Custom form validation directive to compare two fields

You do not need any directive. Just assign the “min” value of max to min-value. Like:

<input name="min" type="number" ng-model="field.min"/>
<input name="max" type="number" ng-model="field.max" min=" {{ field.min }}"/>

And you do not need any customization.
More: you can do min=" {{ field.min + 1}}"