struct generator_input {};


template <typename OutputType, typename InputType>
struct generator {
   struct promise_type;
   using coro_handle = std::coroutine_handle<promise_type>;

   struct passthru_value
   {
       InputType &ret_;

       bool await_ready() {return true;}
       void await_suspend(coro_handle) {}
       InputType &await_resume() { return ret_; }
   };

   struct promise_type {
       OutputType current_value;
       InputType input_value;


       auto get_return_object() { return generator{coro_handle::from_promise(*this)}; }
       auto initial_suspend() { return std::suspend_always{}; }
       auto final_suspend() { return std::suspend_always{}; }
       void unhandled_exception() { std::terminate(); }
       auto yield_value(OutputType value) {
           current_value = value;
           return std::suspend_always{};
       }

       void return_void() {}

       auto await_transform(generator_input)
       {
           return passthru_value{input_value};
…