I'd like to propose a slightly different API. We can have a function that would accept the necessary details (e.g. lView, tNode, etc) and also a trigger type and return whether those triggers should be attached, for example:

export function shouldAttachTrigger(triggerType: TriggerType, lView: LView, tNode: TNode): boolean {
  // ... determine whether a given trigger type should be activated ...
}

The actual usage would look like this:

export function ɵɵdeferWhen(rawValue: unknown) {
  const lView = getLView();
  const tNode = getSelectedTNode();

  if (shouldAttachTrigger(TriggerType.Regular, lView, tNode)) {
    // ... the code that attaches the trigger ...
  }

For a "hydrate" trigger:

export function ɵɵdeferHydrateWhen(rawValue: unknown) {
  const lView = getLView();
  const tNode = getSelectedTNode();

  if (shouldAttachTrigger(TriggerType.Hydrate, lView, tNode)) {
    // ... the code that attaches the trigger ...
  }

We'd need to ensure that the TriggerType.Regular and TriggerType.Hydrate are mutually exclusive, i.e. we can compute a value based on the rules mentioned in the code below and return true/false depending on a trigger type.

It looks like this function is missing an early exit (similar to other functions).

I guess the intention was to check for the SSR_UNIQUE_ID field value presence?

I also think that there should be no conditions in this function and we should move the check to an instruction level (i.e. to a call site).

If we rely on the tDetails.hydrateTriggers presence, we'll make an implicit ordering dependency between deferHydrate* and defer* (non-hydrate) instructions, i.e. hydrate ones must be invoked first. This might be a problem for deferHydrateWhen instruction, which would end up in the "update" block of a template function, when the other functions (e.g. deferOnIdle) would be in the "create" block. I'm wondering if actually checking the SSR_UNIQUE_ID field value presence in LDetails should be sufficient. That would mean that this block was triggered on the server, thus it has "hydrate" triggers.