Build error when starting exec::task in some schedulers, because they miss the scheduler_concept typedef

[miwechner]

Hi,

we recently noticed that you cannot build a program where an exec::task is started in some schedulers. In particular exec::static_thread_pool and exec::timed_thread_context are affecting us.

This was the most minimal example that i could find:

#include <exec/static_thread_pool.hpp>
#include <exec/task.hpp>
#include <stdexec/execution.hpp>

auto main() -> int
{
  exec::static_thread_pool ctx;
  stdexec::sync_wait(stdexec::starts_on(ctx.get_scheduler(), [&]()-> exec::task<void>{
    co_return;
  }()));
  return 0;
}

Compile error

[main] Building folder: /home/coder/stdexec/build/base-debug example.hello_world
[build] Starting build
[proc] Executing command: /usr/bin/cmake --build /home/coder/stdexec/build/base-debug --target example.hello_world --
[build] [0/2] Re-checking globbed directories...
[build] [1/3] Building CXX object examples/CMakeFiles/example.hello_world.dir/hello_world.cpp.o
[build] FAILED: examples/CMakeFiles/example.hello_world.dir/hello_world.cpp.o 
[build] /usr/bin/g++ -DTBB_USE_DEBUG -isystem /home/coder/stdexec/include -isystem /home/coder/stdexec/build/base-debug/include -g -std=gnu++20 -Wno-maybe-uninitialized -fcoroutines -fconcepts-diagnostics-depth=10 -Wall -Werror=unused-parameter -Wno-non-template-friend -MD -MT examples/CMakeFiles/example.hello_world.dir/hello_world.cpp.o -MF examples/CMakeFiles/example.hello_world.dir/hello_world.cpp.o.d -o examples/CMakeFiles/example.hello_world.dir/hello_world.cpp.o -c /home/coder/stdexec/examples/hello_world.cpp
[build] In file included from /home/coder/stdexec/examples/hello_world.cpp:19:
[build] /home/coder/stdexec/include/exec/task.hpp: In instantiation of ‘constexpr experimental::execution::__task::__default_task_context_impl<_SchedulerAffinity>::__default_task_context_impl(_ParentPromise&) [with _ParentPromise = stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >; experimental::execution::__task::__scheduler_affinity _SchedulerAffinity = experimental::execution::__task::__scheduler_affinity::__sticky]’:
[build] /home/coder/stdexec/include/stdexec/__detail/__manual_lifetime.hpp:52:28:   required from ‘constexpr _Ty& stdexec::__manual_lifetime<_Ty>::__construct(_Args&& ...) [with _Args = {stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >&}; _Ty = experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky>]’
[build]    52 |       return *std::launder(::new (static_cast<void*>(__buffer_))
[build]       |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build]    53 |                              _Ty{static_cast<_Args&&>(__args)...});
[build]       |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build] /home/coder/stdexec/include/stdexec/__detail/__optional.hpp:110:41:   required from ‘constexpr _Tp& stdexec::__opt::__optional<_Tp>::emplace(_Us&& ...) [with _Us = {stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >&}; _Tp = experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky>]’
[build]   110 |         _Tp& __t  = __value_.__construct(static_cast<_Us&&>(__us)...);
[build]       |                     ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[build] /home/coder/stdexec/include/exec/task.hpp:676:47:   required from ‘constexpr std::__n4861::coroutine_handle<void> experimental::execution::__task::basic_task<_Ty, _Context>::__task_awaiter<_ParentPromise>::await_suspend(std::__n4861::coroutine_handle<_ParentPromise2>) [with _ParentPromise2 = stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >; _ParentPromise = stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >; _Ty = void; _Context = experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky>]’
[build]   676 |           __coro_.promise().__context_.emplace(__parent.promise());
[build]       |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
[build] /home/coder/stdexec/include/stdexec/__detail/__connect_awaitable.hpp:136:41:   required from ‘constexpr auto stdexec::__connect_await::__awaitable_wrapper<_Derived>::await_suspend(std::__n4861::coroutine_handle<_Promise>) [with _Promise = stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >; _Derived = stdexec::__connect_await::__awaitable_state<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > > >::__state]’
[build]   136 |         return __awaiter().await_suspend(__h);
[build]       |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
[build] /home/coder/stdexec/include/stdexec/__detail/__connect_awaitable.hpp:134:52:   required from ‘constexpr auto stdexec::__connect_await::__awaitable_wrapper<_Derived>::await_suspend(std::__n4861::coroutine_handle<_Promise>) [with _Promise = stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >; _Derived = stdexec::__connect_await::__awaitable_state<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__connect_await::__promise<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > > >]’
[build]   134 |         noexcept(noexcept(__awaiter().await_suspend(__h)))
[build]       |                           ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
[build] /home/coder/stdexec/include/stdexec/__detail/__connect_awaitable.hpp:408:73:   required from ‘void stdexec::__connect_await::__opstate<_Awaiter, _Receiver>::start() & [with _Awaitable = experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >; _Receiver = stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > >]’
[build]   408 |             using __suspend_result_t = decltype(__awaiter_.await_suspend(__coro));
[build]       |                                                 ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
[build] /home/coder/stdexec/include/stdexec/__detail/__operation_states.hpp:133:17:   required from ‘constexpr void stdexec::start_t::operator()(_Op&) const [with _Op = stdexec::__connect_await::__opstate<experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >, stdexec::__seq::__rcvr2<stdexec::__sync_wait::__receiver<>, stdexec::__sched_env<experimental::execution::_pool_::_static_thread_pool::scheduler, stdexec::__env::__fwd<const stdexec::__sync_wait::__env&> > > >]’
[build]   133 |       __op.start();
[build]       |       ~~~~~~~~~~^~
[build] /home/coder/stdexec/include/stdexec/__detail/__sequence.hpp:290:25:   required from ‘constexpr void stdexec::__seq::__opstate<_CvSender1, _Sender2, _Receiver>::__start_next() [with _CvSender1 = stdexec::{anonymous}::__sexpr<<lambda closure object>stdexec::<lambda()>()>; _Sender2 = experimental::execution::__task::basic_task<void, experimental::execution::__task::__default_task_context_impl<experimental::execution::__task::__scheduler_affinity::__sticky> >; _Receiver = stdexec::__sync_wait::__receiver<>]’
[build]   290 |           STDEXEC::start(__op);
[build]       |                         ^
[build] /home/coder/stdexec/include/stdexec/__detail/__sequence.hpp:284:22:   required from here
[build]   284 |       constexpr void __start_next() noexcept
[build]       |                      ^~~~~~~~~~~~
[build] /home/coder/stdexec/include/exec/task.hpp:163:33: error: no match for ‘operator=’ (operand types are ‘experimental::execution::__task::__any_scheduler’ and ‘experimental::execution::_pool_::_static_thread_pool::scheduler’)
[build]   163 |             this->__scheduler_  = __parent_sched;
[build]       |             ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
[build] /home/coder/stdexec/include/exec/task.hpp:163:33: note: there are 2 candidates
[build] /home/coder/stdexec/include/exec/task.hpp:64:12: note: candidate 1: ‘constexpr experimental::execution::__task::__any_scheduler& experimental::execution::__task::__any_scheduler::operator=(const experimental::execution::__task::__any_scheduler&)’
[build]    64 |     struct __any_scheduler
[build]       |            ^~~~~~~~~~~~~~~
[build] /home/coder/stdexec/include/exec/task.hpp:64:12: note: no known conversion for argument 1 from ‘experimental::execution::_pool_::_static_thread_pool::scheduler’ to ‘const experimental::execution::__task::__any_scheduler&’
[build] /home/coder/stdexec/include/exec/task.hpp:64:12: note: candidate 2: ‘constexpr experimental::execution::__task::__any_scheduler& experimental::execution::__task::__any_scheduler::operator=(experimental::execution::__task::__any_scheduler&&)’
[build] /home/coder/stdexec/include/exec/task.hpp:64:12: note: no known conversion for argument 1 from ‘experimental::execution::_pool_::_static_thread_pool::scheduler’ to ‘experimental::execution::__task::__any_scheduler&&’
[build] ninja: build stopped: subcommand failed.
[proc] The command: /usr/bin/cmake --build /home/coder/stdexec/build/base-debug --target example.hello_world -- exited with code: 1
[driver] Build completed: 00:00:02.833
[build] Build finished with exit code 1

With the scheduler from exec::timed_thread_context the same problem occurs. For exec::single_thread_context, which returns the scheduler for stdexec::run_loop, it compiles.

I guess it comes from the __semi_scheduler concept not being fulfilled:

stdexec/include/exec/task.hpp

Lines 55 to 62 in 711da59

	// A scheduler concept that does not check for copyability since that creates a cycle
	// in the type system.
	template <class _Scheduler>
	concept __semi_scheduler = requires(_Scheduler& __sched) {
	typename _Scheduler::scheduler_concept;
	requires __std::derived_from<typename _Scheduler::scheduler_concept, scheduler_tag>;
	{ schedule(__sched) } -> sender;
	};

The affected schedulers do not have this scheduler_concept typedef - hence the concept is not fulfilled. When the scheduler_concept typedef is added, the build error is also fixed.

Have these particular scheduler implementations just not been updated or is it a problem of the task-implementation that those schedulers cannot be used?

Thanks in advance for any answers.