Add 7-zip presence check to diagnose command

src/Composer/Command/DiagnoseCommand.php

@@ -24,6 +24,7 @@ use Composer\Util\IniHelper;
 use Composer\Util\ProcessExecutor;
 use Composer\Util\HttpDownloader;
 use Composer\Util\StreamContextFactory;
+use Composer\Util\Platform;
 use Composer\SelfUpdate\Keys;
 use Composer\SelfUpdate\Versions;
 use Composer\IO\NullIO;
@@ -176,9 +177,17 @@ EOT
 
         $finder = new ExecutableFinder;
         $hasSystemUnzip = (bool) $finder->find('unzip');
+        if (Platform::isWindows()) {
+            $hasSystem7zip = (bool) $finder->find('7z', null, array('C:\Program Files\7-Zip'));
+            $windows7z = ', ' . ($hasSystem7zip ? '<comment>7-Zip present</comment>' : '<comment>7-Zip not available</comment>');
+        } else {
+            $windows7z = '';
+        }
+
         $io->write(
             'zip: ' . (extension_loaded('zip') ? '<comment>extension present</comment>' : '<comment>extension not loaded</comment>')
             . ', ' . ($hasSystemUnzip ? '<comment>unzip present</comment>' : '<comment>unzip not available</comment>')
+            . $windows7z
         );
 
         return $this->exitCode;