feat(input): support constant expressions for ngTrueValue/ngFalseValue

ngTrueValue and ngFalseValue now support parsed expressions which the parser determines to be constant values.

BREAKING CHANGE:

Previously, these attributes would always be treated as strings. However, they are now parsed as
expressions, and will throw if an expression is non-constant.

To convert non-constant strings into constant expressions, simply wrap them in an extra pair of quotes, like so:

    <input type="checkbox" ng-model="..." ng-true-value="'truthyValue'">

Closes #8041
Closes #5346
Closes #1199

docs/content/error/ngModel/constexpr.ngdoc

@@ -0,0 +1,21 @@
+@ngdoc error
+@name ngModel:constexpr
+@fullName Non-Constant Expression
+@description
+
+Some attributes used in conjunction with ngModel (such as ngTrueValue or ngFalseValue) will only
+accept constant expressions.
+
+Examples using constant expressions include:
+
+```
+<input type="checkbox" ng-model="..." ng-true-value="'truthyValue'">
+<input type="checkbox" ng-model="..." ng-false-value="0">
+```
+
+Examples of non-constant expressions include:
+
+```
+<input type="checkbox" ng-model="..." ng-true-value="someValue">
+<input type="checkbox" ng-model="..." ng-false-value="{foo: someScopeValue}">
+```

src/ng/directive/input.js

@@ -807,8 +807,8 @@ var inputType = {
    *
    * @param {string} ngModel Assignable angular expression to data-bind to.
    * @param {string=} name Property name of the form under which the control is published.
-   * @param {string=} ngTrueValue The value to which the expression should be set when selected.
-   * @param {string=} ngFalseValue The value to which the expression should be set when not selected.
+   * @param {expression=} ngTrueValue The value to which the expression should be set when selected.
+   * @param {expression=} ngFalseValue The value to which the expression should be set when not selected.
    * @param {string=} ngChange Angular expression to be executed when input changes due to user
    *    interaction with the input element.
    *
@@ -824,7 +824,7 @@ var inputType = {
          <form name="myForm" ng-controller="Ctrl">
            Value1: <input type="checkbox" ng-model="value1"> <br/>
            Value2: <input type="checkbox" ng-model="value2"
-                          ng-true-value="YES" ng-false-value="NO"> <br/>
+                          ng-true-value="'YES'" ng-false-value="'NO'"> <br/>
            <tt>value1 = {{value1}}</tt><br/>
            <tt>value2 = {{value2}}</tt><br/>
           </form>
@@ -1183,12 +1183,22 @@ function radioInputType(scope, element, attr, ctrl) {
   attr.$observe('value', ctrl.$render);
 }
 
-function checkboxInputType(scope, element, attr, ctrl) {
-  var trueValue = attr.ngTrueValue,
-      falseValue = attr.ngFalseValue;
+function parseConstantExpr($parse, context, name, expression, fallback) {
+  var parseFn;
+  if (isDefined(expression)) {
+    parseFn = $parse(expression);
+    if (!parseFn.constant) {
+      throw new minErr('ngModel')('constexpr', 'Expected constant expression for `{0}`, but saw ' +
+                                   '`{1}`.', name, expression);
+    }
+    return parseFn(context);
+  }
+  return fallback;
+}
 
-  if (!isString(trueValue)) trueValue = true;
-  if (!isString(falseValue)) falseValue = false;
+function checkboxInputType(scope, element, attr, ctrl, $sniffer, $browser, $filter, $parse) {
+  var trueValue = parseConstantExpr($parse, scope, 'ngTrueValue', attr.ngTrueValue, true);
+  var falseValue = parseConstantExpr($parse, scope, 'ngFalseValue', attr.ngFalseValue, false);
 
   var listener = function(ev) {
     scope.$apply(function() {
@@ -1208,7 +1218,7 @@ function checkboxInputType(scope, element, attr, ctrl) {
   };
 
   ctrl.$formatters.push(function(value) {
-    return value === trueValue;
+    return equals(value, trueValue);
   });
 
   ctrl.$parsers.push(function(value) {
@@ -1356,14 +1366,15 @@ function checkboxInputType(scope, element, attr, ctrl) {
       </file>
     </example>
  */
-var inputDirective = ['$browser', '$sniffer', '$filter', function($browser, $sniffer, $filter) {
+var inputDirective = ['$browser', '$sniffer', '$filter', '$parse',
+    function($browser, $sniffer, $filter, $parse) {
   return {
     restrict: 'E',
     require: ['?ngModel'],
     link: function(scope, element, attr, ctrls) {
       if (ctrls[0]) {
         (inputType[lowercase(attr.type)] || inputType.text)(scope, element, attr, ctrls[0], $sniffer,
-                                                            $browser, $filter);
+                                                            $browser, $filter, $parse);
       }
     }
   };

test/ng/directive/inputSpec.js

@@ -2528,8 +2528,8 @@ describe('input', function() {
 
 
     it('should allow custom enumeration', function() {
-      compileInput('<input type="checkbox" ng-model="name" ng-true-value="y" ' +
-          'ng-false-value="n">');
+      compileInput('<input type="checkbox" ng-model="name" ng-true-value="\'y\'" ' +
+          'ng-false-value="\'n\'">');
 
       scope.$apply(function() {
         scope.name = 'y';
@@ -2554,6 +2554,27 @@ describe('input', function() {
     });
 
 
+    it('should throw if ngTrueValue is present and not a constant expression', function() {
+      expect(function() {
+        compileInput('<input type="checkbox" ng-model="value" ng-true-value="yes" />');
+      }).toThrowMinErr('ngModel', 'constexpr', "Expected constant expression for `ngTrueValue`, but saw `yes`.");
+    });
+
+
+    it('should throw if ngFalseValue is present and not a constant expression', function() {
+      expect(function() {
+        compileInput('<input type="checkbox" ng-model="value" ng-false-value="no" />');
+      }).toThrowMinErr('ngModel', 'constexpr', "Expected constant expression for `ngFalseValue`, but saw `no`.");
+    });
+
+
+    it('should not throw if ngTrueValue or ngFalseValue are not present', function() {
+      expect(function() {
+        compileInput('<input type="checkbox" ng-model="value" />');
+      }).not.toThrow();
+    });
+
+
     it('should be required if false', function() {
       compileInput('<input type="checkbox" ng:model="value" required />');