Add failing test to show that overloaded extensions are broken in Extensible

tests/php/Core/Injector/InjectorTest.php

@@ -21,12 +21,13 @@ use SilverStripe\Core\Tests\Injector\InjectorTest\NeedsBothCirculars;
 use SilverStripe\Core\Tests\Injector\InjectorTest\NewRequirementsBackend;
 use SilverStripe\Core\Tests\Injector\InjectorTest\OriginalRequirementsBackend;
 use SilverStripe\Core\Tests\Injector\InjectorTest\OtherTestObject;
+use SilverStripe\Core\Tests\Injector\InjectorTest\SomeCustomisedExtension;
+use SilverStripe\Core\Tests\Injector\InjectorTest\SomeExtension;
 use SilverStripe\Core\Tests\Injector\InjectorTest\TestObject;
 use SilverStripe\Core\Tests\Injector\InjectorTest\TestSetterInjections;
 use SilverStripe\Core\Tests\Injector\InjectorTest\TestStaticInjections;
 use SilverStripe\Dev\SapphireTest;
-use SilverStripe\Dev\TestOnly;
-use stdClass;
+use SilverStripe\Security\Member;
 
 define('TEST_SERVICES', __DIR__ . '/AopProxyServiceTest');
 
@@ -1047,4 +1048,23 @@ class InjectorTest extends SapphireTest
         Injector::unnest();
         $this->nestingLevel--;
     }
+
+    /**
+     * Tests that overloaded extensions work, see {@link Extensible::getExtensionInstance()}
+     */
+    public function testExtendedExtensions()
+    {
+        Config::modify()
+            ->set(Injector::class, SomeExtension::class, [
+                'class' => SomeCustomisedExtension::class,
+            ])
+            ->merge(Member::class, 'extensions', [
+                SomeExtension::class,
+            ]);
+
+        /** @var Member|SomeExtension $member */
+        $member = new Member();
+        $this->assertTrue($member->hasExtension(SomeExtension::class));
+        $this->assertSame('bar', $member->someMethod());
+    }
 }

tests/php/Core/Injector/InjectorTest/SomeCustomisedExtension.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace SilverStripe\Core\Tests\Injector\InjectorTest;
+
+class SomeCustomisedExtension extends SomeExtension
+{
+    public function someMethod()
+    {
+        return 'bar';
+    }
+}

tests/php/Core/Injector/InjectorTest/SomeExtension.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace SilverStripe\Core\Tests\Injector\InjectorTest;
+
+use SilverStripe\ORM\DataExtension;
+
+class SomeExtension extends DataExtension
+{
+    public function someMethod()
+    {
+        return 'foo';
+    }
+}