added nothrow support.

Author: klemens-morgenstern

Diff for: include/boost/cobalt/detail/generator.hpp

@@ -355,7 +355,8 @@ struct generator_promise
       return generator->receiver && generator->receiver->awaited_from.get() == nullptr;
     }
 
-    auto await_suspend(std::coroutine_handle<generator_promise> h) noexcept
+    template<std::derived_from<generator_promise> Promise>
+    auto await_suspend(std::coroutine_handle<Promise> h) noexcept
     {
       std::coroutine_handle<void> res = std::noop_coroutine();
       if (generator->receiver && generator->receiver->awaited_from.get() != nullptr)

Diff for: include/boost/cobalt/detail/promise.hpp

@@ -351,7 +351,8 @@ struct cobalt_promise
       return promise->receiver && promise->receiver->awaited_from.get() == nullptr;
     }
 
-    std::coroutine_handle<void> await_suspend(std::coroutine_handle<cobalt_promise> h) noexcept
+    template<std::derived_from<cobalt_promise> Promise_>
+    std::coroutine_handle<void> await_suspend(std::coroutine_handle<Promise_> h) noexcept
     {
       std::coroutine_handle<void> res = std::noop_coroutine();
       if (promise->receiver && promise->receiver->awaited_from.get() != nullptr)

Diff for: include/boost/cobalt/detail/task.hpp

@@ -336,8 +336,9 @@ struct task_promise
       return promise->receiver && promise->receiver->awaited_from.get() == nullptr;
     }
 
+    template<std::derived_from<task_promise> Promise>
     BOOST_NOINLINE
-    auto await_suspend(std::coroutine_handle<task_promise> h) noexcept
+    auto await_suspend(std::coroutine_handle<Promise> h) noexcept
     {
       std::coroutine_handle<void> res = std::noop_coroutine();
       if (promise->receiver && promise->receiver->awaited_from.get() != nullptr)

Diff for: include/boost/cobalt/nothrow.hpp

@@ -0,0 +1,65 @@
+//
+// Copyright (c) 2023 Klemens Morgenstern ([email protected])
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BOOST_COBALT_NOTHROW_HPP
+#define BOOST_COBALT_NOTHROW_HPP
+
+#include <boost/cobalt/config.hpp>
+
+#include <boost/system/result.hpp>
+#include <boost/system/error_code.hpp>
+#include <boost/system/errc.hpp>
+
+#include <coroutine>
+
+namespace boost::cobalt
+{
+
+template<typename Handle>
+struct nothrow : boost::system::result<Handle>
+{
+  nothrow(Handle && h) : boost::system::result<Handle>(boost::system::in_place_value, std::move(h)) {}
+  nothrow() : boost::system::result<Handle>(boost::system::in_place_error,
+                                            make_error_code(boost::system::errc::not_enough_memory))
+  {}
+};
+
+}
+
+template<typename Handle, typename ... Args>
+struct std::coroutine_traits<boost::cobalt::nothrow<Handle>, Args...>
+{
+  struct promise_type : std::coroutine_traits<Handle, Args...>::promise_type
+  {
+    static void* operator new(std::size_t sz) noexcept
+    {
+      return ::operator new(sz, std::nothrow);
+    }
+
+    static void operator delete(void * raw, const std::size_t size) noexcept
+    {
+#if defined(__cpp_sized_deallocation)
+      ::operator delete(raw, size);
+#else
+      ::operator delete(raw, size);
+#endif
+    }
+
+    boost::cobalt::nothrow<Handle> get_return_object()
+    {
+      return std::coroutine_traits<Handle, Args...>::promise_type::get_return_object();
+    }
+
+    static boost::cobalt::nothrow<Handle> get_return_object_on_allocation_failure()
+    {
+      return {};
+    }
+
+  };
+};
+
+#endif //BOOST_COBALT_NOTHROW_HPP

Diff for: test/CMakeLists.txt

@@ -10,7 +10,7 @@ add_executable(boost_cobalt_main         main.cpp)
 add_executable(boost_cobalt_main_compile main_compile.cpp)
 add_executable(boost_cobalt_basic_tests
       async_for.cpp test_main.cpp promise.cpp with.cpp op.cpp handler.cpp join.cpp race.cpp this_coro.cpp leaf.cpp
-      channel.cpp generator.cpp run.cpp task.cpp gather.cpp wait_group.cpp wrappers.cpp left_race.cpp
+      channel.cpp generator.cpp run.cpp task.cpp gather.cpp wait_group.cpp wrappers.cpp left_race.cpp nothrow.cpp
       strand.cpp fork.cpp thread.cpp any_completion_handler.cpp detached.cpp monotonic_resource.cpp sbo_resource.cpp)
 
 target_link_libraries(boost_cobalt_main         Boost::cobalt)

Diff for: test/nothrow.cpp

@@ -0,0 +1,27 @@
+//
+// Copyright (c) 2024 Klemens Morgenstern ([email protected])
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/cobalt/nothrow.hpp>
+
+#include <boost/cobalt/task.hpp>
+
+#include <boost/test/unit_test.hpp>
+#include "test.hpp"
+
+boost::cobalt::nothrow<boost::cobalt::task<void>> p1() {co_return ;}
+boost::cobalt::nothrow<boost::cobalt::task<void>> p2()
+{
+  volatile char data[1000000000000ull];
+  co_return ;
+}
+
+BOOST_AUTO_TEST_CASE(nothrow_)
+{
+  BOOST_CHECK(p1().has_value());
+  BOOST_CHECK(p2().has_error());
+}
+