diff --git a/composer.json b/composer.json
index 8fcd1ea93..f45775853 100644
--- a/composer.json
+++ b/composer.json
@@ -45,7 +45,8 @@
         }
     },
     "suggest": {
-        "ext-zip": "Enabling the zip extension allows you to unzip archives, and allows gzip compression of all internet traffic",
+        "ext-zip": "Enabling the zip extension allows you to unzip archives",
+        "ext-zlib": "Allow gzip compression of HTTP requests",
         "ext-openssl": "Enabling the openssl extension allows you to access https URLs for repositories and packages"
     },
     "autoload": {
diff --git a/composer.lock b/composer.lock
index caeba6397..31543e65c 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "hash": "fdf4b487fa59607376721ebec4ff4783",
+    "hash": "31b3c13c89f8d6c810637ca1fe8fc6ae",
     "content-hash": "454148e20b837d9755dee7862f9c7a5d",
     "packages": [
         {
diff --git a/doc/00-intro.md b/doc/00-intro.md
index 1d09f2339..872bdff2c 100644
--- a/doc/00-intro.md
+++ b/doc/00-intro.md
@@ -109,7 +109,7 @@ mv composer.phar /usr/local/bin/composer
 A quick copy-paste version including sudo:
 
 ```sh
-curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer
+curl -sS https://getcomposer.org/installer | sudo -H php -- --install-dir=/usr/local/bin --filename=composer
 ```
 
 > **Note:** On some versions of OSX the `/usr` directory does not exist by
diff --git a/doc/03-cli.md b/doc/03-cli.md
index 94a7a963c..b35d8f805 100644
--- a/doc/03-cli.md
+++ b/doc/03-cli.md
@@ -413,7 +413,7 @@ If you have installed Composer for your entire system (see [global installation]
 you may have to run the command with `root` privileges
 
 ```sh
-sudo composer self-update
+sudo -H composer self-update
 ```
 
 ### Options
diff --git a/src/Composer/Command/ConfigCommand.php b/src/Composer/Command/ConfigCommand.php
index 94fccc2e5..722bc94cc 100644
--- a/src/Composer/Command/ConfigCommand.php
+++ b/src/Composer/Command/ConfigCommand.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Command;
 
+use Composer\Util\Platform;
 use Composer\Util\Silencer;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Input\InputArgument;
@@ -184,7 +185,7 @@ EOT
         if ($input->getOption('editor')) {
             $editor = escapeshellcmd(getenv('EDITOR'));
             if (!$editor) {
-                if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+                if (Platform::isWindows()) {
                     $editor = 'notepad';
                 } else {
                     foreach (array('vim', 'vi', 'nano', 'pico', 'ed') as $candidate) {
@@ -197,7 +198,7 @@ EOT
             }
 
             $file = $input->getOption('auth') ? $this->authConfigFile->getPath() : $this->configFile->getPath();
-            system($editor . ' ' . $file . (defined('PHP_WINDOWS_VERSION_BUILD') ? '' : ' > `tty`'));
+            system($editor . ' ' . $file . (Platform::isWindows() ? '' : ' > `tty`'));
 
             return 0;
         }
diff --git a/src/Composer/Command/HomeCommand.php b/src/Composer/Command/HomeCommand.php
index fff1f86eb..150b8c71b 100644
--- a/src/Composer/Command/HomeCommand.php
+++ b/src/Composer/Command/HomeCommand.php
@@ -16,6 +16,7 @@ use Composer\Factory;
 use Composer\Package\CompletePackageInterface;
 use Composer\Repository\RepositoryInterface;
 use Composer\Repository\ArrayRepository;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputOption;
@@ -117,7 +118,7 @@ EOT
     {
         $url = ProcessExecutor::escape($url);
 
-        if (defined('PHP_WINDOWS_VERSION_MAJOR')) {
+        if (Platform::isWindows()) {
             return passthru('start "web" explorer "' . $url . '"');
         }
 
diff --git a/src/Composer/Command/SelfUpdateCommand.php b/src/Composer/Command/SelfUpdateCommand.php
index fff159158..16b98020a 100644
--- a/src/Composer/Command/SelfUpdateCommand.php
+++ b/src/Composer/Command/SelfUpdateCommand.php
@@ -88,9 +88,6 @@ EOT
         if (!is_writable($tmpDir)) {
             throw new FilesystemException('Composer update failed: the "'.$tmpDir.'" directory used to download the temp file could not be written');
         }
-        if (!is_writable($localFilename)) {
-            throw new FilesystemException('Composer update failed: the "'.$localFilename.'" file could not be written');
-        }
 
         if ($input->getOption('rollback')) {
             return $this->rollback($output, $rollbackDir, $localFilename);
@@ -271,10 +268,6 @@ TAGSPUBKEY
             throw new \UnexpectedValueException('Composer rollback failed: no installation to roll back to in "'.$rollbackDir.'"');
         }
 
-        if (!is_writable($rollbackDir)) {
-            throw new FilesystemException('Composer rollback failed: the "'.$rollbackDir.'" dir could not be written to');
-        }
-
         $old = $rollbackDir . '/' . $rollbackVersion . self::OLD_INSTALL_EXT;
 
         if (!is_file($old)) {
diff --git a/src/Composer/Command/ShowCommand.php b/src/Composer/Command/ShowCommand.php
index e3e1ffd71..5ddce2954 100644
--- a/src/Composer/Command/ShowCommand.php
+++ b/src/Composer/Command/ShowCommand.php
@@ -20,6 +20,7 @@ use Composer\Semver\VersionParser;
 use Composer\Plugin\CommandEvent;
 use Composer\Plugin\PluginEvents;
 use Composer\Package\PackageInterface;
+use Composer\Util\Platform;
 use Symfony\Component\Console\Formatter\OutputFormatterStyle;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Input\InputArgument;
@@ -232,7 +233,7 @@ EOT
                     // outside of a real terminal, use space without a limit
                     $width = PHP_INT_MAX;
                 }
-                if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+                if (Platform::isWindows()) {
                     $width--;
                 }
 
diff --git a/src/Composer/Console/Application.php b/src/Composer/Console/Application.php
index f77876100..07d517eae 100644
--- a/src/Composer/Console/Application.php
+++ b/src/Composer/Console/Application.php
@@ -12,6 +12,7 @@
 
 namespace Composer\Console;
 
+use Composer\Util\Platform;
 use Composer\Util\Silencer;
 use Symfony\Component\Console\Application as BaseApplication;
 use Symfony\Component\Console\Input\InputInterface;
@@ -219,7 +220,7 @@ class Application extends BaseApplication
         }
         Silencer::restore();
 
-        if (defined('PHP_WINDOWS_VERSION_BUILD') && false !== strpos($exception->getMessage(), 'The system cannot find the path specified')) {
+        if (Platform::isWindows() && false !== strpos($exception->getMessage(), 'The system cannot find the path specified')) {
             $io->writeError('<error>The following exception may be caused by a stale entry in your cmd.exe AutoRun</error>', true, IOInterface::QUIET);
             $io->writeError('<error>Check https://getcomposer.org/doc/articles/troubleshooting.md#-the-system-cannot-find-the-path-specified-windows- for details</error>', true, IOInterface::QUIET);
         }
diff --git a/src/Composer/Downloader/GitDownloader.php b/src/Composer/Downloader/GitDownloader.php
index 7db561889..48e0d4b39 100644
--- a/src/Composer/Downloader/GitDownloader.php
+++ b/src/Composer/Downloader/GitDownloader.php
@@ -14,6 +14,7 @@ namespace Composer\Downloader;
 
 use Composer\Package\PackageInterface;
 use Composer\Util\Git as GitUtil;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\IO\IOInterface;
 use Composer\Util\Filesystem;
@@ -43,7 +44,7 @@ class GitDownloader extends VcsDownloader
         $path = $this->normalizePath($path);
 
         $ref = $package->getSourceReference();
-        $flag = defined('PHP_WINDOWS_VERSION_MAJOR') ? '/D ' : '';
+        $flag = Platform::isWindows() ? '/D ' : '';
         $command = 'git clone --no-checkout %s %s && cd '.$flag.'%2$s && git remote add composer %1$s && git fetch composer';
         $this->io->writeError("    Cloning ".$ref);
 
@@ -353,7 +354,7 @@ class GitDownloader extends VcsDownloader
 
     protected function normalizePath($path)
     {
-        if (defined('PHP_WINDOWS_VERSION_MAJOR') && strlen($path) > 0) {
+        if (Platform::isWindows() && strlen($path) > 0) {
             $basePath = $path;
             $removed = array();
 
diff --git a/src/Composer/Downloader/GzipDownloader.php b/src/Composer/Downloader/GzipDownloader.php
index ae7d7f17a..9969578e5 100644
--- a/src/Composer/Downloader/GzipDownloader.php
+++ b/src/Composer/Downloader/GzipDownloader.php
@@ -16,6 +16,7 @@ use Composer\Config;
 use Composer\Cache;
 use Composer\EventDispatcher\EventDispatcher;
 use Composer\Package\PackageInterface;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\RemoteFilesystem;
 use Composer\IO\IOInterface;
@@ -40,25 +41,26 @@ class GzipDownloader extends ArchiveDownloader
         $targetFilepath = $path . DIRECTORY_SEPARATOR . basename(substr($file, 0, -3));
 
         // Try to use gunzip on *nix
-        if (!defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (!Platform::isWindows()) {
             $command = 'gzip -cd ' . ProcessExecutor::escape($file) . ' > ' . ProcessExecutor::escape($targetFilepath);
 
             if (0 === $this->process->execute($command, $ignoredOutput)) {
                 return;
             }
 
+            if (extension_loaded('zlib')) {
+                // Fallback to using the PHP extension.
+                $this->extractUsingExt($file, $targetFilepath);
+
+                return;
+            }
+
             $processError = 'Failed to execute ' . $command . "\n\n" . $this->process->getErrorOutput();
             throw new \RuntimeException($processError);
         }
 
         // Windows version of PHP has built-in support of gzip functions
-        $archiveFile = gzopen($file, 'rb');
-        $targetFile = fopen($targetFilepath, 'wb');
-        while ($string = gzread($archiveFile, 4096)) {
-            fwrite($targetFile, $string, strlen($string));
-        }
-        gzclose($archiveFile);
-        fclose($targetFile);
+        $this->extractUsingExt($file, $targetFilepath);
     }
 
     /**
@@ -68,4 +70,15 @@ class GzipDownloader extends ArchiveDownloader
     {
         return $path.'/'.pathinfo(parse_url($package->getDistUrl(), PHP_URL_PATH), PATHINFO_BASENAME);
     }
+
+    private function extractUsingExt($file, $targetFilepath)
+    {
+        $archiveFile = gzopen($file, 'rb');
+        $targetFile = fopen($targetFilepath, 'wb');
+        while ($string = gzread($archiveFile, 4096)) {
+            fwrite($targetFile, $string, strlen($string));
+        }
+        gzclose($archiveFile);
+        fclose($targetFile);
+    }
 }
diff --git a/src/Composer/Downloader/RarDownloader.php b/src/Composer/Downloader/RarDownloader.php
index 81e11785e..2a0c98cf9 100644
--- a/src/Composer/Downloader/RarDownloader.php
+++ b/src/Composer/Downloader/RarDownloader.php
@@ -15,6 +15,7 @@ namespace Composer\Downloader;
 use Composer\Config;
 use Composer\Cache;
 use Composer\EventDispatcher\EventDispatcher;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\RemoteFilesystem;
 use Composer\IO\IOInterface;
@@ -42,7 +43,7 @@ class RarDownloader extends ArchiveDownloader
         $processError = null;
 
         // Try to use unrar on *nix
-        if (!defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (!Platform::isWindows()) {
             $command = 'unrar x ' . ProcessExecutor::escape($file) . ' ' . ProcessExecutor::escape($path) . ' && chmod -R u+w ' . ProcessExecutor::escape($path);
 
             if (0 === $this->process->execute($command, $ignoredOutput)) {
@@ -65,7 +66,7 @@ class RarDownloader extends ArchiveDownloader
             $error = "Could not decompress the archive, enable the PHP rar extension or install unrar.\n"
                 . $iniMessage . "\n" . $processError;
 
-            if (!defined('PHP_WINDOWS_VERSION_BUILD')) {
+            if (!Platform::isWindows()) {
                 $error = "Could not decompress the archive, enable the PHP rar extension.\n" . $iniMessage;
             }
 
diff --git a/src/Composer/Downloader/ZipDownloader.php b/src/Composer/Downloader/ZipDownloader.php
index 6faaaaa4f..5f483975c 100644
--- a/src/Composer/Downloader/ZipDownloader.php
+++ b/src/Composer/Downloader/ZipDownloader.php
@@ -15,6 +15,7 @@ namespace Composer\Downloader;
 use Composer\Config;
 use Composer\Cache;
 use Composer\EventDispatcher\EventDispatcher;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\RemoteFilesystem;
 use Composer\IO\IOInterface;
@@ -38,7 +39,7 @@ class ZipDownloader extends ArchiveDownloader
         $processError = null;
 
         // try to use unzip on *nix
-        if (!defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (!Platform::isWindows()) {
             $command = 'unzip '.ProcessExecutor::escape($file).' -d '.ProcessExecutor::escape($path) . ' && chmod -R u+w ' . ProcessExecutor::escape($path);
             try {
                 if (0 === $this->process->execute($command, $ignoredOutput)) {
@@ -64,7 +65,7 @@ class ZipDownloader extends ArchiveDownloader
             $error = "Could not decompress the archive, enable the PHP zip extension or install unzip.\n"
                 . $iniMessage . "\n" . $processError;
 
-            if (!defined('PHP_WINDOWS_VERSION_BUILD')) {
+            if (!Platform::isWindows()) {
                 $error = "Could not decompress the archive, enable the PHP zip extension.\n" . $iniMessage;
             }
 
diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index 058709275..2f3491fd4 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -20,6 +20,7 @@ use Composer\Package\Version\VersionGuesser;
 use Composer\Repository\RepositoryManager;
 use Composer\Repository\WritableRepositoryInterface;
 use Composer\Util\Filesystem;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\RemoteFilesystem;
 use Composer\Util\Silencer;
@@ -51,7 +52,7 @@ class Factory
             return $home;
         }
 
-        if (defined('PHP_WINDOWS_VERSION_MAJOR')) {
+        if (Platform::isWindows()) {
             if (!getenv('APPDATA')) {
                 throw new \RuntimeException('The APPDATA or COMPOSER_HOME environment variable must be set for composer to run correctly');
             }
@@ -90,7 +91,7 @@ class Factory
             return $homeEnv . '/cache';
         }
 
-        if (defined('PHP_WINDOWS_VERSION_MAJOR')) {
+        if (Platform::isWindows()) {
             if ($cacheDir = getenv('LOCALAPPDATA')) {
                 $cacheDir .= '/Composer';
             } else {
@@ -125,7 +126,7 @@ class Factory
             return $homeEnv;
         }
 
-        if (defined('PHP_WINDOWS_VERSION_MAJOR')) {
+        if (Platform::isWindows()) {
             return strtr($home, '\\', '/');
         }
 
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index 3172e596f..78ea92c03 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -1195,6 +1195,7 @@ class Installer
 
                     foreach ($requirePackages as $requirePackage) {
                         if (isset($skipPackages[$requirePackage->getName()])) {
+                            $this->io->writeError('<warning>Dependency "' . $requirePackage->getName() . '" is also a root requirement, but is not explicitly whitelisted. Ignoring.</warning>');
                             continue;
                         }
                         $packageQueue->enqueue($requirePackage);
diff --git a/src/Composer/Installer/LibraryInstaller.php b/src/Composer/Installer/LibraryInstaller.php
index b14659f7b..31090de00 100644
--- a/src/Composer/Installer/LibraryInstaller.php
+++ b/src/Composer/Installer/LibraryInstaller.php
@@ -17,6 +17,7 @@ use Composer\IO\IOInterface;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
 use Composer\Util\Filesystem;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\Silencer;
 
@@ -241,7 +242,7 @@ class LibraryInstaller implements InstallerInterface
             }
 
             if ($this->binCompat === "auto") {
-                if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+                if (Platform::isWindows()) {
                     $this->installFullBinaries($binPath, $link, $bin, $package);
                 } else {
                     $this->installSymlinkBinaries($binPath, $link);
diff --git a/src/Composer/Installer/PearInstaller.php b/src/Composer/Installer/PearInstaller.php
index 35f7855de..0e16fcb32 100644
--- a/src/Composer/Installer/PearInstaller.php
+++ b/src/Composer/Installer/PearInstaller.php
@@ -17,6 +17,7 @@ use Composer\Composer;
 use Composer\Downloader\PearPackageExtractor;
 use Composer\Repository\InstalledRepositoryInterface;
 use Composer\Package\PackageInterface;
+use Composer\Util\Platform;
 use Composer\Util\ProcessExecutor;
 
 /**
@@ -53,7 +54,7 @@ class PearInstaller extends LibraryInstaller
         parent::installCode($package);
         parent::initializeBinDir();
 
-        $isWindows = defined('PHP_WINDOWS_VERSION_BUILD');
+        $isWindows = Platform::isWindows();
         $php_bin = $this->binDir . ($isWindows ? '/composer-php.bat' : '/composer-php');
 
         if (!$isWindows) {
diff --git a/src/Composer/Package/Loader/RootPackageLoader.php b/src/Composer/Package/Loader/RootPackageLoader.php
index 0fc7c49af..565ff39c4 100644
--- a/src/Composer/Package/Loader/RootPackageLoader.php
+++ b/src/Composer/Package/Loader/RootPackageLoader.php
@@ -113,6 +113,11 @@ class RootPackageLoader extends ArrayLoader
             }
         }
 
+        if (isset($links[$config['name']])) {
+            throw new \InvalidArgumentException(sprintf('Root package \'%s\' cannot require itself in its composer.json' . PHP_EOL .
+                        'Did you accidentally name your root package after an external package?', $config['name']));
+        }
+
         $realPackage->setAliases($aliases);
         $realPackage->setStabilityFlags($stabilityFlags);
         $realPackage->setReferences($references);
diff --git a/src/Composer/Repository/PathRepository.php b/src/Composer/Repository/PathRepository.php
index c3266543b..7fa004eb5 100644
--- a/src/Composer/Repository/PathRepository.php
+++ b/src/Composer/Repository/PathRepository.php
@@ -125,17 +125,16 @@ class PathRepository extends ArrayRepository implements ConfigurableRepositoryIn
             $package['dist'] = array(
                 'type' => 'path',
                 'url' => $url,
-                'reference' => '',
+                'reference' => sha1($json),
             );
 
             if (!isset($package['version'])) {
                 $package['version'] = $this->versionGuesser->guessVersion($package, $path) ?: 'dev-master';
             }
+
             $output = '';
             if (is_dir($path . DIRECTORY_SEPARATOR . '.git') && 0 === $this->process->execute('git log -n1 --pretty=%H', $output, $path)) {
                 $package['dist']['reference'] = trim($output);
-            } else {
-                $package['dist']['reference'] = Locker::getContentHash($json);
             }
 
             $package = $this->loader->load($package);
diff --git a/src/Composer/Repository/PlatformRepository.php b/src/Composer/Repository/PlatformRepository.php
index 833c82c20..27f52aa65 100644
--- a/src/Composer/Repository/PlatformRepository.php
+++ b/src/Composer/Repository/PlatformRepository.php
@@ -114,6 +114,7 @@ class PlatformRepository extends ArrayRepository
         // relying on them.
         foreach ($loadedExtensions as $name) {
             $prettyVersion = null;
+            $description = 'The '.$name.' PHP library';
             switch ($name) {
                 case 'curl':
                     $curlVersion = curl_version();
@@ -146,9 +147,27 @@ class PlatformRepository extends ArrayRepository
                     break;
 
                 case 'openssl':
-                    $prettyVersion = preg_replace_callback('{^(?:OpenSSL\s*)?([0-9.]+)([a-z]?).*}', function ($match) {
-                        return $match[1] . (empty($match[2]) ? '' : '.'.(ord($match[2]) - 96));
+                    $prettyVersion = preg_replace_callback('{^(?:OpenSSL\s*)?([0-9.]+)([a-z]*).*}', function ($match) {
+                        if (empty($match[2])) {
+                            return $match[1];
+                        }
+
+                        // OpenSSL versions add another letter when they reach Z.
+                        // e.g. OpenSSL 0.9.8zh 3 Dec 2015
+
+                        if (!preg_match('{^z*[a-z]$}', $match[2])) {
+                            // 0.9.8abc is garbage
+                            return 0;
+                        }
+
+                        $len = strlen($match[2]);
+                        $patchVersion = ($len - 1) * 26; // All Z
+                        $patchVersion += ord($match[2][$len - 1]) - 96;
+
+                        return $match[1].'.'.$patchVersion;
                     }, OPENSSL_VERSION_TEXT);
+
+                    $description = OPENSSL_VERSION_TEXT;
                     break;
 
                 case 'pcre':
@@ -175,7 +194,7 @@ class PlatformRepository extends ArrayRepository
             }
 
             $lib = new CompletePackage('lib-'.$name, $version, $prettyVersion);
-            $lib->setDescription('The '.$name.' PHP library');
+            $lib->setDescription($description);
             $this->addPackage($lib);
         }
 
diff --git a/src/Composer/Util/Filesystem.php b/src/Composer/Util/Filesystem.php
index c3b888163..8e7ac3d04 100644
--- a/src/Composer/Util/Filesystem.php
+++ b/src/Composer/Util/Filesystem.php
@@ -115,7 +115,7 @@ class Filesystem
             return $this->removeDirectoryPhp($directory);
         }
 
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (Platform::isWindows()) {
             $cmd = sprintf('rmdir /S /Q %s', ProcessExecutor::escape(realpath($directory)));
         } else {
             $cmd = sprintf('rm -rf %s', ProcessExecutor::escape($directory));
@@ -186,10 +186,10 @@ class Filesystem
     {
         if (!@$this->unlinkImplementation($path)) {
             // retry after a bit on windows since it tends to be touchy with mass removals
-            if (!defined('PHP_WINDOWS_VERSION_BUILD') || (usleep(350000) && !@$this->unlinkImplementation($path))) {
+            if (!Platform::isWindows() || (usleep(350000) && !@$this->unlinkImplementation($path))) {
                 $error = error_get_last();
                 $message = 'Could not delete '.$path.': ' . @$error['message'];
-                if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+                if (Platform::isWindows()) {
                     $message .= "\nThis can be due to an antivirus or the Windows Search Indexer locking the file while they are analyzed";
                 }
 
@@ -211,10 +211,10 @@ class Filesystem
     {
         if (!@rmdir($path)) {
             // retry after a bit on windows since it tends to be touchy with mass removals
-            if (!defined('PHP_WINDOWS_VERSION_BUILD') || (usleep(350000) && !@rmdir($path))) {
+            if (!Platform::isWindows() || (usleep(350000) && !@rmdir($path))) {
                 $error = error_get_last();
                 $message = 'Could not delete '.$path.': ' . @$error['message'];
-                if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+                if (Platform::isWindows()) {
                     $message .= "\nThis can be due to an antivirus or the Windows Search Indexer locking the file while they are analyzed";
                 }
 
@@ -269,7 +269,7 @@ class Filesystem
             return $this->copyThenRemove($source, $target);
         }
 
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (Platform::isWindows()) {
             // Try to copy & delete - this is a workaround for random "Access denied" errors.
             $command = sprintf('xcopy %s %s /E /I /Q /Y', ProcessExecutor::escape($source), ProcessExecutor::escape($target));
             $result = $this->processExecutor->execute($command, $output);
@@ -465,7 +465,7 @@ class Filesystem
 
     public static function getPlatformPath($path)
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (Platform::isWindows()) {
             $path = preg_replace('{^(?:file:///([a-z])/)}i', 'file://$1:/', $path);
         }
 
@@ -503,7 +503,7 @@ class Filesystem
      */
     private function unlinkImplementation($path)
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD') && is_dir($path) && is_link($path)) {
+        if (Platform::isWindows() && is_dir($path) && is_link($path)) {
             return rmdir($path);
         }
 
diff --git a/src/Composer/Util/Perforce.php b/src/Composer/Util/Perforce.php
index c1eaeebe9..b57a64a1f 100644
--- a/src/Composer/Util/Perforce.php
+++ b/src/Composer/Util/Perforce.php
@@ -51,10 +51,7 @@ class Perforce
 
     public static function create($repoConfig, $port, $path, ProcessExecutor $process, IOInterface $io)
     {
-        $isWindows = defined('PHP_WINDOWS_VERSION_BUILD');
-        $perforce = new Perforce($repoConfig, $port, $path, $process, $isWindows, $io);
-
-        return $perforce;
+        return new Perforce($repoConfig, $port, $path, $process, Platform::isWindows(), $io);
     }
 
     public static function checkServerExists($url, ProcessExecutor $processExecutor)
diff --git a/src/Composer/Util/Platform.php b/src/Composer/Util/Platform.php
new file mode 100644
index 000000000..eafb88b7a
--- /dev/null
+++ b/src/Composer/Util/Platform.php
@@ -0,0 +1,28 @@
+<?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\Util;
+
+/**
+ * Platform helper for uniform platform-specific tests.
+ *
+ * @author Niels Keurentjes <niels.keurentjes@omines.com>
+ */
+class Platform
+{
+    /**
+     * @return bool Whether the host machine is running a Windows OS
+     */
+    public static function isWindows()
+    {
+        return defined('PHP_WINDOWS_VERSION_BUILD');
+    }
+}
diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index f9499f09f..6b778e5eb 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -50,7 +50,7 @@ class ProcessExecutor
 
         // make sure that null translate to the proper directory in case the dir is a symlink
         // and we call a git command, because msysgit does not handle symlinks properly
-        if (null === $cwd && defined('PHP_WINDOWS_VERSION_BUILD') && false !== strpos($command, 'git') && getcwd()) {
+        if (null === $cwd && Platform::isWindows() && false !== strpos($command, 'git') && getcwd()) {
             $cwd = realpath(getcwd());
         }
 
diff --git a/src/Composer/Util/TlsHelper.php b/src/Composer/Util/TlsHelper.php
index 6ea5cf591..721e93825 100644
--- a/src/Composer/Util/TlsHelper.php
+++ b/src/Composer/Util/TlsHelper.php
@@ -175,7 +175,7 @@ final class TlsHelper
             return self::$useOpensslParse = true;
         }
 
-        if ('\\' === DIRECTORY_SEPARATOR) {
+        if (Platform::isWindows()) {
             // Windows is probably insecure in this case.
             return self::$useOpensslParse = false;
         }
diff --git a/tests/Composer/Test/Downloader/GitDownloaderTest.php b/tests/Composer/Test/Downloader/GitDownloaderTest.php
index 26437ada5..9c851fe9a 100644
--- a/tests/Composer/Test/Downloader/GitDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/GitDownloaderTest.php
@@ -16,6 +16,7 @@ use Composer\Downloader\GitDownloader;
 use Composer\Config;
 use Composer\TestCase;
 use Composer\Util\Filesystem;
+use Composer\Util\Platform;
 
 class GitDownloaderTest extends TestCase
 {
@@ -353,7 +354,7 @@ class GitDownloaderTest extends TestCase
 
     private function winCompat($cmd)
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (Platform::isWindows()) {
             $cmd = str_replace('cd ', 'cd /D ', $cmd);
             $cmd = str_replace('composerPath', getcwd().'/composerPath', $cmd);
 
diff --git a/tests/Composer/Test/Downloader/HgDownloaderTest.php b/tests/Composer/Test/Downloader/HgDownloaderTest.php
index 75dfa84aa..6b660e383 100644
--- a/tests/Composer/Test/Downloader/HgDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/HgDownloaderTest.php
@@ -15,6 +15,7 @@ namespace Composer\Test\Downloader;
 use Composer\Downloader\HgDownloader;
 use Composer\TestCase;
 use Composer\Util\Filesystem;
+use Composer\Util\Platform;
 
 class HgDownloaderTest extends TestCase
 {
@@ -156,10 +157,6 @@ class HgDownloaderTest extends TestCase
 
     private function getCmd($cmd)
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
-            return strtr($cmd, "'", '"');
-        }
-
-        return $cmd;
+        return Platform::isWindows() ? strtr($cmd, "'", '"') : $cmd;
     }
 }
diff --git a/tests/Composer/Test/Downloader/XzDownloaderTest.php b/tests/Composer/Test/Downloader/XzDownloaderTest.php
index 418776d75..d8e77a2cb 100644
--- a/tests/Composer/Test/Downloader/XzDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/XzDownloaderTest.php
@@ -15,6 +15,7 @@ namespace Composer\Test\Downloader;
 use Composer\Downloader\XzDownloader;
 use Composer\TestCase;
 use Composer\Util\Filesystem;
+use Composer\Util\Platform;
 use Composer\Util\RemoteFilesystem;
 
 class XzDownloaderTest extends TestCase
@@ -31,7 +32,7 @@ class XzDownloaderTest extends TestCase
 
     public function setUp()
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (Platform::isWindows()) {
             $this->markTestSkipped('Skip test on Windows');
         }
         $this->testDir = $this->getUniqueTmpDirectory();
diff --git a/tests/Composer/Test/Fixtures/installer/github-issues-4795.test b/tests/Composer/Test/Fixtures/installer/github-issues-4795.test
new file mode 100644
index 000000000..6dc3ced3d
--- /dev/null
+++ b/tests/Composer/Test/Fixtures/installer/github-issues-4795.test
@@ -0,0 +1,47 @@
+--TEST--
+
+See Github issue #4795 ( github.com/composer/composer/issues/4795 ).
+
+Composer\Installer::whitelistUpdateDependencies intentionally ignores root requirements even if said package is also a
+dependency of one the requirements that is whitelisted for update.
+
+--COMPOSER--
+{
+    "repositories": [
+        {
+            "type": "package",
+            "package": [
+                { "name": "a", "version": "1.0.0" },
+                { "name": "a", "version": "1.1.0" },
+                { "name": "b", "version": "1.0.0", "require": { "a": "~1.0" } },
+                { "name": "b", "version": "1.1.0", "require": { "a": "~1.1" } },
+                { "name": "c", "version": "1.0.0", "require": { "a": "~1.0" } }
+            ]
+        }
+    ],
+    "require": {
+        "a": "~1.0",
+        "b": "~1.0",
+        "c": "~1.0"
+    }
+}
+
+--INSTALLED--
+[
+    { "name": "a", "version": "1.0.0" },
+    { "name": "b", "version": "1.0.0", "require": { "a": "~1.0" } },
+    { "name": "c", "version": "1.0.0", "require": { "a": "~1.0" } }
+]
+
+--RUN--
+update B --with-dependencies
+
+--EXPECT-OUTPUT--
+<warning>Dependency "a" is also a root requirement, but is not explicitly whitelisted. Ignoring.</warning>
+Loading composer repositories with package information
+Updating dependencies (including require-dev)
+Nothing to install or update
+Writing lock file
+Generating autoload files
+
+--EXPECT--
diff --git a/tests/Composer/Test/Fixtures/installer/install-self-from-root.test b/tests/Composer/Test/Fixtures/installer/install-self-from-root.test
new file mode 100644
index 000000000..82092c77f
--- /dev/null
+++ b/tests/Composer/Test/Fixtures/installer/install-self-from-root.test
@@ -0,0 +1,16 @@
+--TEST--
+Tries to require a package with the same name as the root package
+--COMPOSER--
+{
+    "name": "foo/bar",
+    "require": {
+        "foo/bar": "@dev"
+    }
+}
+--RUN--
+install
+--EXPECT-EXCEPTION--
+InvalidArgumentException
+--EXPECT--
+Root package 'foo/bar' cannot require itself in its composer.json
+Did you accidentally name your root package after an external package?
diff --git a/tests/Composer/Test/InstallerTest.php b/tests/Composer/Test/InstallerTest.php
index b0a51fea7..eaf6caa03 100644
--- a/tests/Composer/Test/InstallerTest.php
+++ b/tests/Composer/Test/InstallerTest.php
@@ -140,7 +140,7 @@ class InstallerTest extends TestCase
     /**
      * @dataProvider getIntegrationTests
      */
-    public function testIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectOutput, $expect, $expectExitCode)
+    public function testIntegration($file, $message, $condition, $composerConfig, $lock, $installed, $run, $expectLock, $expectOutput, $expect, $expectResult)
     {
         if ($condition) {
             eval('$res = '.$condition.';');
@@ -150,6 +150,14 @@ class InstallerTest extends TestCase
         }
 
         $io = new BufferIO('', OutputInterface::VERBOSITY_NORMAL, new OutputFormatter(false));
+
+        // Prepare for exceptions
+        if (!is_int($expectResult)) {
+            $normalizedOutput = rtrim(str_replace("\n", PHP_EOL, $expect));
+            $this->setExpectedException($expectResult, $normalizedOutput);
+        }
+
+        // Create Composer mock object according to configuration
         $composer = FactoryMock::create($io, $composerConfig);
 
         $jsonMock = $this->getMockBuilder('Composer\Json\JsonFile')->disableOriginalConstructor()->getMock();
@@ -225,9 +233,14 @@ class InstallerTest extends TestCase
         $appOutput = fopen('php://memory', 'w+');
         $result = $application->run(new StringInput($run), new StreamOutput($appOutput));
         fseek($appOutput, 0);
-        $output = str_replace("\r", '', $io->getOutput());
-        $this->assertEquals($expectExitCode, $result, $output . stream_get_contents($appOutput));
 
+        // Shouldn't check output and results if an exception was expected by this point
+        if (!is_int($expectResult)) {
+            return;
+        }
+
+        $output = str_replace("\r", '', $io->getOutput());
+        $this->assertEquals($expectResult, $result, $output . stream_get_contents($appOutput));
         if ($expectLock) {
             unset($actualLock['hash']);
             unset($actualLock['content-hash']);
@@ -259,7 +272,7 @@ class InstallerTest extends TestCase
             $installedDev = array();
             $lock = array();
             $expectLock = array();
-            $expectExitCode = 0;
+            $expectResult = 0;
 
             try {
                 $message = $testData['TEST'];
@@ -296,12 +309,21 @@ class InstallerTest extends TestCase
                 }
                 $expectOutput = isset($testData['EXPECT-OUTPUT']) ? $testData['EXPECT-OUTPUT'] : null;
                 $expect = $testData['EXPECT'];
-                $expectExitCode = isset($testData['EXPECT-EXIT-CODE']) ? (int) $testData['EXPECT-EXIT-CODE'] : 0;
+                if (!empty($testData['EXPECT-EXCEPTION'])) {
+                    $expectResult = $testData['EXPECT-EXCEPTION'];
+                    if (!empty($testData['EXPECT-EXIT-CODE'])) {
+                        throw new \LogicException('EXPECT-EXCEPTION and EXPECT-EXIT-CODE are mutually exclusive');
+                    }
+                } elseif (!empty($testData['EXPECT-EXIT-CODE'])) {
+                    $expectResult = (int) $testData['EXPECT-EXIT-CODE'];
+                } else {
+                    $expectResult = 0;
+                }
             } catch (\Exception $e) {
                 die(sprintf('Test "%s" is not valid: '.$e->getMessage(), str_replace($fixturesDir.'/', '', $file)));
             }
 
-            $tests[basename($file)] = array(str_replace($fixturesDir.'/', '', $file), $message, $condition, $composer, $lock, $installed, $run, $expectLock, $expectOutput, $expect, $expectExitCode);
+            $tests[basename($file)] = array(str_replace($fixturesDir.'/', '', $file), $message, $condition, $composer, $lock, $installed, $run, $expectLock, $expectOutput, $expect, $expectResult);
         }
 
         return $tests;
@@ -321,6 +343,7 @@ class InstallerTest extends TestCase
             'EXPECT-LOCK' => false,
             'EXPECT-OUTPUT' => false,
             'EXPECT-EXIT-CODE' => false,
+            'EXPECT-EXCEPTION' => false,
             'EXPECT' => true,
         );
 
diff --git a/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php b/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php
index c2ae497ca..881b86ea2 100644
--- a/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/SvnDriverTest.php
@@ -16,6 +16,7 @@ use Composer\Repository\Vcs\SvnDriver;
 use Composer\Config;
 use Composer\TestCase;
 use Composer\Util\Filesystem;
+use Composer\Util\Platform;
 
 class SvnDriverTest extends TestCase
 {
@@ -71,7 +72,7 @@ class SvnDriverTest extends TestCase
 
     private function getCmd($cmd)
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
+        if (Platform::isWindows()) {
             return strtr($cmd, "'", '"');
         }
 
diff --git a/tests/Composer/Test/Util/PlatformTest.php b/tests/Composer/Test/Util/PlatformTest.php
new file mode 100644
index 000000000..3d82fb96f
--- /dev/null
+++ b/tests/Composer/Test/Util/PlatformTest.php
@@ -0,0 +1,29 @@
+<?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\Test\Util;
+
+use Composer\Util\Platform;
+
+/**
+ * PlatformTest
+ *
+ * @author Niels Keurentjes <niels.keurentjes@omines.com>
+ */
+class PlatformTest extends \PHPUnit_Framework_TestCase
+{
+    public function testWindows()
+    {
+        // Compare 2 common tests for Windows to the built-in Windows test
+        $this->assertEquals(('\\' === DIRECTORY_SEPARATOR), Platform::isWindows());
+        $this->assertEquals(defined('PHP_WINDOWS_VERSION_MAJOR'), Platform::isWindows());
+    }
+}
diff --git a/tests/Composer/Test/Util/SvnTest.php b/tests/Composer/Test/Util/SvnTest.php
index 55a116376..c16b0e6ce 100644
--- a/tests/Composer/Test/Util/SvnTest.php
+++ b/tests/Composer/Test/Util/SvnTest.php
@@ -14,6 +14,7 @@ namespace Composer\Test\Util;
 
 use Composer\Config;
 use Composer\IO\NullIO;
+use Composer\Util\Platform;
 use Composer\Util\Svn;
 
 class SvnTest extends \PHPUnit_Framework_TestCase
@@ -131,10 +132,6 @@ class SvnTest extends \PHPUnit_Framework_TestCase
 
     private function getCmd($cmd)
     {
-        if (defined('PHP_WINDOWS_VERSION_BUILD')) {
-            return strtr($cmd, "'", '"');
-        }
-
-        return $cmd;
+        return Platform::isWindows() ? strtr($cmd, "'", '"') : $cmd;
     }
 }