asio completion_token error with async_read and streambuf

[SidneyCogdill]

because asioexec completion token implementation doesn't perfect forward the reference, and asio::streambuf is non-copyable and non-movable.

stdexec/include/asioexec/completion_token.hpp

Lines 352 to 366 in 394a7c5

	template <typename Signatures, typename Initiation, typename... Args>
	class sender {
	using args_type_ = std::tuple<std::decay_t<Args>...>;
	public:
	using sender_concept = ::stdexec::sender_t;
	template <typename T, typename... Us>
	requires std::constructible_from<Initiation, T>
	&& std::constructible_from<args_type_, Us...>
	explicit constexpr sender(T&& t, Us&&... us) noexcept(
	std::is_nothrow_constructible_v<Initiation, T>
	&& std::is_nothrow_constructible_v<args_type_, Us...>)
	: init_(static_cast<T&&>(t))
	, args_(static_cast<Us&&>(us)...) {
	}

Code snippet to reproduce:

#include <asioexec/use_sender.hpp>
#include <boost/asio.hpp>
#include <stdexec/execution.hpp>

auto f(boost::asio::readable_pipe pipe, boost::asio::streambuf buf)
{
    auto p = boost::asio::async_read(pipe, buf, asioexec::use_sender);
}

compiler error messages

[build] ~/.cache/CPM/stdexec/ab26/stdexec/include/asioexec/completion_token.hpp:532:14: error: no matching constructor for initialization of '::asioexec::detail::completion_token::sender< ::asioexec::detail::completion_token::completion_signatures<void (boost::system::error_code, unsigned long)>, std::remove_cvref_t<boost::asio::detail::initiate_async_read_dynbuf_v1<boost::asio::basic_readable_pipe<boost::asio::any_io_executor>>>, boost::asio::basic_streambuf<std::allocator<char>> &, boost::asio::detail::transfer_all_t>' (aka '::asioexec::detail::completion_token::sender< ::stdexec::completion_signatures< ::stdexec::set_value_t (boost::system::error_code, unsigned long), ::stdexec::set_error_t (std::exception_ptr), ::stdexec::set_stopped_t ()>, boost::asio::detail::initiate_async_read_dynbuf_v1<boost::asio::basic_readable_pipe<boost::asio::any_io_executor>>, boost::asio::basic_streambuf<std::allocator<char>> &, boost::asio::detail::transfer_all_t>') [clang-diagnostic-error]
[build]   532 |       return ::asioexec::detail::completion_token::sender<
[build]       |              ^
[build]   533 |         ::asioexec::detail::completion_token::completion_signatures<Signatures...>,
[build]   534 |         std::remove_cvref_t<Initiation>,
[build]   535 |         Args...
[build]   536 |       >(static_cast<Initiation&&>(i), static_cast<Args&&>(args)...);
[build]       |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build] ~/.cache/CPM/stdexec/ab26/stdexec/include/asioexec/use_sender.hpp:218:70: note: in instantiation of function template specialization 'boost::asio::async_result<asioexec::completion_token_t, void (boost::system::error_code, unsigned long)>::initiate<boost::asio::detail::initiate_async_read_dynbuf_v1<boost::asio::basic_readable_pipe<>>, boost::asio::basic_streambuf<> &, boost::asio::detail::transfer_all_t>' requested here
[build]   218 |         async_result<::asioexec::completion_token_t, Signatures...>::initiate(
[build]       |                                                                      ^
[build] ./build/vcpkg_installed/x64-linux-dynamic/include/boost/asio/async_result.hpp:644:61: note: in instantiation of function template specialization 'boost::asio::async_result<asioexec::use_sender_t, void (boost::system::error_code, unsigned long)>::initiate<boost::asio::detail::initiate_async_read_dynbuf_v1<boost::asio::basic_readable_pipe<>>, boost::asio::basic_streambuf<> &, boost::asio::detail::transfer_all_t>' requested here
[build]   644 |     async_result<decay_t<CompletionToken>, Signatures...>>::initiate(
[build]       |                                                             ^
[build] ./build/vcpkg_installed/x64-linux-dynamic/include/boost/asio/read.hpp:1038:5: note: while substituting deduced template arguments into function template 'async_initiate' [with CompletionToken = const asioexec::use_sender_t &, Signatures = (no value), Initiation = boost::asio::detail::initiate_async_read_dynbuf_v1<boost::asio::basic_readable_pipe<>>, Args = <boost::asio::basic_streambuf<> &, detail::transfer_all_t>]
[build]  1038 |     async_initiate<ReadToken,
[build]       |     ^
[build] ./source/main.cpp:21:14: note: while substituting deduced template arguments into function template 'async_read' [with AsyncReadStream = boost::asio::readable_pipe, DynamicBuffer_v1 = boost::asio::streambuf &, ReadToken = const use_sender_t &]
[build]    21 |     auto p = boost::asio::async_read(pipe, buf, asioexec::use_sender);
[build]       |              ^
[build] ~/.cache/CPM/stdexec/ab26/stdexec/include/asioexec/completion_token.hpp:361:26: note: candidate template ignored: constraints not satisfied [with T = boost::asio::detail::initiate_async_read_dynbuf_v1<boost::asio::basic_readable_pipe<>>, Us = <boost::asio::basic_streambuf<> &, boost::asio::detail::transfer_all_t>]
[build]   361 |       explicit constexpr sender(T&& t, Us&&... us) noexcept(
[build]       |                          ^
[build] ~/.cache/CPM/stdexec/ab26/stdexec/include/asioexec/completion_token.hpp:360:20: note: because 'std::constructible_from<args_type_, boost::asio::basic_streambuf<> &, boost::asio::detail::transfer_all_t>' evaluated to false
[build]   360 |                 && std::constructible_from<args_type_, Us...>
[build]       |                    ^
[build] /usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/concepts:162:30: note: because 'is_constructible_v<std::tuple<boost::asio::basic_streambuf<>, boost::asio::detail::transfer_all_t>, boost::asio::basic_streambuf<> &, boost::asio::detail::transfer_all_t>' evaluated to false
[build]   162 |       = destructible<_Tp> && is_constructible_v<_Tp, _Args...>;
[build]       |                              ^

It works fine with boost::asio::use_awaitable but not asioexec::use_sender.

[SidneyCogdill]

Changing using args_type_ = std::tuple<std::decay_t<Args>...>; to using args_type_ = std::tuple<Args...>; seems to fix this particular case. I'm not sure if the decay_t is intended, but it causes reference types to become value types.

[RobertLeahy]

Chris described the intent of async_initiate in a committee paper wherein it was said that:

If initiation is deferred, the initiation and args objects must be decay-copied and moved as required.

So the decay-copying which completion_token performs is both intentional and correct (note that under the covers the async_read overloads which traffic in a basic_streambuf launder it via basic_streambuf_ref when passing off to the completion token machinery to workaround the above).

The issue here appears to be that:

• Substitution into async_result<::asioexec::completion_token_t, ...>::initiate isn't SFINAE-friendly, and
• The compiler performs such substitution trying to build the async_read overload set

I'll add a reproducing unit test and make a change which addresses the above.