Async validator not working without Validator.required

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

When you declare a FormControl without Validator.required and an async validator, there is an error.

ERROR Error: Expected validator to return Promise or Observable.

with: new FormControl(question.value || '', null,this.AsyncValidator(question))

But it’s ok if you have: new FormControl(question.value || '', Validators.required, this.AsyncValidator(question))

Expected behavior

the expected behavior would be defining async validators in formcontrol without a required sync validator.

What is the motivation / use case for changing the behavior?

I need a field without required validation but with an async validation

Environment

angular version: 4.3.1

Browser:
- [ x] Chrome (desktop)

 
For Tooling issues:
- Node version: 7.2.0
- Platform:  Linux

[kara]

Async validators only run if sync validators pass. So if you add Validators.required with an empty value, it will fail and the async validators won’t run (and therefore won’t throw an error).

[ericmartinezr]

The problem here is not what you think it is. The error clearly says that you’re not returning a Promise nor an Observable in your Async Validador. What’s weird is that when you add the Validator.required validator the error dissapears, when imho it should still be there.

Reproduction. The sync validator is null, the async validator is not null returning a promise resolving to nothing really, it’s just to reproduce the problem. Comment the return and you’ll see the error.