feature #11709 [Validator] Expression validator now processes null va…

…lues (tommygnr)

This PR was merged into the 2.6-dev branch.

Discussion
----------

[Validator] Expression validator now processes null values

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes(minor)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

The ExpressionValidator was incorrectly skipping validation of null or empty string values.

For example the following was (incorrectly) considered valid if hairColour is null because the validator was skipped
```php
<?php

namespace Acme\DemoBundle\Model\Person;

use Symfony\Component\Validator\Constraints as Assert;

class Person
{
    private $hasHair;

    /**
     * @Assert\Expression(
     *     "!(this.hasHair() and value == null)",
     *     message="If you have hair you must pick its colour!"
     * )
     */
    private $hairColour;
}
```

This is a follow on from #11590 but is targeted against master as the BC break introduced was considered undesirable for currently released versions of symfony.

I will squash and create a documentation PR once there is consensus that this is ready to be merged.

Commits
-------

580e1a7 [Validator] fixed: Expressions always valid for null values

UPGRADE-2.6.md

@@ -17,3 +17,14 @@ Validator
    {
    }
    ```
+
+ * Prior to 2.6 `Symfony\Component\Validator\Constraints\ExpressionValidator`
+   would not execute the Expression if it was attached to a property on an
+   object and that property was set to `null` or an empty string.
+
+   To emulate the old behaviour change your expression to something like
+   this:
+
+   ```
+   value == null or (YOUR_EXPRESSION)
+   ```

src/Symfony/Component/Validator/CHANGELOG.md

@@ -8,6 +8,7 @@ CHANGELOG
  * [BC BREAK] `UserPasswordValidator` source message change
  * [BC BREAK] added internal `ExecutionContextInterface::setConstraint()`
  * added `ConstraintViolation::getConstraint()`
+ * [BC BREAK] The `ExpressionValidator` will now evaluate the Expression even when the property value is null or an empty string
 
 2.5.0
 -----

src/Symfony/Component/Validator/Constraints/ExpressionValidator.php

@@ -60,10 +60,6 @@ public function validate($value, Constraint $constraint)
             throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Expression');
         }
 
-        if (null === $value || '' === $value) {
-            return;
-        }
-
         $variables = array();
 
         // Symfony 2.5+

src/Symfony/Component/Validator/Tests/Constraints/ExpressionValidatorTest.php

@@ -29,18 +29,28 @@ protected function createValidator()
         return new ExpressionValidator(PropertyAccess::createPropertyAccessor());
     }
 
-    public function testNullIsValid()
+    public function testExpressionIsEvaluatedWithNullValue()
     {
-        $this->validator->validate(null, new Expression('value == 1'));
+        $constraint = new Expression(array(
+            'expression' => 'false',
+            'message' => 'myMessage',
+        ));
 
-        $this->assertNoViolation();
+        $this->validator->validate('', $constraint);
+
+        $this->assertViolation('myMessage');
     }
 
-    public function testEmptyStringIsValid()
+    public function testExpressionIsEvaluatedWithEmptyStringValue()
     {
-        $this->validator->validate('', new Expression('value == 1'));
+        $constraint = new Expression(array(
+            'expression' => 'false',
+            'message' => 'myMessage',
+        ));
 
-        $this->assertNoViolation();
+        $this->validator->validate('', $constraint);
+
+        $this->assertViolation('myMessage');
     }
 
     public function testSucceedingExpressionAtObjectLevel()