Merge branch '2.7' into 2.8

* 2.7:
  [HttpFoundation] Use UPSERT for sessions stored in PgSql >= 9.5
  [Console] fixed PHPDoc
  [travis] HHVM 3.12 LTS
  Fix feature detection for IE
  [Form] Fixed collapsed choice attributes
  [Console] added explanation of messages usage in a progress bar
  force enabling the external XML entity loaders
  [Yaml] properly count skipped comment lines

Conflicts:
	src/Symfony/Component/Translation/Loader/XliffFileLoader.php

Author: nicolas-grekas

HttpFoundation/DbalSessionHandler.php

@@ -180,7 +180,7 @@ public function write($sessionId, $data)
             $updateStmt->bindValue(':time', time(), \PDO::PARAM_INT);
             $updateStmt->execute();
 
-            // When MERGE is not supported, like in Postgres, we have to use this approach that can result in
+            // When MERGE is not supported, like in Postgres < 9.5, we have to use this approach that can result in
             // duplicate key errors when the same session is written simultaneously. We can just catch such an
             // error and re-execute the update. This is similar to a serializable transaction with retry logic
             // on serialization failures but without the overhead and without possible false positives due to
@@ -224,11 +224,11 @@ private function getMergeSql()
     {
         $platform = $this->con->getDatabasePlatform()->getName();
 
-        switch ($platform) {
-            case 'mysql':
+        switch (true) {
+            case 'mysql' === $platform:
                 return "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) ".
                     "ON DUPLICATE KEY UPDATE $this->dataCol = VALUES($this->dataCol), $this->timeCol = VALUES($this->timeCol)";
-            case 'oracle':
+            case 'oracle' === $platform:
                 // DUAL is Oracle specific dummy table
                 return "MERGE INTO $this->table USING DUAL ON ($this->idCol = :id) ".
                     "WHEN NOT MATCHED THEN INSERT ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) ".
@@ -239,8 +239,11 @@ private function getMergeSql()
                 return "MERGE INTO $this->table WITH (HOLDLOCK) USING (SELECT 1 AS dummy) AS src ON ($this->idCol = :id) ".
                     "WHEN NOT MATCHED THEN INSERT ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) ".
                     "WHEN MATCHED THEN UPDATE SET $this->dataCol = :data, $this->timeCol = :time;";
-            case 'sqlite':
+            case 'sqlite' === $platform:
                 return "INSERT OR REPLACE INTO $this->table ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time)";
+            case 'postgresql' === $platform && version_compare($this->con->getServerVersion(), '9.5', '>='):
+                return "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time) ".
+                    "ON CONFLICT ($this->idCol) DO UPDATE SET ($this->dataCol, $this->timeCol) = (:data, :time) WHERE $this->idCol = :id";
         }
     }
 }