fix(input): always format viewValue as a string inputs with text controls

Backported from 1eda18365a

NgModel will format all scope-based values to string when setting the viewValue for
the associated input element. The formatting, however, only applies to input elements
that contain a text, email, url or blank input type. In the event of a null or undefined
scope or model value, the viewValue will be set to null or undefined instead of being
converted to an empty string.

Closes #5936
Closes #9277

src/ng/directive/input.js

@@ -475,7 +475,18 @@ function addNativeHtml5Validators(ctrl, validatorName, badFlags, ignoreFlags, va
   }
 }
 
-function textInputType(scope, element, attr, ctrl, $sniffer, $browser) {
+function stringBasedInputType(ctrl) {
+  ctrl.$formatters.push(function stringifier(value) {
+    return ctrl.$isEmpty(value) ? value : value.toString();
+  });
+}
+
+function textInputType(scope, element, attr,ctrl, $sniffer, $browser) {
+  baseInputType(scope, element, attr, ctrl, $sniffer, $browser);
+  stringBasedInputType(ctrl);
+}
+
+function baseInputType(scope, element, attr, ctrl, $sniffer, $browser) {
   var validity = element.prop(VALIDITY_STATE_PROPERTY);
   var placeholder = element[0].placeholder, noevent = {};
   var type = lowercase(element[0].type);
@@ -1535,6 +1546,8 @@ var requiredDirective = function() {
  */
 var ngListDirective = function() {
   return {
+    restrict: 'A',
+    priority: 100,
     require: 'ngModel',
     link: function(scope, element, attr, ctrl) {
       var match = /\/(.*)\//.exec(attr.ngList),

test/ng/directive/inputSpec.js

@@ -310,6 +310,87 @@ describe('ngModel', function() {
   }));
 
 
+  it('should always format the viewValue as a string for a blank input type when the value is present',
+    inject(function($compile, $rootScope, $sniffer) {
+
+    var form = $compile('<form name="form"><input name="field" ng-model="val" /></form>')($rootScope);
+
+    $rootScope.val = 123;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe('123');
+
+    $rootScope.val = null;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe(null);
+
+    dealoc(form);
+  }));
+
+
+  it('should always format the viewValue as a string for a `text` input type when the value is present',
+    inject(function($compile, $rootScope, $sniffer) {
+
+    var form = $compile('<form name="form"><input type="text" name="field" ng-model="val" /></form>')($rootScope);
+    $rootScope.val = 123;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe('123');
+
+    $rootScope.val = null;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe(null);
+
+    $rootScope.val = undefined;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBeUndefined();
+
+    dealoc(form);
+  }));
+
+
+  it('should always format the viewValue as a string for an `email` input type when the value is present',
+    inject(function($compile, $rootScope, $sniffer) {
+
+    var fakeEmail = {};
+    fakeEmail.toString = function() { return 'fake@email'; };
+    var form = $compile('<form name="form"><input type="email" name="field" ng-model="val" /></form>')($rootScope);
+    $rootScope.val = fakeEmail;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe('fake@email');
+
+    $rootScope.val = null;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe(null);
+
+    $rootScope.val = undefined;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBeUndefined();
+
+    dealoc(form);
+  }));
+
+
+  it('should always format the viewValue as a string for a `url` input type when the value is present',
+    inject(function($compile, $rootScope, $sniffer) {
+
+    var fakeUrl = {};
+    fakeUrl.toString = function() { return 'https://www.angularjs.org'; };
+    var form = $compile('<form name="form"><input type="url" name="field" ng-model="val" /></form>')($rootScope);
+    $rootScope.val = fakeUrl;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe('https://www.angularjs.org');
+
+    $rootScope.val = null;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBe(null);
+
+    $rootScope.val = undefined;
+    $rootScope.$digest();
+    expect($rootScope.form.field.$viewValue).toBeUndefined();
+
+    dealoc(form);
+  }));
+
+
   it('should register/deregister a nested ngModel with parent form when entering or leaving DOM',
       inject(function($compile, $rootScope) {