std::coroutine_traits(3)

NAME
std::coroutine_traits - std::coroutine_traits

Synopsis
   Defined in header <coroutine>
   template< class R, class... Args > (since C++20)
   struct coroutine_traits;
   Determines the promise type from the return type and parameter types of a coroutine.
   The standard library implementation provides a publicly accessible member type
   promise_type same as R::promise_type if the qualified-id is valid and denotes a
   type. Otherwise, it has no such member.
   Program-defined specializations of coroutine_traits shall define a publicly
   accessible member type promise_type; otherwise, the behavior is undefined.

Template parameters
   R - return type of the coroutine
   Args - parameter types of the coroutine, including the implicit object parameter if
          the coroutine is a non-static member function

Member types
   Type Definition
   promise_type R::promise_type if it is valid, or provided by program-defined
                specializations

Possible implementation
   template<class, class...>
   struct coroutine_traits {};
   template<class R, class... Args>
   requires requires { typename R::promise_type; }
   struct coroutine_traits<R, Args...> {
     using promise_type = typename R::promise_type;
   };

Notes
   If the coroutine is a non-static member function, then the first type in Args... is
   the type of the implicit object parameter, and the rest are parameter types of the
   function (if any).
   If std::coroutine_traits<R, Args...>::promise_type does not exist or is not a class
   type, the corresponding coroutine definition is ill-formed.
   Users may define explicit or partial specializations of coroutine_traits dependent
   on program-defined types to avoid modification to return types.

Example
// Run this code
 #include <chrono>
 #include <coroutine>
 #include <exception>
 #include <future>
 #include <iostream>
 #include <thread>
 #include <type_traits>
 // A program-defined type on which the coroutine_traits specializations below depend
 struct as_coroutine {};
 // Enable the use of std::future<T> as a coroutine type
 // by using a std::promise<T> as the promise type.
 template <typename T, typename... Args>
 requires(!std::is_void_v<T> && !std::is_reference_v<T>)
 struct std::coroutine_traits<std::future<T>, as_coroutine, Args...> {
   struct promise_type : std::promise<T> {
     std::future<T> get_return_object() noexcept {
       return this->get_future();
     }
     std::suspend_never initial_suspend() const noexcept { return {}; }
     std::suspend_never final_suspend() const noexcept { return {}; }
     void return_value(const T &value)
     noexcept(std::is_nothrow_copy_constructible_v<T>) {
       this->set_value(value);
     }
     void return_value(T &&value)
     noexcept(std::is_nothrow_move_constructible_v<T>) {
       this->set_value(std::move(value));
     }
     void unhandled_exception() noexcept {
       this->set_exception(std::current_exception());
     }
   };
 };
 // Same for std::future<void>.
 template <typename... Args>
 struct std::coroutine_traits<std::future<void>, as_coroutine, Args...> {
   struct promise_type : std::promise<void> {
     std::future<void> get_return_object() noexcept {
       return this->get_future();
     }
     std::suspend_never initial_suspend() const noexcept { return {}; }
     std::suspend_never final_suspend() const noexcept { return {}; }
     void return_void() noexcept {
       this->set_value();
     }
     void unhandled_exception() noexcept {
       this->set_exception(std::current_exception());
     }
   };
 };
 // Allow co_await'ing std::future<T> and std::future<void>
 // by naively spawning a new thread for each co_await.
 template <typename T>
 auto operator co_await(std::future<T> future) noexcept
 requires(!std::is_reference_v<T>) {
   struct awaiter : std::future<T> {
     bool await_ready() const noexcept {
       using namespace std::chrono_literals;
       return this->wait_for(0s) != std::future_status::timeout;
     }
     void await_suspend(std::coroutine_handle<> cont) const {
       std::thread([this, cont] {
         this->wait();
         cont();
       }).detach();
     }
     T await_resume() { return this->get(); }
   };
   return awaiter{std::move(future)};
 }
 // Utilize the infrastructure we have established.
 std::future<int> compute(as_coroutine) {
   int a = co_await std::async([] { return 6; });
   int b = co_await std::async([] { return 7; });
   co_return a * b;
 }
 std::future<void> fail(as_coroutine) {
   throw std::runtime_error("bleah");
   co_return;
 }
 int main() {
   std::cout << compute({}).get() << '\n';
   try {
     fail({}).get();
   } catch (const std::runtime_error &e) {
     std::cout << "error: " << e.what() << '\n';
   }
 }

Output:
 42
 error: bleah