Use default endopoints as fallbacks if Composer endpoint is specified as a string

Author: emodric

src/Downloader.php

@@ -59,12 +59,17 @@ public function __construct(Composer $composer, IoInterface $io, $rfs)
         if (null === $endpoint = $composer->getPackage()->getExtra()['symfony']['endpoint'] ?? null) {
             $this->endpoints = self::DEFAULT_ENDPOINTS;
 
-            if (!filter_var(getenv('FLEX_SERVERLESS'), \FILTER_VALIDATE_BOOLEAN)) {
+            if (!$this->isServerlessEnabled()) {
                 $this->endpoints = null;
                 $this->legacyEndpoint = 'https://flex.symfony.com';
             }
         } elseif (\is_array($endpoint) || false !== strpos($endpoint, '.json') || 'flex://defaults' === $endpoint) {
             $this->endpoints = array_values((array) $endpoint);
+            if (false !== strpos($endpoint, '.json')) {
+                $this->endpoints[] = 'flex://defaults';
+            }
+        } elseif ($this->isServerlessEnabled() && \is_string($endpoint)) {
+            $this->endpoints = [$endpoint, 'flex://defaults'];
         } else {
             $this->legacyEndpoint = rtrim($endpoint, '/');
         }
@@ -414,4 +419,9 @@ private function initialize()
             $this->endpoints[$endpoint] = $config;
         }
     }
+
+    private function isServerlessEnabled(): bool
+    {
+        return filter_var(getenv('FLEX_SERVERLESS'), \FILTER_VALIDATE_BOOLEAN);
+    }
 }