std::coroutine_handle, std::noop_coroutine_handle

#include <coroutine>
#include <iostream>
#include <optional>
 
template<std::movable T>
class Generator
{
public:
 struct promise_type
 {
 Generator<T> get_return_object()
 {
 return Generator{Handle::from_promise(*this)};
 }
 static std::suspend_always initial_suspend() noexcept
 {
 return {};
 }
 static std::suspend_always final_suspend() noexcept
 {
 return {};
 }
 std::suspend_always yield_value(T value) noexcept
 {
 current_value = std::move(value);
 return {};
 }
 // Disallow co_await in generator coroutines.
 void await_transform() = delete;
 [[noreturn]]
 static void unhandled_exception() { throw; }
 
 std::optional <T> current_value;
 };
 
 using Handle = std::coroutine_handle<promise_type>;
 
 explicit Generator(const Handle coroutine) :
 m_coroutine{coroutine}
 {}
 
 Generator() = default;
 ~Generator()
 {
 if (m_coroutine)
 m_coroutine.destroy();
 }
 
 Generator(const Generator&) = delete;
 Generator& operator=(const Generator&) = delete;
 
 Generator(Generator&& other) noexcept :
 m_coroutine{other.m_coroutine}
 {
 other.m_coroutine = {};
 }
 Generator& operator=(Generator&& other) noexcept
 {
 if (this != &other)
 {
 if (m_coroutine)
 m_coroutine.destroy();
 m_coroutine = other.m_coroutine;
 other.m_coroutine = {};
 }
 return *this;
 }
 
 // Range-based for loop support.
 class Iter
 {
 public:
 void operator++()
 {
 m_coroutine.resume();
 }
 const T& operator*() const
 {
 return *m_coroutine.promise().current_value;
 }
 bool operator==(std::default_sentinel_t ) const
 {
 return !m_coroutine || m_coroutine.done();
 }
 
 explicit Iter(const Handle coroutine) :
 m_coroutine{coroutine}
 {}
 
 private:
 Handle m_coroutine;
 };
 
 Iter begin()
 {
 if (m_coroutine)
 m_coroutine.resume();
 return Iter{m_coroutine};
 }
 
 std::default_sentinel_t end() { return {}; }
 
private:
 Handle m_coroutine;
};
 
template<std::integral T>
Generator<T> range(T first, const T last)
{
 while (first < last)
 co_yield first++;
}
 
int main()
{
 for (const char i : range(65, 91))
 std::cout << i << ' ';
 std::cout << '\n';
}

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