C++20 使用std::coroutine递归实现后根遍历迭代器

查了一下，std::coroutine库没有实现默认的generator，我在github上面找了一个三方实现的std::generator。没咋看代码。但是能用。
主要就是使用协程实现递归。后根遍历二叉树。
代码结构如下，
苹果的clang 12.0.5没有完整实现协程库。所以会标红。
实际在fedora最新镜像中是可以编译通过的。
test/CMakeLists.txt

cmake_minimum_required(VERSION 2.6)

if(APPLE)
  message(STATUS "This is Apple, do nothing.")
  set(CMAKE_MACOSX_RPATH 1)
  set(CMAKE_PREFIX_PATH /Users/aabjfzhu/software/vcpkg/ports/cppwork/vcpkg_installed/x64-osx/share )
elseif(UNIX)
  message(STATUS "This is linux, set CMAKE_PREFIX_PATH.")
  set(CMAKE_PREFIX_PATH /vcpkg/ports/cppwork/vcpkg_installed/x64-linux/share)
endif(APPLE)

project(binary_tree_it_coroutine)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-narrowing")

add_definitions(-g)

find_package(ZLIB)

find_package(OpenCV REQUIRED )
find_package(Arrow CONFIG REQUIRED)

find_package(unofficial-brotli REQUIRED)
find_package(unofficial-utf8proc CONFIG REQUIRED)
find_package(Thrift CONFIG REQUIRED)

find_package(glog REQUIRED)

find_package(OpenSSL REQUIRED)

find_package(Boost REQUIRED COMPONENTS
  system
  filesystem
  serialization
  program_options
  thread
  )

find_package(DataFrame REQUIRED)

if(APPLE)
  MESSAGE(STATUS "This is APPLE, set INCLUDE_DIRS")
set(INCLUDE_DIRS ${Boost_INCLUDE_DIRS} /usr/local/include /usr/local/iODBC/include /opt/snowflake/snowflakeodbc/include/ ${CMAKE_CURRENT_SOURCE_DIR}/../include/ ${CMAKE_CURRENT_SOURCE_DIR}/../../../include)
elseif(UNIX)
  MESSAGE(STATUS "This is linux, set INCLUDE_DIRS")
  set(INCLUDE_DIRS ${Boost_INCLUDE_DIRS} /usr/local/include ${CMAKE_CURRENT_SOURCE_DIR}/../include/   ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/)
endif(APPLE)


if(APPLE)
  MESSAGE(STATUS "This is APPLE, set LINK_DIRS")
  set(LINK_DIRS /usr/local/lib /usr/local/iODBC/lib /opt/snowflake/snowflakeodbc/lib/universal)
elseif(UNIX)
  MESSAGE(STATUS "This is linux, set LINK_DIRS")
  set(LINK_DIRS ${Boost_INCLUDE_DIRS} /usr/local/lib /vcpkg/ports/cppwork/vcpkg_installed/x64-linux/lib)
endif(APPLE)

if(APPLE)
  MESSAGE(STATUS "This is APPLE, set ODBC_LIBS")
  set(ODBC_LIBS iodbc iodbcinst)
elseif(UNIX)
  MESSAGE(STATUS "This is linux, set LINK_DIRS")
  set(ODBC_LIBS odbc odbcinst ltdl)
endif(APPLE)

include_directories(${INCLUDE_DIRS})
LINK_DIRECTORIES(${LINK_DIRS})

file( GLOB test_file_list ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp) 

file( GLOB APP_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/../impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../include/*.h ${CMAKE_CURRENT_SOURCE_DIR}/../include/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/arr_/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/http/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/yaml/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/df/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/death_handler/impl/*.cpp)

add_library(${PROJECT_NAME}_lib SHARED ${APP_SOURCES} ${test_file})
target_link_libraries(${PROJECT_NAME}_lib ${Boost_LIBRARIES} ZLIB::ZLIB glog::glog DataFrame::DataFrame ${OpenCV_LIBS})
target_link_libraries(${PROJECT_NAME}_lib OpenSSL::SSL OpenSSL::Crypto libgtest.a pystring libyaml-cpp.a libgmock.a ${ODBC_LIBS} libnanodbc.a pthread dl backtrace libzstd.a libbz2.a libsnappy.a re2::re2 parquet lz4 unofficial::brotli::brotlidec-static unofficial::brotli::brotlienc-static unofficial::brotli::brotlicommon-static utf8proc thrift::thrift  arrow arrow_dataset)

foreach( test_file ${test_file_list} )
  file(RELATIVE_PATH filename ${CMAKE_CURRENT_SOURCE_DIR} ${test_file})
  string(REPLACE ".cpp" "" file ${filename})
  add_executable(${file}  ${test_file})
  target_link_libraries(${file} ${PROJECT_NAME}_lib)
endforeach( test_file ${test_file_list})

test/binary_tree_iterator_coroutine_test.cpp

#include "binary_tree_iterator_coroutine.hpp"

#include 
#include 

#include 
#include 

#include "death_handler/death_handler.h"

int main(int argc, char** argv) {
  FLAGS_log_dir = "./";
  FLAGS_alsologtostderr = true;
  // 日志级别 INFO, WARNING, ERROR, FATAL 的值分别为0、1、2、3
  FLAGS_minloglevel = 0;

  Debug::DeathHandler dh;

  google::InitGoogleLogging("./logs.log");
  testing::InitGoogleTest(&argc, argv);
  int ret = RUN_ALL_TESTS();
  return ret;
}

// 后根遍历迭代器
GTEST_TEST(BinaryTreeIteratorTests, BinaryTreeIt) {
  BinaryTree family (
      new Node("me",
      new Node("mother",
          new Node("Mother's mother"),
          new Node("Mother's father")
      ),
      new Node("father"))
  );

  for(auto it: family.post_order()) {
      std::cout << (*it).value << "\n";
  }
}

include/binary_tree_iterator_coroutine.hpp

#ifndef _FREDRIC_BINARY_TREE_ITERATOR_COROUTINE_HPP_
#define _FREDRIC_BINARY_TREE_ITERATOR_COROUTINE_HPP_

#include 
#include 
#include 
#include 
#include 
#include 
#include 


template 
struct BinaryTree;

template 
struct Node {
  T value{};

  Node* left{nullptr}, *right{nullptr}, *parent{nullptr};

  BinaryTree* tree{nullptr};

  Node(T value_) : value{value_} {}
  Node(T value_, Node* left_, Node* right_)
      : value(value_), left(left_), right{right_} {
      // 初始化左子节点和右子节点的树为同一颗树
      this->left->tree = this->right->tree = tree;
      // 初始化左子节点和右子节点的父节点为当前节点
      this->left->parent = this->right->parent = this;
  }

  void set_tree(BinaryTree* t) {
      tree = t;
      if (left) left->set_tree(t);
      if (right) right->set_tree(t);
  }

  ~Node() {
      if(left) delete left;
      if(right) delete right;
  } 
};


template 
struct BinaryTree {
  Node* root{nullptr};
  BinaryTree(Node* root_) : root(root_) {
      root->set_tree(this);
  }

  ~BinaryTree() {
      if(root) {
          delete root;
      }
  }

  std::generator*> post_order() {
      return post_order_impl(root);
  }

private:
  std::generator*> post_order_impl(Node* node) {
      // 递归yield 一个node，左右根，后续遍历
      if(node) {
          for(auto x: post_order_impl(node->left)) {
              co_yield x;
          }
          for(auto y: post_order_impl(node->right)) {
              co_yield y;
          }
          co_yield node;
      }
  }
};
#endif

include/generator/generator头文件

#ifndef __STD_GENERATOR_INCLUDED
#define __STD_GENERATOR_INCLUDED
///
// Reference implementation of std::generator proposal P2168.
//
// See https://wg21.link/P2168 for details.
//
///
// Copyright Lewis Baker, Corentin Jabot
//
// Use, modification and distribution is subject to the Boost Software License,
// Version 1.0.
// (See accompanying file LICENSE or http://www.boost.org/LICENSE_1_0.txt)
///

#pragma once

#if __has_include()
#include 
#else
// Fallback for older experimental implementations of coroutines.
#include 
namespace std {
using std::experimental::coroutine_handle;
using std::experimental::coroutine_traits;
using std::experimental::noop_coroutine;
using std::experimental::suspend_always;
using std::experimental::suspend_never;
} // namespace std
#endif

#include 
#include 
#include 
#include 
#include 
#include 
#include 

#if __has_include()
#  include 
#else

// Placeholder implementation of the bits we need from  header
// when we don't have the  header (e.g. Clang 12 and earlier).
namespace std {

// Don't create naming conflicts with recent libc++ which defines std::iter_reference_t
// in  but doesn't yet provide a  header.
template 
using __iter_reference_t = decltype(*std::declval<_T&>());

template 
using iter_value_t =
  typename std::iterator_traits>::value_type;

namespace ranges {

namespace __begin {
void begin();

struct _fn {
  template 
  requires requires(_Range& __r) {
      __r.begin();
  }
  auto operator()(_Range&& __r) const
      noexcept(noexcept(__r.begin()))
      -> decltype(__r.begin()) {
      return __r.begin();
  }

  template 
  requires
      (!requires(_Range& __r) { __r.begin(); }) &&
      requires(_Range& __r) { begin(__r); }
  auto operator()(_Range&& __r) const
      noexcept(noexcept(begin(__r)))
      -> decltype(begin(__r)) {
      return begin(__r);
  }
};

} // namespace __begin

inline namespace __begin_cpo {
inline constexpr __begin::_fn begin = {};
}

namespace __end {
void end();

struct _fn {
  template 
  requires requires(_Range& __r) { __r.end(); }
  auto operator()(_Range&& __r) const
      noexcept(noexcept(__r.end()))
      -> decltype(__r.end()) {
      return __r.end();
  }

  template 
  requires
      (!requires(_Range& __r) { __r.end(); }) &&
      requires(_Range& __r) { end(__r); }
  auto operator()(_Range&& __r) const
      noexcept(noexcept(end(__r)))
      -> decltype(end(__r)) {
      return end(__r);
  }
};
} // namespace __end

inline namespace _end_cpo {
inline constexpr __end::_fn end = {};
}

template 
using iterator_t = decltype(begin(std::declval<_Range>()));

template 
using sentinel_t = decltype(end(std::declval<_Range>()));

template 
using range_reference_t = __iter_reference_t>;

template 
using range_value_t = iter_value_t>;

template 
concept range = requires(_T& __t) {
  ranges::begin(__t);
  ranges::end(__t);
};

} // namespace ranges
} // namespace std

#endif // !__has_include()


namespace std {

template 
class __manual_lifetime {
public:
  __manual_lifetime() noexcept {}
  ~__manual_lifetime() {}

  template 
  _T& construct(_Args&&... __args) noexcept(std::is_nothrow_constructible_v<_T, _Args...>) {
      return *::new (static_cast(std::addressof(__value_))) _T((_Args&&)__args...);
  }

  void destruct() noexcept(std::is_nothrow_destructible_v<_T>) {
      __value_.~_T();
  }

  _T& get() & noexcept {
      return __value_;
  }
  _T&& get() && noexcept {
      return static_cast<_T&&>(__value_);
  }
  const _T& get() const & noexcept {
      return __value_;
  }
  const _T&& get() const && noexcept {
      return static_cast(__value_);
  }

private:
  union {
      std::remove_const_t<_T> __value_;
  };
};

template 
class __manual_lifetime<_T&> {
public:
  __manual_lifetime() noexcept : __value_(nullptr) {}
  ~__manual_lifetime() {}

  _T& construct(_T& __value) noexcept {
      __value_ = std::addressof(__value);
      return __value;
  }

  void destruct() noexcept {}

  _T& get() const noexcept {
      return *__value_;
  }

private:
  _T* __value_;
};

template 
class __manual_lifetime<_T&&> {
public:
  __manual_lifetime() noexcept : __value_(nullptr) {}
  ~__manual_lifetime() {}

  _T&& construct(_T&& __value) noexcept {
      __value_ = std::addressof(__value);
      return static_cast<_T&&>(__value);
  }

  void destruct() noexcept {}

  _T&& get() const noexcept {
      return static_cast<_T&&>(*__value_);
  }

private:
  _T* __value_;
};

struct use_allocator_arg {};

namespace ranges {

template 
struct elements_of {
  explicit constexpr elements_of(_Rng&& __rng) noexcept
  requires std::is_default_constructible_v<_Allocator>
  : __range(static_cast<_Rng&&>(__rng))
  {}

  constexpr elements_of(_Rng&& __rng, _Allocator&& __alloc) noexcept
  : __range((_Rng&&)__rng), __alloc((_Allocator&&)__alloc) {}

  constexpr elements_of(elements_of&&) noexcept = default;

  constexpr elements_of(const elements_of &) = delete;
  constexpr elements_of &operator=(const elements_of &) = delete;
  constexpr elements_of &operator=(elements_of &&) = delete;

  constexpr _Rng&& get() noexcept {
      return static_cast<_Rng&&>(__range);
  }

  constexpr _Allocator get_allocator() const noexcept {
      return __alloc;
  }

private:
  [[no_unique_address]] _Allocator __alloc; // \expos
  _Rng && __range; // \expos
};

template 
elements_of(_Rng &&) -> elements_of<_Rng>;

template 
elements_of(_Rng &&, Allocator&&) -> elements_of<_Rng, Allocator>;

} // namespace ranges

template 
static constexpr bool __allocator_needs_to_be_stored =
  !std::allocator_traits<_Alloc>::is_always_equal::value ||
  !std::is_default_constructible_v<_Alloc>;

// Round s up to next multiple of a.
constexpr size_t __aligned_allocation_size(size_t s, size_t a) {
  return (s + a - 1) & ~(a - 1);
}


template ,
        typename _Allocator = use_allocator_arg>
class generator;

template
class __promise_base_alloc {
  static constexpr std::size_t __offset_of_allocator(std::size_t __frameSize) noexcept {
      return __aligned_allocation_size(__frameSize, alignof(_Alloc));
  }

  static constexpr std::size_t __padded_frame_size(std::size_t __frameSize) noexcept {
      return __offset_of_allocator(__frameSize) + sizeof(_Alloc);
  }

  static _Alloc& __get_allocator(void* __frame, std::size_t __frameSize) noexcept {
      return *reinterpret_cast<_Alloc*>(
          static_cast(__frame) + __offset_of_allocator(__frameSize));
  }

public:
  template
  static void* operator new(std::size_t __frameSize, std::allocator_arg_t, _Alloc __alloc, _Args&...) {
      void* __frame = __alloc.allocate(__padded_frame_size(__frameSize));

      // Store allocator at end of the coroutine frame.
      // Assuming the allocator's move constructor is non-throwing (a requirement for allocators)
      ::new (static_cast(std::addressof(__get_allocator(__frame, __frameSize)))) _Alloc(std::move(__alloc));

      return __frame;
  }

  template
  static void* operator new(std::size_t __frameSize, _This&, std::allocator_arg_t, _Alloc __alloc, _Args&...) {
      return __promise_base_alloc::operator new(__frameSize, std::allocator_arg, std::move(__alloc));
  }

  static void operator delete(void* __ptr, std::size_t __frameSize) noexcept {
      _Alloc& __alloc = __get_allocator(__ptr, __frameSize);
      _Alloc __localAlloc(std::move(__alloc));
      __alloc.~Alloc();
      __localAlloc.deallocate(static_cast(__ptr), __padded_frame_size(__frameSize));
  }
};

template
  requires (!__allocator_needs_to_be_stored<_Alloc>)
class __promise_base_alloc<_Alloc> {
public:
  static void* operator new(std::size_t __size) {
      _Alloc __alloc;
      return __alloc.allocate(__size);
  }

  static void operator delete(void* __ptr, std::size_t __size) noexcept {
      _Alloc __alloc;
      __alloc.deallocate(static_cast(__ptr), __size);
  }
};

template
struct __generator_promise_base
{
  template 
  friend class generator;

  __generator_promise_base* __root_;
  std::coroutine_handle<> __parentOrLeaf_;
  // Note: Using manual_lifetime here to avoid extra calls to exception_ptr
  // constructor/destructor in cases where it is not needed (i.e. where this
  // generator coroutine is not used as a nested coroutine).
  // This member is lazily constructed by the __yield_sequence_awaiter::await_suspend()
  // method if this generator is used as a nested generator.
  __manual_lifetime __exception_;
  __manual_lifetime<_Ref> __value_;

  explicit __generator_promise_base(std::coroutine_handle<> thisCoro) noexcept
      : __root_(this)
      , __parentOrLeaf_(thisCoro)
  {}

  ~__generator_promise_base() {
      if (__root_ != this) {
          // This coroutine was used as a nested generator and so will
          // have constructed its __exception_ member which needs to be
          // destroyed here.
          __exception_.destruct();
      }
  }

  std::suspend_always initial_suspend() noexcept {
      return {};
  }

  void return_void() noexcept {}

  void unhandled_exception() {
      if (__root_ != this) {
          __exception_.get() = std::current_exception();
      } else {
          throw;
      }
  }

  // Transfers control back to the parent of a nested coroutine
  struct __final_awaiter {
      bool await_ready() noexcept {
          return false;
      }

      template 
      std::coroutine_handle<>
      await_suspend(std::coroutine_handle<_Promise> __h) noexcept {
          _Promise& __promise = __h.promise();
          __generator_promise_base& __root = *__promise.__root_;
          if (&__root != &__promise) {
              auto __parent = __promise.__parentOrLeaf_;
              __root.__parentOrLeaf_ = __parent;
              return __parent;
          }
          return std::noop_coroutine();
      }

      void await_resume() noexcept {}
  };

  __final_awaiter final_suspend() noexcept {
      return {};
  }

  std::suspend_always yield_value(_Ref&& __x)
          noexcept(std::is_nothrow_move_constructible_v<_Ref>) {
      __root_->__value_.construct((_Ref&&)__x);
      return {};
  }

  template 
  requires
      (!std::is_reference_v<_Ref>) &&
      std::is_convertible_v<_T, _Ref>
  std::suspend_always yield_value(_T&& __x)
          noexcept(std::is_nothrow_constructible_v<_Ref, _T>) {
      __root_->__value_.construct((_T&&)__x);
      return {};
  }

  template 
  struct __yield_sequence_awaiter {
      _Gen __gen_;

      __yield_sequence_awaiter(_Gen&& __g) noexcept
          // Taking ownership of the generator ensures frame are destroyed
          // in the reverse order of their execution.
          : __gen_((_Gen&&)__g) {
      }

      bool await_ready() noexcept {
          return false;
      }

      // set the parent, root and exceptions pointer and
      // resume the nested
      template
      std::coroutine_handle<>
      await_suspend(std::coroutine_handle<_Promise> __h) noexcept {
          __generator_promise_base& __current = __h.promise();
          __generator_promise_base& __nested = *__gen_.__get_promise();
          __generator_promise_base& __root = *__current.__root_;

          __nested.__root_ = __current.__root_;
          __nested.__parentOrLeaf_ = __h;

          // Lazily construct the __exception_ member here now that we
          // know it will be used as a nested generator. This will be
          // destroyed by the promise destructor.
          __nested.__exception_.construct();
          __root.__parentOrLeaf_ = __gen_.__get_coro();

          // Immediately resume the nested coroutine (nested generator)
          return __gen_.__get_coro();
      }

      void await_resume() {
          __generator_promise_base& __nestedPromise = *__gen_.__get_promise();
          if (__nestedPromise.__exception_.get()) {
              std::rethrow_exception(std::move(__nestedPromise.__exception_.get()));
          }
      }
  };

  template 
  __yield_sequence_awaiter>
  yield_value(std::ranges::elements_of> __g) noexcept {
      return std::move(__g).get();
  }

  template 
  __yield_sequence_awaiter, _Allocator>>
  yield_value(std::ranges::elements_of<_Rng, _Allocator> && __x) {
      return [](allocator_arg_t, _Allocator alloc, auto && __rng) -> generator<_Ref, std::remove_cvref_t<_Ref>, _Allocator> {
          for(auto && e: __rng)
              co_yield static_cast(e);
      }(std::allocator_arg, __x.get_allocator(), std::forward<_Rng>(__x.get()));
  }

  void resume() {
      __parentOrLeaf_.resume();
  }

  // Disable use of co_await within this coroutine.
  void await_transform() = delete;
};

template
struct __generator_promise;

template
struct __generator_promise, _ByteAllocator, _ExplicitAllocator> final
  : public __generator_promise_base<_Ref>
  , public __promise_base_alloc<_ByteAllocator> {
  __generator_promise() noexcept
  : __generator_promise_base<_Ref>(std::coroutine_handle<__generator_promise>::from_promise(*this))
  {}

  generator<_Ref, _Value, _Alloc> get_return_object() noexcept {
      return generator<_Ref, _Value, _Alloc>{
          std::coroutine_handle<__generator_promise>::from_promise(*this)
      };
  }

  using __generator_promise_base<_Ref>::yield_value;

  template 
  typename __generator_promise_base<_Ref>::template __yield_sequence_awaiter>
  yield_value(std::ranges::elements_of<_Rng> && __x) {
      static_assert (!_ExplicitAllocator,
      "This coroutine has an explicit allocator specified with std::allocator_arg so an allocator needs to be passed "
      "explicitely to std::elements_of");
      return [](auto && __rng) -> generator<_Ref, _Value, _Alloc> {
          for(auto && e: __rng)
              co_yield static_cast(e);
      }(std::forward<_Rng>(__x.get()));
  }
};

template
using __byte_allocator_t = typename std::allocator_traits>::template rebind_alloc;


// Type-erased allocator with default allocator behaviour.
template
struct coroutine_traits, _Args...> {
  using promise_type = __generator_promise, std::allocator>;
};

// Type-erased allocator with std::allocator_arg parameter
template
struct coroutine_traits, allocator_arg_t, _Alloc, _Args...> {
private:
  using __byte_allocator = __byte_allocator_t<_Alloc>;
public:
  using promise_type = __generator_promise, __byte_allocator, true /*explicit Allocator*/>;
};

// Type-erased allocator with std::allocator_arg parameter (non-static member functions)
template
struct coroutine_traits, _This, allocator_arg_t, _Alloc, _Args...> {
private:
  using __byte_allocator = __byte_allocator_t<_Alloc>;
public:
  using promise_type = __generator_promise, __byte_allocator,  true /*explicit Allocator*/>;
};

// Generator with specified allocator type
template
struct coroutine_traits, _Args...> {
  using __byte_allocator = __byte_allocator_t<_Alloc>;
public:
  using promise_type = __generator_promise, __byte_allocator>;
};


// TODO :  make layout compatible promise casts possible
template 
class generator {
  using __byte_allocator = __byte_allocator_t<_Alloc>;
public:
  using promise_type = __generator_promise, __byte_allocator>;
  friend promise_type;
private:
  using __coroutine_handle = std::coroutine_handle;
public:

  generator() noexcept = default;

  generator(generator&& __other) noexcept
      : __coro_(std::exchange(__other.__coro_, {}))
      , __started_(std::exchange(__other.__started_, false)) {
  }

  ~generator() noexcept {
      if (__coro_) {
          if (__started_ && !__coro_.done()) {
              __coro_.promise().__value_.destruct();
          }
          __coro_.destroy();
      }
  }

  generator& operator=(generator && g) noexcept {
      swap(g);
      return *this;
  }

  void swap(generator& __other) noexcept {
      std::swap(__coro_, __other.__coro_);
      std::swap(__started_, __other.__started_);
  }

  struct sentinel {};

  class iterator {
    public:
      using iterator_category = std::input_iterator_tag;
      using difference_type = std::ptrdiff_t;
      using value_type = _Value;
      using reference = _Ref;
      using pointer = std::add_pointer_t<_Ref>;

      iterator() noexcept = default;
      iterator(const iterator &) = delete;

      iterator(iterator&& __other) noexcept
      : __coro_(std::exchange(__other.__coro_, {})) {
      }

      iterator& operator=(iterator&& __other) {
          std::swap(__coro_, __other.__coro_);
          return *this;
      }

      ~iterator() {
      }

      friend bool operator==(const iterator &it, sentinel) noexcept {
          return it.__coro_.done();
      }

      iterator &operator++() {
          __coro_.promise().__value_.destruct();
          __coro_.promise().resume();
          return *this;
      }
      void operator++(int) {
          (void)operator++();
      }

      reference operator*() const noexcept {
          return static_cast(__coro_.promise().__value_.get());
      }

    private:
      friend generator;

      explicit iterator(__coroutine_handle __coro) noexcept
      : __coro_(__coro) {}

      __coroutine_handle __coro_;
  };

  iterator begin() {
      assert(__coro_);
      assert(!__started_);
      __started_ = true;
      __coro_.resume();
      return iterator{__coro_};
  }

  sentinel end() noexcept {
      return {};
  }

private:
  explicit generator(__coroutine_handle __coro) noexcept
      : __coro_(__coro) {
  }

public: // to get around access restrictions for __yield_sequence_awaitable
  std::coroutine_handle<> __get_coro() noexcept { return __coro_; }
  promise_type* __get_promise() noexcept { return std::addressof(__coro_.promise()); }

private:
  __coroutine_handle __coro_;
  bool __started_ = false;
};

// Specialisation for type-erased allocator implementation.
template 
class generator<_Ref, _Value, use_allocator_arg> {
  using __promise_base = __generator_promise_base<_Ref>;
public:

  generator() noexcept 
      : __promise_(nullptr)
      , __coro_()
      , __started_(false)
  {}

  generator(generator&& __other) noexcept
      : __promise_(std::exchange(__other.__promise_, nullptr))
      , __coro_(std::exchange(__other.__coro_, {}))
      , __started_(std::exchange(__other.__started_, false)) {
  }

  ~generator() noexcept {
      if (__coro_) {
          if (__started_ && !__coro_.done()) {
              __promise_->__value_.destruct();
          }
          __coro_.destroy();
      }
  }

  generator& operator=(generator g) noexcept {
      swap(g);
      return *this;
  }

  void swap(generator& __other) noexcept {
      std::swap(__promise_, __other.__promise_);
      std::swap(__coro_, __other.__coro_);
      std::swap(__started_, __other.__started_);
  }

  struct sentinel {};

  class iterator {
    public:
      using iterator_category = std::input_iterator_tag;
      using difference_type = std::ptrdiff_t;
      using value_type = _Value;
      using reference = _Ref;
      using pointer = std::add_pointer_t<_Ref>;

      iterator() noexcept = default;
      iterator(const iterator &) = delete;

      iterator(iterator&& __other) noexcept
      : __promise_(std::exchange(__other.__promise_, nullptr))
      , __coro_(std::exchange(__other.__coro_, {}))
      {}

      iterator& operator=(iterator&& __other) {
          __promise_ = std::exchange(__other.__promise_, nullptr);
          __coro_ = std::exchange(__other.__coro_, {});
          return *this;
      }

      ~iterator() = default;

      friend bool operator==(const iterator &it, sentinel) noexcept {
          return it.__coro_.done();
      }

      iterator& operator++() {
          __promise_->__value_.destruct();
          __promise_->resume();
          return *this;
      }

      void operator++(int) {
          (void)operator++();
      }

      reference operator*() const noexcept {
          return static_cast(__promise_->__value_.get());
      }

    private:
      friend generator;

      explicit iterator(__promise_base* __promise, std::coroutine_handle<> __coro) noexcept
      : __promise_(__promise)
      , __coro_(__coro)
      {}

      __promise_base* __promise_;
      std::coroutine_handle<> __coro_;
  };

  iterator begin() {
      assert(__coro_);
      assert(!__started_);
      __started_ = true;
      __coro_.resume();
      return iterator{__promise_, __coro_};
  }

  sentinel end() noexcept {
      return {};
  }

private:
  template
  friend struct __generator_promise;

  template
  explicit generator(std::coroutine_handle<_Promise> __coro) noexcept
      : __promise_(std::addressof(__coro.promise()))
      , __coro_(__coro)
  {}

public: // to get around access restrictions for __yield_sequence_awaitable
  std::coroutine_handle<> __get_coro() noexcept { return __coro_; }
  __promise_base* __get_promise() noexcept { return __promise_; }

private:
  __promise_base* __promise_;
  std::coroutine_handle<> __coro_;
  bool __started_ = false;
};

#if __has_include()
namespace ranges {

template 
constexpr inline bool enable_view> = true;

} // namespace ranges
#endif

} // namespace std

#endif // __STD_GENERATOR_INCLUDED

程序输出如下，