  @returns @c *this
	       *
	       *  The target of @a __x is moved to @c *this. If @a __x has no
	       *  target, then @c *this will be empty.
	       *
	       *  If @a __x targets a function pointer or a reference to a function
	       *  object, then this operation will not throw an %exception.
	       */
	      function&
	      operator=(function&& __x) noexcept
	      {
		function(std::move(__x)).swap(*this);
		return *this;
	      }
	
	      /**
	       *  @brief %Function assignment to zero.
	       *  @post @c !(bool)*this
	       *  @returns @c *this
	       *
	       *  The target of @c *this is deallocated, leaving it empty.
	       */
	      function&
	      operator=(nullptr_t) noexcept
	      {
		if (_M_manager)
		  {
		    _M_manager(_M_functor, _M_functor, __destroy_functor);
		    _M_manager = nullptr;
		    _M_invoker = nullptr;
		  }
		return *this;
	      }
	
	      /**
	       *  @brief %Function assignment to a new target.
	       *  @param __f A %function object that is callable with parameters of
	       *  type @c T1, @c T2, ..., @c TN and returns a value convertible
	       *  to @c Res.
	       *  @return @c *this
	       *
	       *  This  %function object wrapper will target a copy of @a
	       *  __f. If @a __f is @c reference_wrapper<F>, then this function
	       *  object will contain a reference to the function object @c
	       *  __f.get(). If @a __f is a NULL function pointer or NULL
	       *  pointer-to-member, @c this object will be empty.
	       *
	       *  If @a __f is a non-NULL function pointer or an object of type @c
	       *  reference_wrapper<F>, this function will not throw.
	       */
	      template<typename _Functor>
		_Requires<_Callable<typename decay<_Functor>::type>, function&>
		operator=(_Functor&& __f)
		{
		  function(std::forward<_Functor>(__f)).swap(*this);
		  return *this;
		}
	
	      /// @overload
	      template<typename _Functor>
		function&
		operator=(reference_wrapper<_Functor> __f) noexcept
		{
		  function(__f).swap(*this);
		  return *this;
		}
	
	      // [3.7.2.2] function modifiers
	
	      /**
	       *  @brief Swap the targets of two %function objects.
	       *  @param __x A %function with identical call signature.
	       *
	       *  Swap the targets of @c this function object and @a __f. This
	       *  function will not throw an %exception.
	       */
	      void swap(function& __x) noexcept
	      {
		std::swap(_M_functor, __x._M_functor);
		std::swap(_M_manager, __x._M_manager);
		std::swap(_M_invoker, __x._M_invoker);
	      }
	
	      // [3.7.2.3] function capacity
	
	      /**
	       *  @brief Determine if the %function wrapper has a target.
	       *
	       *  @return @c true when this %function object contains a target,
	       *  or @c false when it is empty.
	       *
	       *  This function will not throw an %exception.
	       */
	      explicit operator bool() const noexcept
	      { return !_M_empty(); }
	
	      // [3.7.2.4] function invocation
	
	      /**
	       *  @brief Invokes the function targeted by @c *this.
	       *  @returns the result of the target.
	       *  @throws bad_function_call when @c !(bool)*this
	       *
	       *  The function call operator invokes the target function object
	       *  stored by @c this.
	       */
	      _Res operator()(_ArgTypes... __args) const;
	
	#if __cpp_rtti
	      // [3.7.2.5] function target access
	      /**
	       *  @brief Determine the type of the target of this function object
	       *  wrapper.
	       *
	       *  @returns the type identifier of the target function object, or
	       *  @c typeid(void) if @c !(bool)*this.
	       *
	       *  This function will not throw an %exception.
	       */
	      const type_info& target_type() const noexcept;
	
	      /**
	       *  @brief Access the stored target function object.
	       *
	       *  @return Returns a pointer to the stored target function object,
	       *  if @c typeid(_Functor).equals(target_type()); otherwise, a NULL
	       *  pointer.
	       *
	       * This function does not throw exceptions.
	       *
	       * @{
	       */
	      template<typename _Functor>       _Functor* target() noexcept;
	
	      template<typename _Functor> const _Functor* target() const noexcept;
	      // @}
	#endif
	
	    private:
	      using _Invoker_type = _Res (*)(const _Any_data&, _ArgTypes&&...);

	      _Invoker_type _M_invoker;
	  };
	
	#if __cpp_deduction_guides >= 201606
	  template<typename>
	    struct __function_guide_helper
	    { };
	
	  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
	    struct __function_guide_helper<
	      _Res (_Tp::*) (_Args...) noexcept(_Nx)
	    >
	    { using type = _Res(_Args...); };
	
	  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
	    struct __function_guide_helper<
	      _Res (_Tp::*) (_Args...) & noexcept(_Nx)
	    >
	    { using type = _Res(_Args...); };
	
	  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
	    struct __function_guide_helper<
	      _Res (_Tp::*) (_Args...) const noexcept(_Nx)
	    >
	    { using type = _Res(_Args...); };
	
	  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
	    struct __function_guide_helper<
	      _Res (_Tp::*) (_Args...) const & noexcept(_Nx)
	    >
	    { using type = _Res(_Args...); };
	
	  template<typename _Res, typename... _ArgTypes>
	    function(_Res(*)(_ArgTypes...)) -> function<_Res(_ArgTypes...)>;
	
	  template<typename _Functor, typename _Signature = typename
		   __function_guide_helper<decltype(&_Functor::operator())>::type>
	    function(_Functor) -> function<_Signature>;
	#endif
	
	  // Out-of-line member definitions.
	  template<typename _Res, typename... _ArgTypes>
	    function<_Res(_ArgTypes...)>::
	    function(const function& __x)
	    : _Function_base()
	    {
	      if (static_cast<bool>(__x))
		{
		  __x._M_manager(_M_functor, __x._M_functor, __clone_functor);
		  _M_invoker = __x._M_invoker;
		  _M_manager = __x._M_manager;
		}
	    }
	
	  template<typename _Res, typename... _ArgTypes>
	    template<typename _Functor, typename, typename>
	      function<_Res(_ArgTypes...)>::
	      function(_Functor __f)
	      : _Function_base()
	      {
		typedef _Function_handler<_Res(_ArgTypes...), _Functor> _My_handler;
	

		if (_My_handler::_M_not_empty_function(__f))
		  {
		    _My_handler::_M_init_functor(_M_functor, std::move(__f));
		    _M_invoker = &_My_handler::_M_invoke;
		    _M_manager = &_My_handler::_M_manager;

		  }

	      }
	
	  template<typename _Res, typename... _ArgTypes>
	    _Res
	    function<_Res(_ArgTypes...)>::
	    operator()(_ArgTypes... __args) const
	    {
	      if (_M_empty())
		__throw_bad_function_call();
	      return _M_invoker(_M_functor, std::forward<_ArgTypes>(__args)...);
	    }
	
	#if __cpp_rtti
	  template<typename _Res, typename... _ArgTypes>
	    const type_info&
	    function<_Res(_ArgTypes...)>::
	    target_type() const noexcept
	    {
	      if (_M_manager)
		{
		  _Any_data __typeinfo_result;
		  _M_manager(__typeinfo_result, _M_functor, __get_type_info);
		  return *__typeinfo_result._M_access<const type_info*>();
		}
	      else
		return typeid(void);
	    }
	
	  template<typename _Res, typename... _ArgTypes>
	    template<typename _Functor>
	      _Functor*
	      function<_Res(_ArgTypes...)>::
	      target() noexcept
	      {
		const function* __const_this = this;
		const _Functor* __func = __const_this->template target<_Functor>();
		return const_cast<_Functor*>(__func);
	      }
	
	  template<typename _Res, typename... _ArgTypes>
	    template<typename _Functor>
	      const _Functor*
	      function<_Res(_ArgTypes...)>::
	      target() const noexcept
	      {
		if (typeid(_Functor) == target_type() && _M_manager)
		  {
		    _Any_data __ptr;
		    _M_manager(__ptr, _M_functor, __get_functor_ptr);
		    return __ptr._M_access<const _Functor*>();
		  }
		else
		  return nullptr;
	      }
	#endif
	
	  // [20.7.15.2.6] null pointer comparisons
	
	  /**
	   *  @brief Compares a polymorphic function object wrapper against 0
	   *  (the NULL pointer).
	   *  @returns @c true if the wrapper has no target, @c false otherwise
	   *
	   *  This function will not throw an %exception.
	   */
	  template<typename _Res, typename... _Args>
	    inline bool
	    operator==(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
	    { return !static_cast<bool>(__f); }
	
	  /// @overload
	  template<typename _Res, typename... _Args>
	    inline bool
	    operator==(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
	    { return !static_cast<bool>(__f); }
	
	  /**
	   *  @brief Compares a polymorphic function object wrapper against 0
	   *  (the NULL pointer).
	   *  @returns @c false if the wrapper has no target, @c true otherwise
	   *
	   *  This function will not throw an %exception.
	   */
	  template<typename _Res, typename... _Args>
	    inline bool
	    operator!=(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
	    { return static_cast<bool>(__f); }
	
	  /// @overload
	  template<typename _Res, typename... _Args>
	    inline bool
	    operator!=(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
	    { return static_cast<bool>(__f); }
	
	
	  // [20.7.15.2.7] specialized algorithms
	
	  /**
	   *  @brief Swap the targets of two polymorphic function object wrappers.
	   *
	   *  This function will not throw an %exception.
	   */
	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
	  // 2062. Effect contradictions w/o no-throw guarantee of std::function swaps
	  template<typename _Res, typename... _Args>
	    inline void
	    swap(function<_Res(_Args...)>& __x, function<_Res(_Args...)>& __y) noexcept
	    { __x.swap(__y); }
	
	#if __cplusplus >= 201703L
	  namespace __detail::__variant
	  {
	    template<typename> struct _Never_valueless_alt; // see <variant>
	
	    // Provide the strong exception-safety guarantee when emplacing a
	    // function into a variant.
	    template<typename _Signature>
	      struct _Never_valueless_alt<std::function<_Signature>>
	      : std::true_type
	      { };
	  }  // namespace __detail::__variant
	#endif // C++17
	
	_GLIBCXX_END_NAMESPACE_VERSION
	} // namespace std
	
	#endif // C++11
	#endif // _GLIBCXX_STD_FUNCTION_H