template<typename S>
{
s.defer_for(duration{}, std::declval<optional_delay_from_now (*)(const details::fake_schedulable_handler&)>(), handler)
} -> std::same_as<void>;
{
s.defer_for(duration{}, std::declval<optional_delay_from_this_timepoint (*)(const details::fake_schedulable_handler&)>(), handler)
} -> std::same_as<void>;
{
s.defer_for(duration{}, std::declval<optional_delay_to (*)(const details::fake_schedulable_handler&)>(), handler)
} -> std::same_as<void>;
}