sync_wait / co_await of starts_on(task_scheduler{...}, task) fails a completion-domain static_assert

[kyusic]

Starting a stdexec::task with starts_on on a stdexec::task_scheduler produces a sender
that I cannot sync_wait, and cannot co_await from another task. The same code with the
concrete scheduler compiles.

#include <stdexec/execution.hpp>

namespace ex = stdexec;

auto child() -> ex::task<void> { co_return; }

int main() {
  ex::run_loop loop;
  ex::sync_wait(
      ex::starts_on(ex::task_scheduler{loop.get_scheduler()}, child()));
}

g++ -std=c++26 -Iinclude -fsyntax-only repro.cpp

fails with:

include/stdexec/__detail/__domain.hpp:297:21: error: static assertion failed: the sender claims to complete on a domain that is not the domain of its completion scheduler
  297 |       static_assert(__same_as<_Domain, __detail::__scheduler_domain_t<_Sch, _Env const &...>>,

clang 21.1.8 reports the same static_assert at the same line.

The same failure appears when the sender is awaited from another task:

auto parent() -> ex::task<void> {
  auto sched = co_await ex::get_start_scheduler();
  co_await ex::starts_on(sched, child());
}

int main() {
  ex::run_loop loop;
  ex::sync_wait(ex::starts_on(loop.get_scheduler(), parent()));
}

Here sched deduces to stdexec::task_scheduler, even though parent() itself is
started on the concrete run_loop::scheduler.

Variants, all with the same child():

variant	result
sync_wait(starts_on(task_scheduler{s}, child()))	fails
co_await starts_on(task_scheduler{s}, child()) inside a task	fails
sync_wait(starts_on(s, child())) with the concrete scheduler s	compiles
sync_wait(starts_on(task_scheduler{s}, just()))	compiles
sync_wait(starts_on(task_scheduler{s}, just() | let_value(child)))	compiles
sync_wait(child() | write_env(prop{get_start_scheduler, task_scheduler{s}}))	compiles
connect(starts_on(task_scheduler{s}, child()), some_receiver{})	compiles
spawn(starts_on(task_scheduler{s}, child()) | upon_error(...), scope.get_token())	compiles
completion_signatures_of_t<decltype(starts_on(task_scheduler{s}, child())), env<>>	compiles

Is starts_on with a task_scheduler and a task child expected to be usable with
sync_wait / co_await, or is the type-erased scheduler meant to be used differently here?

Tested at f91f6363 with g++ 15.2.0 and clang 21.1.8, both -std=c++26.