bug #21267 [Form] Fix ChoiceType to ensure submitted data is not nested unnecessarily (issei-m)

This PR was squashed before being merged into the 2.7 branch (closes #21267).

Discussion
----------

[Form] Fix ChoiceType to ensure submitted data is not nested unnecessarily

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Fixed ChoiceType to protect against some problem caused by treating of array.

Let's say we have the choice-form like:

```php
$form = $factory->create(ChoiceType, null, [
    'choices' => [
        'A',
        'B',
        'C',
    ],
    'expanded' => true,
    'multiple' => true,
]);
```

Then, submit data like this:

```php
$form->submit([
    [], // unnecessality nested
]);
```

(Yes, I agree in most cases these situation doesn't happen, but can be)

Then, we get `array_flip(): Can only flip STRING and INTEGER values!` error at [here](https://github.com/symfony/symfony/blob/6babdb3296a5fbbd9c69d1e3410adbdf749959ef/src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php#L114).
Even if form is not `multiple`, annoying `Array to string conversion` error occurs in [here](https://github.com/symfony/symfony/blob/6babdb3296a5fbbd9c69d1e3410adbdf749959ef/src/Symfony/Component/Form/ChoiceList/ArrayChoiceList.php#L144) (via [ChoicesToValuesTransformer](https://github.com/symfony/symfony/blob/5129c4cf7e294b1a5ea30d6fec6e89b75396dcd2/src/Symfony/Component/Form/Extension/Core/DataTransformer/ChoicesToValuesTransformer.php#L74)).
(As far as I know, non-multiple and non-expanded form has no problem, thanks to [ChoiceToValueTransformer](https://github.com/symfony/symfony/blob/6babdb3296a5fbbd9c69d1e3410adbdf749959ef/src/Symfony/Component/Form/Extension/Core/DataTransformer/ChoiceToValueTransformer.php#L43))

To resolve these problems, I just added a simple-validation listener to choice type.

Commits
-------

64d7a82d28 [Form] Fix ChoiceType to ensure submitted data is not nested unnecessarily

Author: fabpot

Extension/Core/Type/ChoiceType.php

@@ -160,6 +160,22 @@ public function buildForm(FormBuilderInterface $builder, array $options)
             // transformation is merged back into the original collection
             $builder->addEventSubscriber(new MergeCollectionListener(true, true));
         }
+
+        // To avoid issues when the submitted choices are arrays (i.e. array to string conversions),
+        // we have to ensure that all elements of the submitted choice data are strings or null.
+        $builder->addEventListener(FormEvents::PRE_SUBMIT, function (FormEvent $event) {
+            $data = $event->getData();
+
+            if (!is_array($data)) {
+                return;
+            }
+
+            foreach ($data as $v) {
+                if (null !== $v && !is_string($v)) {
+                    throw new TransformationFailedException('All choices submitted must be NULL or strings.');
+                }
+            }
+        }, 256);
     }
 
     /**
@@ -505,8 +521,8 @@ private function createChoiceListView(ChoiceListInterface $choiceList, array $op
      * "choice_label" closure by default.
      *
      * @param array|\Traversable $choices      The choice labels indexed by choices
-     * @param object             $choiceLabels The object that receives the choice labels
-     *                                         indexed by generated keys.
+     * @param object             $choiceLabels the object that receives the choice labels
+     *                                         indexed by generated keys
      * @param int                $nextKey      The next generated key
      *
      * @return array The choices in a normalized array with labels replaced by generated keys

Tests/Extension/Core/Type/ChoiceTypeTest.php

@@ -14,9 +14,10 @@
 use Symfony\Component\Form\ChoiceList\View\ChoiceGroupView;
 use Symfony\Component\Form\ChoiceList\View\ChoiceView;
 use Symfony\Component\Form\Extension\Core\ChoiceList\ObjectChoiceList;
+use Symfony\Component\Form\Test\TypeTestCase;
 use Symfony\Component\Form\Tests\Fixtures\ChoiceSubType;
 
-class ChoiceTypeTest extends \Symfony\Component\Form\Test\TypeTestCase
+class ChoiceTypeTest extends TypeTestCase
 {
     private $choices = array(
         'Bernhard' => 'a',
@@ -2283,4 +2284,30 @@ public function testCustomChoiceTypeDoesNotInheritChoiceLabels()
         // In this case the 'choice_label' closure returns null and not the closure from the first choice type.
         $this->assertNull($form->get('subChoice')->getConfig()->getOption('choice_label'));
     }
+
+    /**
+     * @dataProvider invalidNestedValueTestMatrix
+     */
+    public function testSubmitInvalidNestedValue($multiple, $expanded, $submissionData)
+    {
+        $form = $this->factory->create('choice', null, array(
+            'choices' => $this->choices,
+            'multiple' => $multiple,
+            'expanded' => $expanded,
+        ));
+
+        $form->submit($submissionData);
+        $this->assertFalse($form->isSynchronized());
+        $this->assertEquals('All choices submitted must be NULL or strings.', $form->getTransformationFailure()->getMessage());
+    }
+
+    public function invalidNestedValueTestMatrix()
+    {
+        return array(
+            'non-multiple, non-expanded' => array(false, false, array(array())),
+            'non-multiple, expanded' => array(false, true, array(array())),
+            'multiple, non-expanded' => array(true, false, array(array())),
+            'multiple, expanded' => array(true, true, array(array())),
+        );
+    }
 }