c++ - 为什么 co_await for 循环没有在 C++20 中提供？

Question

我从 github 存储库的 cppcoro 库中看到了这段代码：

cppcoro::async_generator<int> ticker(int count, threadpool& tp)
{
  for (int i = 0; i < count; ++i)
  {
    co_await tp.delay(std::chrono::seconds(1));
    co_yield i;
  }
}

cppcoro::task<> consumer(threadpool& tp)
{
  auto sequence = ticker(10, tp);
  for co_await (std::uint32_t i : sequence)
  {
    std::cout << "Tick " << i << std::endl;
  }
}

其中async_generator产生一个值 while 可以co_await和协程consumer具有for co_await其中的等价性：

for co_await (std::uint32_t i : sequence) {
  std::cout << "Tick " << i << std::endl;
}

是

{
  auto&& __range = sequence;
  auto __begin   = co_await __range.begin();
  auto __end     = __range.end();
  for (; __begin != __end; co_await ++__begin) {
    std::uint32_t i = *__begin;
    std::cout << "Tick " << i << std::endl;
  }
}

根据协程 TS 草案：http ://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4775.pdf

for co_awaitC++20 不发货的原因是什么？