How implement a coroutine suspend-running in a multi-thread trampoline scheduler when using a custom sender?

The issue provides only an inline counter_sender example and names trampoline_scheduler, schedule(), then(), connect(), and sync_wait(); begin by checking those APIs and their operation-state lifetime rules. A useful resolution would define a supported way to suspend and resume the running coroutine across the multi-thread trampoline, demonstrated by a working version of the example with clear completion behavior.

Something like this

#include <unifex/single_thread_context.hpp>
#include <unifex/trampoline_scheduler.hpp>
#include <unifex/let_value.hpp>
#include <unifex/just.hpp>
#include <unifex/sync_wait.hpp>
#include <unifex/then.hpp>

using namespace unifex;

class counter_sender {
public:
    counter_sender(int start, int end, trampoline_scheduler& scheduler)
        : current_(start), end_(end), scheduler_(scheduler) {}

    template<typename Receiver>
    struct operation {
        operation(int current, int end, 
                 trampoline_scheduler& scheduler, Receiver&& receiver)
            : current_(current)
            , end_(end)
            , scheduler_(scheduler)
            , receiver_((Receiver&&)receiver) {}

        void start() noexcept {
            if (current_ >= end_) {
                unifex::set_value(std::move(receiver_), current_);
                return;
            }

            auto next = current_ + 1;
            
            auto reschedule = 
                scheduler_.schedule() | 
                then([this, next]() {
                    auto new_op = operation{
                        next, 
                        end_, 
                        scheduler_, 
                        (Receiver&&)receiver_
                    };
                    new_op.start();
                });

            // restart
            connect(std::move(reschedule), receiver_).start();
        }

    private:
        int current_;
        int end_;
        trampoline_scheduler& scheduler_;
        Receiver receiver_;
    };

    template<typename Receiver>
    auto connect(Receiver&& receiver) {
        return operation<Receiver>{
            current_, end_, scheduler_, (Receiver&&)receiver
        };
    }

private:
    int current_;
    int end_;
    trampoline_scheduler& scheduler_;
};

void example() {
    trampoline_scheduler scheduler{};
    
    auto result = sync_wait(counter_sender{1, 5, scheduler});
    
    if (result) {
        std::cout << "Final count: " << *result << "\n";
    }
}