minor #28789 [Form] fix a return type hint (xabbuh)

This PR was merged into the 4.1 branch.

Discussion
----------

[Form] fix a return type hint

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #26918
| License       | MIT
| Doc PR        |

Commits
-------

095b9f3a30 fix a return type hint

src/Symfony/Component/Form/FormBuilder.php

@@ -235,9 +235,9 @@ class FormBuilder extends FormConfigBuilder implements \IteratorAggregate, FormB
     }
 
     /**
-     * Converts an unresolved child into a {@link FormBuilder} instance.
+     * Converts an unresolved child into a {@link FormBuilderInterface} instance.
      */
-    private function resolveChild(string $name): self
+    private function resolveChild(string $name): FormBuilderInterface
     {
         $info = $this->unresolvedChildren[$name];
         $child = $this->create($name, $info['type'], $info['options']);

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

@@ -13,7 +13,9 @@ namespace Symfony\Component\Form\Tests;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Form\ButtonBuilder;
+use Symfony\Component\Form\Extension\Core\Type\SubmitType;
 use Symfony\Component\Form\FormBuilder;
+use Symfony\Component\Form\FormFactoryBuilder;
 use Symfony\Component\Form\SubmitButtonBuilder;
 
 class FormBuilderTest extends TestCase
@@ -228,6 +230,14 @@ class FormBuilderTest extends TestCase
         $this->assertEmpty($unresolvedChildren->getValue($config));
     }
 
+    public function testGetButtonBuilderBeforeExplicitlyResolvingAllChildren()
+    {
+        $builder = new FormBuilder('name', null, $this->dispatcher, (new FormFactoryBuilder())->getFormFactory());
+        $builder->add('submit', SubmitType::class);
+
+        $this->assertInstanceOf(ButtonBuilder::class, $builder->get('submit'));
+    }
+
     private function getFormBuilder($name = 'name')
     {
         $mock = $this->getMockBuilder('Symfony\Component\Form\FormBuilder')