Function set_error

include/exec/async_scope.hpp:593–599

      template <__movable_value _Error>
      constexpr void set_error(_Error&& __err) noexcept
      {
        auto&            __state = *__state_;
        std::unique_lock __guard{__state.__mutex_};
        __save_completion(set_error_t(), static_cast<_Error&&>(__err));
        __dispatch_result_(__guard);
      }

      constexpr void set_stopped() noexcept
      {