Use only the locked repository when possible for better performance

src/Composer/Installer.php

@@ -255,12 +255,15 @@ class Installer
         $pool = new Pool($minimumStability, $stabilityFlags);
         $pool->addRepository($installedRepo, $aliases);
         if ($installFromLock) {
-            $pool->addRepository($lockedRepository);
+            $pool->addRepository($lockedRepository, $aliases);
         }
+
+        if (!$installFromLock || $this->locker->isLegacyFormat($devMode)) {
             $repositories = $this->repositoryManager->getRepositories();
             foreach ($repositories as $repository) {
                 $pool->addRepository($repository, $aliases);
             }
+        }
 
         // creating requirements request
         $request = new Request($pool);

src/Composer/Package/Locker.php

@@ -87,6 +87,24 @@ class Locker
         return $this->hash === $lock['hash'];
     }
 
+    /**
+     * Checks whether the lock file is in the legacy format
+     *
+     * @param  bool $dev true to check in dev mode
+     * @return bool
+     */
+    public function isLegacyFormat($dev)
+    {
+        $lockData = $this->getLockData();
+        $lockedPackages = $dev ? $lockData['packages-dev'] : $lockData['packages'];
+
+        if (empty($lockedPackages) || isset($lockedPackages[0]['name'])) {
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * Searches and returns an array of locked packages, retrieved from registered repositories.
      *
@@ -129,9 +147,8 @@ class Locker
             if (!$package && !empty($info['alias-version'])) {
                 $package = $this->repositoryManager->findPackage($info['package'], $info['version']);
                 if ($package) {
-                    $alias = new AliasPackage($package, $info['alias-version'], $info['alias-pretty-version']);
-                    $package->getRepository()->addPackage($alias);
-                    $package = $alias;
+                    $package->setAlias($info['alias-version']);
+                    $package->setPrettyAlias($info['alias-pretty-version']);
                 }
             }
 
@@ -142,7 +159,15 @@ class Locker
                 ));
             }
 
-            $packages->addPackage(clone $package);
+            $package = clone $package;
+            if (!empty($info['time'])) {
+                $package->setReleaseDate($info['time']);
+            }
+            if (!empty($info['source-reference'])) {
+                $package->setSourceReference($info['source-reference']);
+            }
+
+            $packages->addPackage($package);
         }
 
         return $packages;

tests/Composer/Test/Fixtures/installer/install-missing-alias-from-lock.test

@@ -0,0 +1,41 @@
+--TEST--
+Installing an old alias that doesn't exist anymore from a lock is possible
+--COMPOSER--
+{
+    "repositories": [
+        {
+            "type": "package",
+            "package": [
+                {
+                    "name": "a/a", "version": "dev-master",
+                    "extra": { "branch-alias": { "dev-master": "2.2.x-dev" } },
+                    "source": { "reference": "master", "type": "git", "url": "" }
+                }
+            ]
+        }
+    ],
+    "require": {
+        "a/a": "2.1.*"
+    },
+    "minimum-stability": "dev"
+}
+--LOCK--
+{
+    "packages": [
+        {
+            "name": "a/a", "version": "dev-master", "version_normalized": "9999999-dev",
+            "extra": { "branch-alias": { "dev-master": "2.1.x-dev" } },
+            "source": { "reference": "oldmaster", "type": "git", "url": "" },
+            "type": "library"
+        }
+    ],
+    "packages-dev": null,
+    "aliases": [],
+    "minimum-stability": "dev",
+    "stability-flags": []
+}
+--RUN--
+install
+--EXPECT--
+Installing a/a (dev-master oldmaster)
+Marking a/a (2.1.x-dev oldmaster) as installed, alias of a/a (dev-master oldmaster)