Accept observable in mergeValidators/setValidators

[rafaelss95]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Support request
[ ] Other... Please describe:

Current behavior

Currently mergeValidators/setValidators doesn't accept observables... we have the validateOn that deals with custom errors, but not with validators.

Expected behavior

I'd expect to see mergeValidators/setValidators accepting observables also, as setValue/patchValue.

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

I have an use case where I set the validators based on another formControl value, e.g.:

this.formGroup.controls.someControl.setValidators(
  this.formGroup.select(({ dependent }) => dependent).pipe(
    map(dependent => dependent === 'X' ? [Validators.required, Validators.minlength(3)], customValidator),
  ),
);

---

Also, I think we could add support to accept observable in mergeErrors/setErrors, that would remove the necessity of having validateOn... but that's just an idea.

Environment

Angular version: 10.x.y

[NetanelBasal]

You're welcome to create a PR.