bug #17787 [Form] Fix choice placeholder edge cases (Tobion)

This PR was merged into the 2.7 branch.

Discussion
----------

[Form] Fix choice placeholder edge cases

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

Fixing several problems with choice placeholder that enhances #9030 for more edge cases:

- A choice with an empty value manually added in the choices array should only be considered a placeholder when it is the first element in the final choice select.
This is part of the HTML spec and how browsers also behave. If you select a choice with an empty value that is not the first option, it will still pass the "required" check
and thus submit the empty value. So it's not a placeholder.
If in the example below you move the empty option to the first place, the browsers will error on submit that you
must select a value. So only then it is a placeholder to show as initial value.

```html
<select id="form_timezone" name="form[timezone]" required="required">
    <option value="Africa/Abidjan">Abidjan</option>
    <option value="">Empty</option>
</select>
```

Also the validator https://validator.w3.org/nu/ will mark the above code as error:
> The first child option element of a select element with a required attribute, and without a multiple attribute, and without a size attribute whose value is greater than 1, must have either an empty value attribute, or must have no text content. Consider either adding a placeholder option label, or adding a size attribute with a value equal to the number of option elements.

This is fixed by replacing`0 !== count($choiceList->getChoicesForValues(array('')))` with `$view->vars['placeholder_in_choices'] = $choiceListView->hasPlaceholder()`.
Which means, the required attribute is removed automatically because the select form element is required implicitly anyway due to the nature of the choice UI.

- As the above quote mentions, the `size` attribute also has impact. Namely for a select with size > 1 it can be possible to have a required attribute even without placeholder.
This is because when the size > 1, there is no default choice selected (similar to select with "multiple").

- A placeholder for required radio buttons or a select with size > 1 does not make sense as it would just be fake data that can be submitted (similar to the ignored placeholder for multi-select and checkboxes).

Commits
-------

0efbc30 [Form] fix edge cases with choice placeholder

src/Symfony/Bridge/Twig/Resources/views/Form/form_div_layout.html.twig

@@ -52,7 +52,7 @@
 {%- endblock choice_widget_expanded -%}
 
 {%- block choice_widget_collapsed -%}
-    {%- if required and placeholder is none and not placeholder_in_choices and not multiple -%}
+    {%- if required and placeholder is none and not placeholder_in_choices and not multiple and (attr.size is not defined or attr.size <= 1) -%}
         {% set required = false %}
     {%- endif -%}
     <select {{ block('widget_attributes') }}{% if multiple %} multiple="multiple"{% endif %}>

src/Symfony/Bundle/FrameworkBundle/Resources/views/Form/choice_widget_collapsed.html.php

@@ -1,5 +1,5 @@
 <select
-    <?php if ($required && null === $placeholder && $placeholder_in_choices === false && $multiple === false):
+    <?php if ($required && null === $placeholder && $placeholder_in_choices === false && $multiple === false && (!isset($attr['size']) || $attr['size'] <= 1)):
         $required = false;
     endif; ?>
     <?php echo $view['form']->block($form, 'widget_attributes', array(

src/Symfony/Component/Form/ChoiceList/Factory/DefaultChoiceListFactory.php

@@ -246,7 +246,7 @@ private static function addChoiceViewGroupedBy($groupBy, $choice, $value, $label
 
         $groupLabel = (string) $groupLabel;
 
-        // Initialize the group views if necessary. Unnnecessarily built group
+        // Initialize the group views if necessary. Unnecessarily built group
         // views will be cleaned up at the end of createView()
         if (!isset($preferredViews[$groupLabel])) {
             $preferredViews[$groupLabel] = new ChoiceGroupView($groupLabel);

src/Symfony/Component/Form/ChoiceList/View/ChoiceListView.php

@@ -48,4 +48,24 @@ public function __construct(array $choices = array(), array $preferredChoices =
         $this->choices = $choices;
         $this->preferredChoices = $preferredChoices;
     }
+
+    /**
+     * Returns whether a placeholder is in the choices.
+     *
+     * A placeholder must be the first child element, not be in a group and have an empty value.
+     *
+     * @return bool
+     */
+    public function hasPlaceholder()
+    {
+        if ($this->preferredChoices) {
+            $firstChoice = reset($this->preferredChoices);
+
+            return $firstChoice instanceof ChoiceView && '' === $firstChoice->value;
+        }
+
+        $firstChoice = reset($this->choices);
+
+        return $firstChoice instanceof ChoiceView && '' === $firstChoice->value;
+    }
 }

src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php

@@ -199,7 +199,7 @@ public function buildView(FormView $view, FormInterface $form, array $options)
         }
 
         // Check if the choices already contain the empty value
-        $view->vars['placeholder_in_choices'] = 0 !== count($options['choice_list']->getChoicesForValues(array('')));
+        $view->vars['placeholder_in_choices'] = $choiceListView->hasPlaceholder();
 
         // Only add the empty value option if this is not the case
         if (null !== $options['placeholder'] && !$view->vars['placeholder_in_choices']) {
@@ -343,6 +343,9 @@ public function configureOptions(OptionsResolver $resolver)
             if ($options['multiple']) {
                 // never use an empty value for this case
                 return;
+            } elseif ($options['required'] && ($options['expanded'] || isset($options['attr']['size']) && $options['attr']['size'] > 1)) {
+                // placeholder for required radio buttons or a select with size > 1 does not make sense
+                return;
             } elseif (false === $placeholder) {
                 // an empty value should be added but the user decided otherwise
                 return;

src/Symfony/Component/Form/Tests/AbstractBootstrap3LayoutTest.php

@@ -232,6 +232,26 @@ public function testSingleChoice()
         );
     }
 
+    public function testSelectWithSizeBiggerThanOneCanBeRequired()
+    {
+        $form = $this->factory->createNamed('name', 'choice', null, array(
+            'choices' => array('a', 'b'),
+            'choices_as_values' => true,
+            'multiple' => false,
+            'expanded' => false,
+            'attr' => array('size' => 2),
+        ));
+
+        $this->assertWidgetMatchesXpath($form->createView(), array('attr' => array('class' => '')),
+'/select
+    [@name="name"]
+    [@required="required"]
+    [@size="2"]
+    [count(./option)=2]
+'
+        );
+    }
+
     public function testSingleChoiceWithoutTranslation()
     {
         $form = $this->factory->createNamed('name', 'choice', '&a', array(
@@ -754,6 +774,7 @@ public function testSingleChoiceExpandedWithPlaceholder()
             'multiple' => false,
             'expanded' => true,
             'placeholder' => 'Test&Me',
+            'required' => false,
         ));
 
         $this->assertWidgetMatchesXpath($form->createView(), array(),

src/Symfony/Component/Form/Tests/AbstractLayoutTest.php

@@ -528,6 +528,26 @@ public function testSingleChoice()
         );
     }
 
+    public function testSelectWithSizeBiggerThanOneCanBeRequired()
+    {
+        $form = $this->factory->createNamed('name', 'choice', null, array(
+            'choices' => array('a', 'b'),
+            'choices_as_values' => true,
+            'multiple' => false,
+            'expanded' => false,
+            'attr' => array('size' => 2),
+        ));
+
+        $this->assertWidgetMatchesXpath($form->createView(), array(),
+'/select
+    [@name="name"]
+    [@required="required"]
+    [@size="2"]
+    [count(./option)=2]
+'
+        );
+    }
+
     public function testSingleChoiceWithoutTranslation()
     {
         $form = $this->factory->createNamed('name', 'choice', '&a', array(
@@ -1001,6 +1021,7 @@ public function testSingleChoiceExpandedWithPlaceholder()
             'multiple' => false,
             'expanded' => true,
             'placeholder' => 'Test&Me',
+            'required' => false,
         ));
 
         $this->assertWidgetMatchesXpath($form->createView(), array(),

src/Symfony/Component/Form/Tests/Extension/Core/Type/ChoiceTypeTest.php

@@ -1689,7 +1689,7 @@ public function testDontPassPlaceholderIfContainedInChoices($multiple, $expanded
             'expanded' => $expanded,
             'required' => $required,
             'placeholder' => $placeholder,
-            'choices' => array('A' => 'a', 'Empty' => ''),
+            'choices' => array('Empty' => '', 'A' => 'a'),
             'choices_as_values' => true,
         ));
         $view = $form->createView();
@@ -1716,9 +1716,9 @@ public function getOptionsWithPlaceholder()
             array(false, true, false, '', 'None'),
             array(false, true, false, null, null),
             array(false, true, false, false, null),
-            array(false, true, true, 'foobar', 'foobar'),
-            // radios should never have an empty label
-            array(false, true, true, '', 'None'),
+            // required radios should never have a placeholder
+            array(false, true, true, 'foobar', null),
+            array(false, true, true, '', null),
             array(false, true, true, null, null),
             array(false, true, true, false, null),
             // multiple non-expanded