A strange compilation error occurs, seemingly related to `set_stopped`

[Cra3z]

The intent of the following program is to print #1 in the main thread, then #2 in thread1, and finally #3 in thread2. Using stdexec, it compiles and runs as intended, but curiously, it fails to compile with beman::execution. see https://godbolt.org/z/71Mv3frGx

namespace {
    std::thread::id main_thread_id = std::this_thread::get_id();

    template<typename... Args>
    void debug(std::format_string<Args...> fmt, Args&&... args) {
        auto thread_id = std::this_thread::get_id();
        auto thread_name = thread_id == main_thread_id ? "main" : (std::stringstream{} << thread_id).str();
        std::clog << std::format("[thread-{}] {}\n", thread_name, std::format(fmt, std::forward<Args>(args)...));
    }
}

int main() {
    ex::run_loop ex_context1;
    std::jthread thread1{
        [&ex_context1] {
            debug("ex_context1 run...");
            ex_context1.run();
        }
    };

    ex::run_loop ex_context2;
    std::jthread thread2{
        [&ex_context2] {
            debug("ex_context2 run...");
            ex_context2.run();
        }
    };

    ex::sender auto snd = ex::just() | ex::then([] {
        debug("#1");
    }) | ex::continues_on(ex_context1.get_scheduler()) | ex::then([] {
        debug("#2");
    }) | ex::continues_on(ex_context2.get_scheduler()) | ex::then([] {
        debug("#3");
    });

    ex::sync_wait(snd);

    ex_context1.finish();
    ex_context2.finish();
}

I may have located where the compilation error occurs:

execution/include/beman/execution/detail/schedule_from.hpp

Lines 146 to 153 in d82ccca

	[]<typename Tag, typename... Args>(auto, auto& state, auto& receiver, Tag, Args&&... args) noexcept -> void {
	using result_t = ::beman::execution::detail::decayed_tuple<Tag, Args...>;
	constexpr bool nothrow = ::std::is_nothrow_constructible_v<result_t, Tag, Args...>;
	try {
	[&]() noexcept(nothrow) {
	state.async_result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
	}();

Here, Tag is beman::execution::set_stopped_t, state.async_result is a std::variant<std::monostate, std::tuple<beman::execution::set_value_t>, std::tuple<beman::execution::set_error_t, std::exception_ptr>>,
while result_t is std::tuple<beman::execution::set_stopped_t>, thus causing the compilation error when attempting to invoke emplace.

[dietmarkuehl]

Thank you for the report! Looking at the implementation for scheduler_from there is a "TODO" to fix the completion signatures. In particular, the completion signatures were lacking set_stopped_t() (which should probably be added conditionally). I have created this PR to fix the immediate issue (which I'll and once CI passes). The computation of completion signatures is going to change when I update the implementation to incorporate P3164 (I hope I get around doing that soon).

[dietmarkuehl]

Thank you for the report! Looking at the implementation for scheduler_from there is a "TODO" to fix the completion signatures. In particular, the completion signatures were lacking set_stopped_t() (which should probably be added conditionally). I have created this PR to fix the immediate issue.

[dietmarkuehl]

The change is landed ⇒ resolving