diff --git a/src/Composer/Command/SelfUpdateCommand.php b/src/Composer/Command/SelfUpdateCommand.php
index d79d22df2..e5550d14e 100644
--- a/src/Composer/Command/SelfUpdateCommand.php
+++ b/src/Composer/Command/SelfUpdateCommand.php
@@ -135,8 +135,36 @@ EOT
 
         $latest = $versionsUtil->getLatest();
         $latestStable = $versionsUtil->getLatest('stable');
+        try {
+            $latestPreview = $versionsUtil->getLatest('preview');
+        } catch (\UnexpectedValueException $e) {
+            $latestPreview = $latestStable;
+        }
         $latestVersion = $latest['version'];
         $updateVersion = $input->getArgument('version') ?: $latestVersion;
+        $currentMajorVersion = preg_replace('{^(\d+).*}', '$1', Composer::getVersion());
+        $updateMajorVersion = preg_replace('{^(\d+).*}', '$1', $updateVersion);
+        $previewMajorVersion = preg_replace('{^(\d+).*}', '$1', $latestPreview['version']);
+
+        if ($versionsUtil->getChannel() === 'stable' && !$input->getArgument('version')) {
+            // if requesting stable channel and no specific version, avoid automatically upgrading to the next major
+            // simply output a warning that the next major stable is available and let users upgrade to it manually
+            if ($currentMajorVersion < $updateMajorVersion) {
+                $skippedVersion = $updateVersion;
+
+                $versionsUtil->setChannel($currentMajorVersion);
+
+                $latest = $versionsUtil->getLatest();
+                $latestStable = $versionsUtil->getLatest('stable');
+                $latestVersion = $latest['version'];
+                $updateVersion = $latestVersion;
+
+                $io->writeError('<warning>A new stable major version of Composer is available ('.$skippedVersion.'), run "composer self-update --'.$updateMajorVersion.'" to update to it. See also https://github.com/composer/composer/releases for changelogs.</warning>');
+            } elseif ($currentMajorVersion < $previewMajorVersion) {
+                // promote next major version if available in preview
+                $io->writeError('<warning>A preview release of the next major version of Composer is available ('.$latestPreview['version'].'), run "composer self-update --preview" to give it a try. See also https://github.com/composer/composer/releases for changelogs.</warning>');
+            }
+        }
 
         if ($requestedChannel && is_numeric($requestedChannel) && substr($latestStable['version'], 0, 1) !== $requestedChannel) {
             $io->writeError('<warning>Warning: You forced the install of '.$latestVersion.' via --'.$requestedChannel.', but '.$latestStable['version'].' is the latest stable version. Updating to it via composer self-update --stable is recommended.</warning>');
diff --git a/src/Composer/DependencyResolver/DefaultPolicy.php b/src/Composer/DependencyResolver/DefaultPolicy.php
index 235987aa3..4f3bdefa3 100644
--- a/src/Composer/DependencyResolver/DefaultPolicy.php
+++ b/src/Composer/DependencyResolver/DefaultPolicy.php
@@ -60,7 +60,7 @@ class DefaultPolicy implements PolicyInterface
             $sortedLiterals = $this->pruneRemoteAliases($pool, $sortedLiterals);
         }
 
-        $selected = \call_user_func_array('array_merge', $packages);
+        $selected = \call_user_func_array('array_merge', array_values($packages));
 
         // now sort the result across all packages to respect replaces across packages
         usort($selected, function ($a, $b) use ($policy, $pool, $requiredPackage) {
diff --git a/src/Composer/Repository/Vcs/BitbucketDriver.php b/src/Composer/Repository/Vcs/BitbucketDriver.php
index 22180b55d..d23b32dd2 100644
--- a/src/Composer/Repository/Vcs/BitbucketDriver.php
+++ b/src/Composer/Repository/Vcs/BitbucketDriver.php
@@ -120,10 +120,14 @@ abstract class BitbucketDriver extends VcsDriver
 
         if (!isset($this->infoCache[$identifier])) {
             if ($this->shouldCache($identifier) && $res = $this->cache->read($identifier)) {
-                return $this->infoCache[$identifier] = JsonFile::parseJson($res);
-            }
+                $composer = JsonFile::parseJson($res);
+            } else {
+                $composer = $this->getBaseComposerInformation($identifier);
 
-            $composer = $this->getBaseComposerInformation($identifier);
+                if ($this->shouldCache($identifier)) {
+                    $this->cache->write($identifier, json_encode($composer));
+                }
+            }
 
             if ($composer) {
                 // specials for bitbucket
@@ -174,10 +178,6 @@ abstract class BitbucketDriver extends VcsDriver
             }
 
             $this->infoCache[$identifier] = $composer;
-
-            if ($this->shouldCache($identifier)) {
-                $this->cache->write($identifier, json_encode($composer));
-            }
         }
 
         return $this->infoCache[$identifier];
diff --git a/src/Composer/Repository/Vcs/GitHubDriver.php b/src/Composer/Repository/Vcs/GitHubDriver.php
index 0a97123c0..83e353321 100644
--- a/src/Composer/Repository/Vcs/GitHubDriver.php
+++ b/src/Composer/Repository/Vcs/GitHubDriver.php
@@ -151,10 +151,14 @@ class GitHubDriver extends VcsDriver
 
         if (!isset($this->infoCache[$identifier])) {
             if ($this->shouldCache($identifier) && $res = $this->cache->read($identifier)) {
-                return $this->infoCache[$identifier] = JsonFile::parseJson($res);
-            }
+                $composer =  JsonFile::parseJson($res);
+            } else {
+                $composer = $this->getBaseComposerInformation($identifier);
 
-            $composer = $this->getBaseComposerInformation($identifier);
+                if ($this->shouldCache($identifier)) {
+                    $this->cache->write($identifier, json_encode($composer));
+                }
+            }
 
             if ($composer) {
                 // specials for github
@@ -173,10 +177,6 @@ class GitHubDriver extends VcsDriver
                 }
             }
 
-            if ($this->shouldCache($identifier)) {
-                $this->cache->write($identifier, json_encode($composer));
-            }
-
             $this->infoCache[$identifier] = $composer;
         }
 
diff --git a/src/Composer/Repository/Vcs/GitLabDriver.php b/src/Composer/Repository/Vcs/GitLabDriver.php
index 8f0ed6f0f..22f884d7f 100644
--- a/src/Composer/Repository/Vcs/GitLabDriver.php
+++ b/src/Composer/Repository/Vcs/GitLabDriver.php
@@ -131,10 +131,14 @@ class GitLabDriver extends VcsDriver
 
         if (!isset($this->infoCache[$identifier])) {
             if ($this->shouldCache($identifier) && $res = $this->cache->read($identifier)) {
-                return $this->infoCache[$identifier] = JsonFile::parseJson($res);
-            }
+                $composer =  JsonFile::parseJson($res);
+            } else {
+                $composer = $this->getBaseComposerInformation($identifier);
 
-            $composer = $this->getBaseComposerInformation($identifier);
+                if ($this->shouldCache($identifier)) {
+                    $this->cache->write($identifier, json_encode($composer));
+                }
+            }
 
             if ($composer) {
                 // specials for gitlab (this data is only available if authentication is provided)
@@ -146,10 +150,6 @@ class GitLabDriver extends VcsDriver
                 }
             }
 
-            if ($this->shouldCache($identifier)) {
-                $this->cache->write($identifier, json_encode($composer));
-            }
-
             $this->infoCache[$identifier] = $composer;
         }
 
@@ -448,7 +448,7 @@ class GitLabDriver extends VcsDriver
                     if (!$moreThanGuestAccess) {
                         $this->io->writeError('<warning>GitLab token with Guest only access detected</warning>');
 
-                        return $this->attemptCloneFallback(); 
+                        return $this->attemptCloneFallback();
                     }
                 }
 
diff --git a/src/Composer/SelfUpdate/Versions.php b/src/Composer/SelfUpdate/Versions.php
index 6d1379d41..21b48d9e5 100644
--- a/src/Composer/SelfUpdate/Versions.php
+++ b/src/Composer/SelfUpdate/Versions.php
@@ -26,6 +26,7 @@ class Versions
     private $httpDownloader;
     private $config;
     private $channel;
+    private $versionsData;
 
     public function __construct(Config $config, HttpDownloader $httpDownloader)
     {
@@ -63,12 +64,7 @@ class Versions
 
     public function getLatest($channel = null)
     {
-        if ($this->config->get('disable-tls') === true) {
-            $protocol = 'http';
-        } else {
-            $protocol = 'https';
-        }
-        $versions = $this->httpDownloader->get($protocol . '://getcomposer.org/versions')->decodeJson();
+        $versions = $this->getVersionsData();
 
         foreach ($versions[$channel ?: $this->getChannel()] as $version) {
             if ($version['min-php'] <= PHP_VERSION_ID) {
@@ -76,6 +72,21 @@ class Versions
             }
         }
 
-        throw new \LogicException('There is no version of Composer available for your PHP version ('.PHP_VERSION.')');
+        throw new \UnexpectedValueException('There is no version of Composer available for your PHP version ('.PHP_VERSION.')');
+    }
+
+    private function getVersionsData()
+    {
+        if (!$this->versionsData) {
+            if ($this->config->get('disable-tls') === true) {
+                $protocol = 'http';
+            } else {
+                $protocol = 'https';
+            }
+
+            $this->versionsData = $this->httpDownloader->get($protocol . '://getcomposer.org/versions')->decodeJson();
+        }
+
+        return $this->versionsData;
     }
 }