Add InstalledArrayRepository to make sure the root package is not purged by the Pool because of a lower stability

src/Composer/Installer.php

@@ -29,6 +29,7 @@ use Composer\Package\Locker;
 use Composer\Package\PackageInterface;
 use Composer\Repository\ArrayRepository;
 use Composer\Repository\CompositeRepository;
+use Composer\Repository\InstalledArrayRepository;
 use Composer\Repository\PlatformRepository;
 use Composer\Repository\RepositoryInterface;
 use Composer\Repository\RepositoryManager;
@@ -139,7 +140,7 @@ class Installer
         $repos = array_merge(
             $this->repositoryManager->getLocalRepositories(),
             array(
-                new ArrayRepository(array($this->package)),
+                new InstalledArrayRepository(array($this->package)),
                 new PlatformRepository(),
             )
         );

src/Composer/Repository/InstalledArrayRepository.php

@@ -0,0 +1,42 @@
+<?php
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer\Repository;
+
+use Composer\Json\JsonFile;
+use Composer\Package\PackageInterface;
+use Composer\Package\Loader\ArrayLoader;
+use Composer\Package\Dumper\ArrayDumper;
+
+/**
+ * Installed array repository.
+ *
+ * This is used for serving the RootPackage inside an in-memory InstalledRepository
+ *
+ * @author Jordi Boggiano <j.boggiano@seld.be>
+ */
+class InstalledArrayRepository extends ArrayRepository implements InstalledRepositoryInterface
+{
+    /**
+     * {@inheritDoc}
+     */
+    public function write()
+    {
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function reload()
+    {
+    }
+}