bug #14715 [Form] Check instance of FormBuilderInterface instead of FormBuilder (dosten)

This PR was merged into the 2.3 branch.

Discussion
----------

[Form] Check instance of FormBuilderInterface instead of FormBuilder

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #14714
| License       | MIT
| Doc PR        | -

Commits
-------

44469d0 Check instance of FormBuilderInterface instead of FormBuilder

src/Symfony/Component/Form/FormBuilder.php

@@ -62,7 +62,7 @@ class FormBuilder extends FormConfigBuilder implements \IteratorAggregate, FormB
             throw new BadMethodCallException('FormBuilder methods cannot be accessed anymore once the builder is turned into a FormConfigInterface instance.');
         }
 
-        if ($child instanceof self) {
+        if ($child instanceof FormBuilderInterface) {
             $this->children[$child->getName()] = $child;
 
             // In case an unresolved child with the same name exists
@@ -72,7 +72,7 @@ class FormBuilder extends FormConfigBuilder implements \IteratorAggregate, FormB
         }
 
         if (!is_string($child) && !is_int($child)) {
-            throw new UnexpectedTypeException($child, 'string, integer or Symfony\Component\Form\FormBuilder');
+            throw new UnexpectedTypeException($child, 'string, integer or Symfony\Component\Form\FormBuilderInterface');
         }
 
         if (null !== $type && !is_string($type) && !$type instanceof FormTypeInterface) {

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

@@ -11,7 +11,9 @@
 
 namespace Symfony\Component\Form\Tests;
 
+use Symfony\Component\Form\ButtonBuilder;
 use Symfony\Component\Form\FormBuilder;
+use Symfony\Component\Form\SubmitButtonBuilder;
 
 class FormBuilderTest extends \PHPUnit_Framework_TestCase
 {
@@ -154,6 +156,12 @@ class FormBuilderTest extends \PHPUnit_Framework_TestCase
         $this->builder->create('foo');
     }
 
+    public function testAddButton()
+    {
+        $this->builder->add(new ButtonBuilder('reset'));
+        $this->builder->add(new SubmitButtonBuilder('submit'));
+    }
+
     public function testGetUnknown()
     {
         $this->setExpectedException('Symfony\Component\Form\Exception\InvalidArgumentException', 'The child with the name "foo" does not exist.');