ng-class allows assignment operator. is it a bug or feature?

I recently noticed that ng-class allows assignment operator is it a bug or a feature.

<li ng-repeat="cat in cats" ng-class="{active: cat = 'some-text'}">{{cat}}</li>

correct usage

<li ng-repeat="cat in cats" ng-class="{active: cat == 'some-text'}">{{cat}}</li>

also look at the plunk behaviour

Allowing assignment operator is a bug or a feature?

Solution

Raised an issue in GitHub and got a clarification