How can I conditionally require form inputs with AngularJS?

For Angular2

<input type='email' 
    [(ngModel)]='contact.email'
    [required]='!contact.phone' >

if you want put a input required if other is written:

   <input type='text'
   name='name'
   ng-model='person.name'/>

   <input type='text'
   ng-model='person.lastname'             
   ng-required='person.name' />  

Regards.

There's no need to write a custom directive. Angular's documentation is good but not complete. In fact, there is a directive called ngRequired, that takes an Angular expression.

<input type='email'
       name='email'
       ng-model='contact.email' 
       placeholder='[email protected]'
       ng-required='!contact.phone' />

<input type='text'
       ng-model='contact.phone'             
       placeholder='(xxx) xxx-xxxx'
       ng-required='!contact.email' />  

Here's a more complete example: http://jsfiddle.net/uptnx/1/