boost::asio::spawn is broken with BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT?

Question

After update boost libraries from 1.71 up to 1.74, my code, which uses boost::asio::spawn(), does not compile with MSVC2019 (19.25.28612 for x64) with predefined macro BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT. AFAIK, this macro is supposed for backward compatibilty and I have to use this macro because I use boost::beast::saved_handler but it uses executor work guards which are not defined for proposed standard executors.

More than that my code successfully compiles with clang-11 (and defined BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT).

Here is a simple code example which fails to compile with MSVC2019 and Boost.Asio:1.74 but will compile with Boost.Asio:1.71 (of course, boost::asio::any_io_executor should be replaced back with boost::asio::executor):

#define BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT
#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>

int main()
{
  boost::asio::io_context ctx;
  boost::asio::any_io_executor executor = ctx.get_executor();
  boost::asio::spawn(executor, [](auto yield){});
  return 0;
}

with following output:

C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): error C2672: 'boost::asio::strand<Executor>::execute': no matching overloaded function found
        with
        [
            Executor=boost::asio::executor
        ]
C:\local\boost_1_74_0\boost/asio/impl/dispatch.hpp(57): note: see reference to function template instantiation 'void asio_execution_execute_fn::impl::operator ()<boost::asio::strand<Executor>,boost::asio::detail::spawn_helper<handler_type,function_type>&>(T &&,F) noexcept(false) const' being compiled
        with
        [
            Executor=boost::asio::executor,
            T=boost::asio::strand<boost::asio::executor>,
            F=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/async_result.hpp(151): note: see reference to function template instantiation 'void boost::asio::detail::initiate_dispatch::operator ()<boost::asio::detail::spawn_helper<handler_type,function_type>&>(CompletionHandler,void *) const' being compiled
        with
        [
            CompletionHandler=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/async_result.hpp(363): note: see reference to function template instantiation 'boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::return_type boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::initiate<Initiation,boost::asio::detail::spawn_helper<handler_type,function_type>&,>(Initiation &&,RawCompletionToken)' being compiled
        with
        [
            Signature=void (void),
            Initiation=boost::asio::detail::initiate_dispatch,
            RawCompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/async_result.hpp(364): note: see reference to function template instantiation 'boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::return_type boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::initiate<Initiation,boost::asio::detail::spawn_helper<handler_type,function_type>&,>(Initiation &&,RawCompletionToken)' being compiled
        with
        [
            Signature=void (void),
            Initiation=boost::asio::detail::initiate_dispatch,
            RawCompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/impl/dispatch.hpp(212): note: see reference to function template instantiation 'void boost::asio::async_initiate<CompletionToken,void(void),boost::asio::detail::initiate_dispatch,>(Initiation &&,CompletionToken)' being compiled
        with
        [
            CompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &,
            Initiation=boost::asio::detail::initiate_dispatch
        ]
C:\local\boost_1_74_0\boost/asio/impl/spawn.hpp(452): note: see reference to function template instantiation 'void boost::asio::dispatch<boost::asio::detail::spawn_helper<handler_type,function_type>&>(CompletionToken)' being compiled
        with
        [
            CompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/impl/spawn.hpp(493): note: see reference to function template instantiation 'void boost::asio::spawn<boost::asio::executor_binder<void (__cdecl *)(void),boost::asio::strand<Executor>>,Function>(Handler &&,Function &&,const boost::coroutines::attributes &,void *)' being compiled
        with
        [
            Executor=boost::asio::executor,
            Function=main::<lambda_c142a977195366454fba7e579637857b>,
            Handler=boost::asio::executor_binder<void (__cdecl *)(void),boost::asio::strand<boost::asio::executor>>
        ]
C:\local\boost_1_74_0\boost/asio/impl/spawn.hpp(483): note: see reference to function template instantiation 'void boost::asio::spawn<Function,Executor>(const boost::asio::strand<Executor> &,Function &&,const boost::coroutines::attributes &)' being compiled
        with
        [
            Function=main::<lambda_c142a977195366454fba7e579637857b>,
            Executor=boost::asio::executor
        ]
C:\Users\Artem Suleimanov\CLionProjects\test\main.cpp(10): note: see reference to function template instantiation 'void boost::asio::spawn<main::<lambda_c142a977195366454fba7e579637857b>,boost::asio::executor>(const Executor &,Function &&,const boost::coroutines::attributes &,void *)' being compiled
        with
        [
            Executor=boost::asio::executor,
            Function=main::<lambda_c142a977195366454fba7e579637857b>
        ]
C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): error C2893: Failed to specialize function template 'enable_if<boost::asio::execution::can_execute<const Executor&,Function>::value,void>::type boost::asio::strand<Executor>::execute(Function &&) const'
        with
        [
            Executor=boost::asio::executor
        ]
C:\local\boost_1_74_0\boost/asio/strand.hpp(276): note: see declaration of 'boost::asio::strand<Executor>::execute'
        with
        [
            Executor=boost::asio::executor
        ]
C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): note: With the following template arguments:
C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): note: 'Function=boost::asio::detail::spawn_helper<handler_type,function_type> &'

Is that a compiler/boost bug or I am doing something wrong?

Answer 1

Seems that this issue was fixed. I applied the patch and code now compiles by MSVC2019.

THe release notes entry for this fix:

• Fixed strand<> adaptation of Networking TS executors when targeting older C++ versions or less conformant compilers.