Merge branch '1.x' into 2.x

* 1.x:
  [Flex] Fix Composer update exception when flex://defaults missing

Author: nicolas-grekas

src/PackageFilter.php

@@ -122,6 +122,9 @@ private function getVersions(): array
         $this->downloader = null;
         $okVersions = [];
 
+        if (!isset($versions['splits'])) {
+            throw new \LogicException('The Flex index is missing a "splits" entry. Did you forget to add "flex://defaults" in the "extra.symfony.endpoint" array of your composer.json?');
+        }
         foreach ($versions['splits'] as $name => $vers) {
             foreach ($vers as $i => $v) {
                 if (!isset($okVersions[$v])) {