Boost.Beast unable to use `asioexec::use_sender`

[SidneyCogdill]

#include <asioexec/use_sender.hpp>
#include <exec/task.hpp>

#include <boost/beast.hpp>

auto foo(boost::beast::tcp_stream stream) -> exec::task<void> {
  boost::beast::flat_buffer buffer;

  boost::beast::http::request<boost::beast::http::string_body> req;
  auto bytes_read = co_await boost::beast::http::async_read(
      stream, buffer, req, asioexec::use_sender);
}

Error occurs within boost/beast/http/impl/read.hpp:

// If you get an error on the following line it means
// that your handler does not meet the documented type
// requirements for the handler.

static_assert(
    beast::detail::is_invocable<ReadHandler,
    void(error_code, std::size_t)>::value,
    "ReadHandler type requirements not met");
...

Compile time error

[proc] Executing command: /home/user/.local/bin/cmake --build ./build --config Debug --target all  --
[build] [1/2   0% :: 0.010] Re-checking globbed directories...
[build] [1/8  12% :: 0.365] Scanning ./main.cpp for CXX dependencies
[build] [2/8  25% :: 0.373] Generating CXX dyndep file CMakeFiles/hello_world.dir/CXX.dd
[build] [3/4  75% :: 6.722] Building CXX object CMakeFiles/hello_world.dir/main.cpp.o
[build] FAILED: CMakeFiles/hello_world.dir/main.cpp.o 
[build] /usr/bin/clang++-21 -I./build/_deps/stdexec-src/include -I./build/_deps/stdexec-build/include -g -std=gnu++23 -MD -MT CMakeFiles/hello_world.dir/main.cpp.o -MF CMakeFiles/hello_world.dir/main.cpp.o.d @CMakeFiles/hello_world.dir/main.cpp.o.modmap -o CMakeFiles/hello_world.dir/main.cpp.o -c ./main.cpp
[build] In file included from ./main.cpp:5:
[build] In file included from /usr/include/boost/beast.hpp:16:
[build] In file included from /usr/include/boost/beast/http.hpp:29:
[build] In file included from /usr/include/boost/beast/http/read.hpp:843:
[build] /usr/include/boost/beast/http/impl/read.hpp:148:13: error: static assertion failed due to requirement 'beast::detail::is_invocable<asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void, exec::__task::__default_task_context_impl<exec::__task::__scheduler_affinity::__sticky>>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>, void (boost::system::error_code, unsigned long)>::value': ReadHandler type requirements not met
[build]   148 |             beast::detail::is_invocable<ReadHandler,
[build]       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build]   149 |             void(error_code, std::size_t)>::value,
[build]       |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build] /usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/bits/invoke.h:63:14: note: in instantiation of function template specialization 'boost::beast::http::detail::run_read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>>::operator()<asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::basic_string_body<char>, std::allocator<char>>' requested here
[build]    63 |     { return std::forward<_Fn>(__f)(std::forward<_Args>(__args)...); }
[build]       |              ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__operation_states.hpp:38:14: note: in instantiation of member function 'asioexec::detail::completion_token::operation_state<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>, boost::beast::http::detail::run_read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>>, std::tuple<boost::beast::basic_flat_buffer<std::allocator<char>> *, boost::beast::http::message<true, boost::beast::http::basic_string_body<char>> *>>::start' requested here
[build]    38 |         __op.start();
[build]       |              ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__tag_invoke.hpp:100:16: note: in instantiation of function template specialization 'stdexec::__start::tag_invoke<stdexec::__start::start_t, asioexec::detail::completion_token::operation_state<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>, boost::beast::http::detail::run_read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>>, std::tuple<boost::beast::basic_flat_buffer<std::allocator<char>> *, boost::beast::http::message<true, boost::beast::http::basic_string_body<char>> *>>>' requested here
[build]   100 |         return tag_invoke(static_cast<_Tag&&>(__tag), static_cast<_Args&&>(__args)...);
[build]       |                ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:128:12: note: in instantiation of function template specialization 'stdexec::__start::start_t::operator()<asioexec::detail::completion_token::operation_state<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>, boost::beast::http::detail::run_read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>>, std::tuple<boost::beast::basic_flat_buffer<std::allocator<char>> *, boost::beast::http::message<true, boost::beast::http::basic_string_body<char>> *>>>' requested here
[build]   128 |           (_StartTag()(__ops), ...);
[build]       |            ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__operation_states.hpp:38:14: note: in instantiation of member function 'stdexec::__op_state<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>::__t>::start' requested here
[build]    38 |         __op.start();
[build]       |              ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__tag_invoke.hpp:100:16: note: in instantiation of function template specialization 'stdexec::__start::tag_invoke<stdexec::__start::start_t, stdexec::__op_state<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>::__t>>' requested here
[build]   100 |         return tag_invoke(static_cast<_Tag&&>(__tag), static_cast<_Args&&>(__args)...);
[build]       |                ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__as_awaitable.hpp:168:11: note: in instantiation of function template specialization 'stdexec::__start::start_t::operator()<stdexec::__op_state<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>::__t>>' requested here
[build]   168 |           stdexec::start(__op_state_);
[build]       |           ^
[build] ./main.cpp:213:14: note: in instantiation of member function 'stdexec::__as_awaitable::__sender_awaitable<exec::__task::basic_task<void>::__promise, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>>::__t::await_suspend' requested here
[build]   213 |     auto a = co_await boost::beast::http::async_read(stream, buffer, req,
[build]       |              ^
[build] In file included from ./main.cpp:5:
[build] In file included from /usr/include/boost/beast.hpp:15:
[build] In file included from /usr/include/boost/beast/core.hpp:15:
[build] /usr/include/boost/beast/core/async_base.hpp:458:9: error: no matching function for call to object of type 'asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>'
[build]   458 |         h_(std::forward<Args>(args)...);
[build]       |         ^~
[build] /usr/include/boost/beast/http/impl/read.hpp:115:15: note: in instantiation of function template specialization 'boost::beast::async_base<asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>, boost::asio::any_io_executor>::complete_now<boost::system::error_code &, unsigned long &>' requested here
[build]   115 |         this->complete_now(ec, bytes_transferred);
[build]       |               ^
[build] /usr/include/boost/asio/composed.hpp:155:5: note: in instantiation of member function 'boost::beast::http::detail::read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::basic_string_body<char>, std::allocator<char>, asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>>::operator()' requested here
[build]   155 |     static_cast<Handler&&>(this->handler_)(static_cast<Args&&>(args)...);
[build]       |     ^
[build] /usr/include/boost/beast/http/impl/read.hpp:325:18: note: in instantiation of member function 'boost::asio::detail::composed_op<boost::beast::http::detail::read_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::detail::parser_is_done>, boost::asio::detail::composed_work<void (boost::asio::any_io_executor)>, boost::beast::http::detail::read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::basic_string_body<char>, std::allocator<char>, asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>>, void (boost::system::error_code, unsigned long)>::complete' requested here
[build]   325 |             self.complete(ec, bytes_transferred_);
[build]       |                  ^
[build] /usr/include/boost/asio/composed.hpp:149:5: note: in instantiation of function template specialization 'boost::beast::http::detail::read_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::detail::parser_is_done>::operator()<boost::asio::detail::composed_op<boost::beast::http::detail::read_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::detail::parser_is_done>, boost::asio::detail::composed_work<void (boost::asio::any_io_executor)>, boost::beast::http::detail::read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::basic_string_body<char>, std::allocator<char>, asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>>, void (boost::system::error_code, unsigned long)>>' requested here
[build]   149 |     this->impl_(*this, static_cast<T&&>(t)...);
[build]       |     ^
[build] /usr/include/boost/asio/composed.hpp:224:5: note: in instantiation of function template specialization 'boost::asio::detail::composed_op<boost::beast::http::detail::read_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::detail::parser_is_done>, boost::asio::detail::composed_work<void (boost::asio::any_io_executor)>, boost::beast::http::detail::read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>, boost::beast::basic_flat_buffer<std::allocator<char>>, true, boost::beast::http::basic_string_body<char>, std::allocator<char>, asioexec::detail::completion_token::completion_handler<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>>>, void (boost::system::error_code, unsigned long)>::operator()<>' requested here
[build]   224 |     composed_op<decay_t<Implementation>, composed_work<Executors>,
[build]       |     ^
[build] /usr/include/boost/asio/async_result.hpp:329:5: note: (skipping 8 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
[build]   329 |     static_cast<Initiation&&>(initiation)(
[build]       |     ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:128:12: note: in instantiation of function template specialization 'stdexec::__start::start_t::operator()<asioexec::detail::completion_token::operation_state<stdexec::completion_signatures<stdexec::__rcvrs::set_value_t (boost::system::error_code, unsigned long), stdexec::__rcvrs::set_error_t (std::__exception_ptr::exception_ptr), stdexec::__rcvrs::set_stopped_t ()>, asioexec::detail::use_sender::receiver<stdexec::__rcvr<stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, 0>::__t>, boost::beast::http::detail::run_read_msg_op<boost::beast::basic_stream<boost::asio::ip::tcp>>, std::tuple<boost::beast::basic_flat_buffer<std::allocator<char>> *, boost::beast::http::message<true, boost::beast::http::basic_string_body<char>> *>>>' requested here
[build]   128 |           (_StartTag()(__ops), ...);
[build]       |            ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__operation_states.hpp:38:14: note: in instantiation of member function 'stdexec::__op_state<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>::__t>::start' requested here
[build]    38 |         __op.start();
[build]       |              ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__tag_invoke.hpp:100:16: note: in instantiation of function template specialization 'stdexec::__start::tag_invoke<stdexec::__start::start_t, stdexec::__op_state<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>::__t>>' requested here
[build]   100 |         return tag_invoke(static_cast<_Tag&&>(__tag), static_cast<_Args&&>(__args)...);
[build]       |                ^
[build] ./build/_deps/stdexec-src/include/stdexec/__detail/__as_awaitable.hpp:168:11: note: in instantiation of function template specialization 'stdexec::__start::start_t::operator()<stdexec::__op_state<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>, stdexec::__as_awaitable::__receiver<exec::__task::basic_task<void>::__promise, unsigned long>::__t>>' requested here
[build]   168 |           stdexec::start(__op_state_);
[build]       |           ^
[build] ./main.cpp:213:14: note: in instantiation of member function 'stdexec::__as_awaitable::__sender_awaitable<exec::__task::basic_task<void>::__promise, stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at ./build/_deps/stdexec-src/include/stdexec/__detail/__basic_sender.hpp:45:7){}>>::__t::await_suspend' requested here
[build]   213 |     auto a = co_await boost::beast::http::async_read(stream, buffer, req,
[build]       |              ^
[build] ./build/_deps/stdexec-src/include/asioexec/completion_token.hpp:246:12: note: candidate function template not viable: expects an rvalue for object argument
[build]   246 |       void operator()(Args&&... args) && noexcept {
[build]       |            ^
[build] 2 errors generated.
[build] ninja: build stopped: subcommand failed.
[proc] The command: /home/user/.local/bin/cmake --build ./build/vscode --config Debug --target all  -- exited with code: 1
[driver] Build completed: 00:00:07.149

Boost version is 1.88

> dpkg -s libboost1.88-all-dev
Package: libboost1.88-all-dev
Status: install ok installed
Priority: optional
Section: libdevel
Installed-Size: 2139
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Architecture: amd64
Source: boost1.88
Version: 1.88.0~beta1-1ubuntu1
Depends: libboost1.88-dev, libboost1.88-tools-dev, libboost-atomic1.88-dev, libboost-chrono1.88-dev, libboost-container1.88-dev, libboost-context1.88-dev, libboost-coroutine1.88-dev, libboost-date-time1.88-dev, libboost-exception1.88-dev, libboost-fiber1.88-dev, libboost-filesystem1.88-dev, libboost-graph-parallel1.88-dev, libboost-graph1.88-dev, libboost-iostreams1.88-dev, libboost-json1.88-dev, libboost-locale1.88-dev, libboost-log1.88-dev, libboost-math1.88-dev, libboost-mpi-python1.88-dev, libboost-mpi1.88-dev, libboost-nowide1.88-dev, libboost-numpy1.88-dev, libboost-program-options1.88-dev, libboost-python1.88-dev, libboost-random1.88-dev, libboost-regex1.88-dev, libboost-serialization1.88-dev, libboost-stacktrace1.88-dev, libboost-system1.88-dev, libboost-test1.88-dev, libboost-thread1.88-dev, libboost-timer1.88-dev, libboost-type-erasure1.88-dev, libboost-url1.88-dev, libboost-wave1.88-dev
Description: Boost C++ Libraries development files (ALL)
 The Boost web site provides free, peer-reviewed, portable C++ source
 libraries. The emphasis is on libraries which work well with the C++
 Standard Library. One goal is to establish "existing practice" and
 provide reference implementations so that the Boost libraries are
 suitable for eventual standardization. Some of the libraries have
 already been proposed for inclusion in the C++ Standards Committee's
 upcoming C++ Standard Library Technical Report.
 .
 This metapackage provides the complete Boost development environment,
 including all separately-packaged libraries.
Original-Maintainer: Debian Boost Team <team+boost@tracker.debian.org>
Homepage: https://www.boost.org/

Compiler version:

> clang++-21 --version
Ubuntu clang version 21.0.0 (++20250501084208+9b1051281e43-1~exp1~20250501084221.26)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-21/bin

stdexec is obtained using CPM:

CPMAddPackage(
  NAME stdexec
  GITHUB_REPOSITORY NVIDIA/stdexec
  GIT_TAG 35a3e31590e736fbb7dd55324b3a7f991a059ce3
  OPTIONS
    "STDEXEC_BUILD_EXAMPLES OFF"
    "STDEXEC_ENABLE_ASIO ON"
)

[pixxon]

I ran into the same problem.

I was able to dig a more into it and it seems like the static assert is fired, because the ReadHandler will be asioexec::detail::completion_token::completion_handler where operator() is rvalue only. Changing it to accept this as lvalue allows the code to compile. ( And in my case it also caused no further issues. )

I am not sure what is the proper solution. With a quick look, I saw that boost.beast uses a base class for the handler, where it is passed through as lvalue. ( Sadly could not fix the problem quickly here, so the problem lies somewhere deeper. )
In the file boost/libs/beast/include/boost/beast/core/async_base.hpp:

    template<class... Args>
    void
    complete_now(Args&&... args)
    {
        this->before_invoke_hook();
        wg1_.reset();
        h_(std::forward<Args>(args)...);
    }

Since asio works and beast does not, I would assume it is the latter where some changes needs to be made.

[RobertLeahy]

Given that invoking a completion handler is consumptive rvalue-ness is correct here and has been modeled by Asio since 1.77.0.

I don't think Boost.Beast has caught up with this convention and now that I'm thinking about it I think this has caused issues with code I maintain at work previously.

I think removing the rvalue qualifier is probably the right solution for compatibility here (that's what we've done in work codebases to support interop with Boost.Beast).