Check the Composer version before downloading the dependencies

composer.json

@@ -44,6 +44,12 @@
     },
     "scripts": {
         "drupal-scaffold": "DrupalComposer\\DrupalScaffold\\Plugin::scaffold",
+        "pre-install-cmd": [
+            "DrupalProject\\composer\\ScriptHandler::checkComposerVersion"
+        ],
+        "pre-update-cmd": [
+            "DrupalProject\\composer\\ScriptHandler::checkComposerVersion"
+        ],
         "post-install-cmd": [
             "DrupalProject\\composer\\ScriptHandler::createRequiredFiles"
         ],

scripts/composer/ScriptHandler.php

@@ -8,6 +8,7 @@
 namespace DrupalProject\composer;
 
 use Composer\Script\Event;
+use Composer\Semver\Comparator;
 use Symfony\Component\Filesystem\Filesystem;
 
 class ScriptHandler {
@@ -57,4 +58,35 @@ public static function createRequiredFiles(Event $event) {
     }
   }
 
+  /**
+   * Checks if the installed version of Composer is compatible.
+   *
+   * Composer 1.0.0 and higher consider a `composer install` without having a
+   * lock file present as equal to `composer update`. We do not ship with a lock
+   * file to avoid merge conflicts downstream, meaning that if a project is
+   * installed with an older version of Composer the scaffolding of Drupal will
+   * not be triggered. We check this here instead of in drupal-scaffold to be
+   * able to give immediate feedback to the end user, rather than failing the
+   * installation after going through the lengthy process of compiling and
+   * downloading the Composer dependencies.
+   *
+   * @see https://github.com/composer/composer/pull/5035
+   */
+  public static function checkComposerVersion(Event $event) {
+    $composer = $event->getComposer();
+    $io = $event->getIO();
+
+    $version = $composer::VERSION;
+
+    // If Composer is installed through git we have no easy way to determine if
+    // it is new enough, just display a warning.
+    if ($version === '@package_version@') {
+      $io->writeError('<warning>You are running a development version of Composer. If you experience problems, please update Composer to the latest stable version.</warning>');
+    }
+    elseif (Comparator::lessThan($version, '1.0.0')) {
+      $io->writeError('<error>Drupal-project requires Composer version 1.0.0 or higher. Please update your Composer before continuing</error>.');
+      exit(1);
+    }
+  }
+
 }