feature symfony#58004 [DependencyInjection] Add ContainerBuilder::registerChild() shortcut method (HypeMC)

This PR was merged into the 7.2 branch.

Discussion
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[DependencyInjection] Add `ContainerBuilder::registerChild()` shortcut method

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Issues        | -
| License       | MIT

Extracted from symfony#56823 as suggested in symfony#56823 (comment)

Commits
-------

7ba430c [DependencyInjection] Add `ContainerBuilder::registerChild()` shortcut method

Author: fabpot

src/Symfony/Component/DependencyInjection/CHANGELOG.md

@@ -5,6 +5,7 @@ CHANGELOG
 ---
 
  * Deprecate `!tagged` tag, use `!tagged_iterator` instead
+ * Add a `ContainerBuilder::registerChild()` shortcut method for registering child definitions
 
 7.1
 ---

src/Symfony/Component/DependencyInjection/ContainerBuilder.php

@@ -937,6 +937,15 @@ public function register(string $id, ?string $class = null): Definition
         return $this->setDefinition($id, new Definition($class));
     }
 
+    /**
+     * This method provides a fluid interface for easily registering a child
+     * service definition of the given parent service.
+     */
+    public function registerChild(string $id, string $parent): ChildDefinition
+    {
+        return $this->setDefinition($id, new ChildDefinition($parent));
+    }
+
     /**
      * Registers an autowired service definition.
      *

src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php

@@ -212,8 +212,18 @@ public function testRegister()
     {
         $builder = new ContainerBuilder();
         $builder->register('foo', 'Bar\FooClass');
-        $this->assertTrue($builder->hasDefinition('foo'), '->register() registers a new service definition');
-        $this->assertInstanceOf(Definition::class, $builder->getDefinition('foo'), '->register() returns the newly created Definition instance');
+        $this->assertTrue($builder->hasDefinition('foo'), '->hasDefinition() returns true if a service definition exists');
+        $this->assertInstanceOf(Definition::class, $builder->getDefinition('foo'), '->getDefinition() returns an instance of Definition');
+    }
+
+    public function testRegisterChild()
+    {
+        $builder = new ContainerBuilder();
+        $builder->register('foo', 'Bar\FooClass');
+        $builder->registerChild('bar', 'foo');
+        $this->assertTrue($builder->hasDefinition('bar'), '->hasDefinition() returns true if a service definition exists');
+        $this->assertInstanceOf(ChildDefinition::class, $definition = $builder->getDefinition('bar'), '->getDefinition() returns an instance of Definition');
+        $this->assertSame('foo', $definition->getParent(), '->getParent() returns the id of the parent service');
     }
 
     public function testAutowire()