MNT Update generic inheritance for DataExtension

- 'static' type should only be used on the bottom level, otherwise it can give a weird type 'DataExtension<DataObject|Hierachy>|DataObject|Hierachy' for example
- phpstan doesn't like intersection types (&) during testing, will possibly need the open source silverstripe-phpstan to complete the types as they currently resolve to 'never'

src/ORM/DataExtension.php

@@ -14,7 +14,7 @@ use Exception;
  * An extension that adds additional functionality to a {@link DataObject}.
  *
  * @template T of DataObject
- * @extends Extension<T|static>
+ * @extends Extension<T>
  */
 abstract class DataExtension extends Extension
 {

src/ORM/Hierarchy/Hierarchy.php

@@ -23,8 +23,8 @@ use SilverStripe\View\ViewableData;
  * obvious example of this is SiteTree.
  *
  * @property int $ParentID
- * @property DataObject|Hierarchy $owner
  * @method DataObject Parent()
+ * @extends DataExtension<DataObject|static>
  */
 class Hierarchy extends DataExtension
 {
@@ -119,8 +119,7 @@ class Hierarchy extends DataExtension
     public function validate(ValidationResult $validationResult)
     {
         // The object is new, won't be looping.
-        /** @var DataObject|Hierarchy $owner */
-        $owner = $this->owner;
+        $owner = $this->getOwner();
         if (!$owner->ID) {
             return;
         }

tests/php/Type/Source/ExtensionMockTwo.php

@@ -2,12 +2,12 @@
 
 namespace SilverStripe\Type\Tests\Source;
 
-use SilverStripe\Core\Extension;
 use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataExtension;
 
 /**
- * @extends Extension<ExtensibleMockOne|ExtensibleMockTwo|static>
+ * @extends DataExtension<ExtensibleMockOne|ExtensibleMockTwo|static>
  */
-class ExtensionMockTwo extends Extension implements TestOnly
+class ExtensionMockTwo extends DataExtension implements TestOnly
 {
 }