fix($parse): stateful interceptors override an undefined expression

[lgalfaso]

When using $parse with a stateful interceptor and the expression is undefined, then return the result from the interceptor

Closes #9821

[lgalfaso]

@rodyhaddad WDYT?

[caitp]

this is wrong, should be {{greeting}}

[lgalfaso]

There is a watcher on the parent scope that will copy greeting to text that is not triggered

[caitp]

Get rid of the watcher in the parent scope, that is bogus and unrelated to the issue, honestly. The problem still happens when the 2-way binding name is used. That's bad.

[lgalfaso]

updated the patch with the comments

[caitp]

yeah --- so I mean this is okay, but it's weird that we're special-casing $stateful for this. It seems like this should be the default behaviour for anything that isn't worried about bind-once.

[caitp]

and $stateful's meaning makes a lot more sense when it comes to figuring out whether to invoke filters or not. It doesn't make a lot of sense when we're talking about overriding a return value.

[caitp]

@tbosch / @jeffbcross what do you think --- leave it as is, or find a way to isolate the bogus behaviour to just bind-once delegates?

[lgalfaso]

the other option would be to add another property to the interceptor to state this interceptor overrides an undefined. We cannot just remove the logic as there are other interceptors (e.g. interpolate that will transform undefined into the empty string) that would be better if left alone

[caitp]

I don't see why we'd want to do that --- the behaviour was added for bind-once, that's where it makes sense. Why add another footgun for people to use?

[caitp]

@IgorMinar did you ever take a look at this?

As I said, I think we could land it and fix it up later, but I would prefer we didn't do it in this fashion

[IgorMinar]

I'm not keen on having another flag.

I'm with @caitp that something smells here.

As @caitp suggested the change above should be the default behavior, but that requires some changes to one-time binding and $interpolate. I think it's doable.

The fix above shouldn't cause other issues AFAICT, so let's land it now and improve next week.

[caitp]

sounds good to me