fix(select): Fail validation if selection is not a valid option.

The md-select currently passes validation even if the model is
not one of the provided options.

Fixes #7954.

Closes #7989

src/components/select/select.js

@@ -685,6 +685,11 @@ function SelectMenuDirective($parse, $mdUtil, $mdTheming) {
       self.ngModel = ngModel;
       self.modelBinding = binding;
 
+      // Setup a more robust version of isEmpty to ensure value is a valid option
+      self.ngModel.$isEmpty = function($viewValue) {
+        return !self.options[$viewValue];
+      };
+
       // Allow users to provide `ng-model="foo" ng-model-options="{trackBy: 'foo.id'}"` so
       // that we can properly compare objects set on the model to the available options
       if (ngModel.$options && ngModel.$options.trackBy) {

src/components/select/select.spec.js

@@ -826,6 +826,29 @@ describe('<md-select>', function() {
           expect($rootScope.testForm.$valid).toBe(false);
       }));
 
+      it('properly validates required attribute based on available options', inject(function($rootScope, $compile) {
+        var template =
+          '<form name="testForm">' +
+          '  <md-select ng-model="model" required="required">' +
+          '    <md-option ng-repeat="opt in opts" ng-value="opt"></md-option>' +
+          '  </md-select>' +
+          '</form>';
+
+        $rootScope.opts = [1, 2, 3, 4];
+
+        $compile(template)($rootScope);
+
+        // Option 0 is not available; should be false
+        $rootScope.model = 0;
+        $rootScope.$digest();
+        expect($rootScope.testForm.$valid).toBe(false);
+
+        // Option 1 is available; should be true
+        $rootScope.model = 1;
+        $rootScope.$digest();
+        expect($rootScope.testForm.$valid).toBe(true);
+      }));
+
       it('should keep the form pristine when model is predefined', inject(function($rootScope, $timeout, $compile) {
         $rootScope.model = [1, 2];
         $rootScope.opts = [1, 2, 3, 4];