bug #15623 Fix the retrieval of the value with property path when using a loader (stof)

This PR was merged into the 2.7 branch.

Discussion
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Fix the retrieval of the value with property path when using a loader

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

the ChoiceType is transforming the form data to its value in case of non-expanded fields.

When creating the choice list from choices, this works fine when using a property path for the choice_value, because it has a safeguard against values which are not object or arrays (a typical case being ``null`` because nothing is selected yet).
However, when loading from a ChoiceLoader, the generated closure was missing the same safeguard, breaking the usage of ChoiceLoader with property path.
This went unnoticed because the only usage of choice_loader in core is for the doctrine type, and this one uses its own logic to get the choice value

I added test on the PropertyAccessDecorator directly, to have fast tests. We could imagine adding tests in the ChoiceTypeTest using a ChoiceLoader too, matching all existing tests for other ways to specify arguments, but I'm not sure it is worth it.

Commits
-------

5df64dc Fix the retrieval of the value with property path when using a loader

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

@@ -141,7 +141,13 @@ class PropertyAccessDecorator implements ChoiceListFactoryInterface
         if ($value instanceof PropertyPath) {
             $accessor = $this->propertyAccessor;
             $value = function ($choice) use ($accessor, $value) {
-                return $accessor->getValue($choice, $value);
+                // The callable may be invoked with a non-object/array value
+                // when such values are passed to
+                // ChoiceListInterface::getValuesForChoices(). Handle this case
+                // so that the call to getValue() doesn't break.
+                if (is_object($choice) || is_array($choice)) {
+                    return $accessor->getValue($choice, $value);
+                }
             };
         }
 

src/Symfony/Component/Form/Tests/ChoiceList/Factory/PropertyAccessDecoratorTest.php

@@ -93,6 +93,36 @@ class PropertyAccessDecoratorTest extends \PHPUnit_Framework_TestCase
         $this->assertSame('value', $this->factory->createListFromLoader($loader, 'property'));
     }
 
+    // https://github.com/symfony/symfony/issues/5494
+    public function testCreateFromChoicesAssumeNullIfValuePropertyPathUnreadable()
+    {
+        $choices = array(null);
+
+        $this->decoratedFactory->expects($this->once())
+            ->method('createListFromChoices')
+            ->with($choices, $this->isInstanceOf('\Closure'))
+            ->will($this->returnCallback(function ($choices, $callback) {
+                return array_map($callback, $choices);
+            }));
+
+        $this->assertSame(array(null), $this->factory->createListFromChoices($choices, 'property'));
+    }
+
+    // https://github.com/symfony/symfony/issues/5494
+    public function testCreateFromChoiceLoaderAssumeNullIfValuePropertyPathUnreadable()
+    {
+        $loader = $this->getMock('Symfony\Component\Form\ChoiceList\Loader\ChoiceLoaderInterface');
+
+        $this->decoratedFactory->expects($this->once())
+            ->method('createListFromLoader')
+            ->with($loader, $this->isInstanceOf('\Closure'))
+            ->will($this->returnCallback(function ($loader, $callback) {
+                return $callback(null);
+            }));
+
+        $this->assertNull($this->factory->createListFromLoader($loader, 'property'));
+    }
+
     public function testCreateFromLoaderPropertyPathInstance()
     {
         $loader = $this->getMock('Symfony\Component\Form\ChoiceList\Loader\ChoiceLoaderInterface');