How to distinguish between cancellation from stop_when and ouside?

[vrecluse]

eg. call rpc with timeout using stop_when, and retry:

        int retryCount = 5;

        while (retryCount > 0) {
            -- retryCount;
            auto result = co_await
                    done_as_optional(stop_when(fakeRpc(), s.schedule_after(2s)));
            if (result) {
                std::cout << "Task done with " << *result << std::endl;
                co_return *result;
            } else {
                std::cout << "Retry..." << std::endl;
            }
        }

But if this coroutine is canceled outside by stop_source, stop_when is canceled, and several retries run instantly.
How to tell if canceled by stop_when?