// Should be set before calling Get().
	updateFn func() (T, error)

	// ttl for a cached value.
	ttl time.Duration

	opts Opts

	// Once can be used to initialize values for lazy initialization.
	// Should be set before calling Get().
	Once sync.Once

	// Managed values.
	valErr atomic.Pointer[struct {
		v T
		e error
	}]
	lastUpdateMs atomic.Int64
	updating     sync.Mutex
}