diff --git a/UPGRADE-4.0.md b/UPGRADE-4.0.md
index 4eb1a0ed2a..4a6d02b374 100644
--- a/UPGRADE-4.0.md
+++ b/UPGRADE-4.0.md
@@ -236,6 +236,17 @@ DependencyInjection
 
  * The `ExtensionCompilerPass` has been moved to before-optimization passes with priority -1000.
 
+ * In 3.4, parameter `container.dumper.inline_class_loader` was introduced. Unless
+   you're using a custom autoloader, you should enable this parameter. This can
+   drastically improve DX by reducing the time to load classes when the `DebugClassLoader`
+   is enabled. If you're using `FrameworkBundle`, this performance improvement will
+   also impact the "dev" environment:
+
+   ```yml
+   parameters:
+       container.dumper.inline_class_loader: true
+   ```
+
 DoctrineBridge
 --------------
 
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/AnalyzeServiceReferencesPass.php b/src/Symfony/Component/DependencyInjection/Compiler/AnalyzeServiceReferencesPass.php
index 95a1e5a928..a67d9b044f 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/AnalyzeServiceReferencesPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/AnalyzeServiceReferencesPass.php
@@ -64,7 +64,8 @@ class AnalyzeServiceReferencesPass extends AbstractRecursivePass implements Repe
         $this->lazy = false;
 
         foreach ($container->getAliases() as $id => $alias) {
-            $this->graph->connect($id, $alias, (string) $alias, $this->getDefinition((string) $alias), null);
+            $targetId = $this->getDefinitionId((string) $alias);
+            $this->graph->connect($id, $alias, $targetId, $this->getDefinition($targetId), null);
         }
 
         parent::process($container);
@@ -87,12 +88,13 @@ class AnalyzeServiceReferencesPass extends AbstractRecursivePass implements Repe
             return $value;
         }
         if ($value instanceof Reference) {
-            $targetDefinition = $this->getDefinition((string) $value);
+            $targetId = $this->getDefinitionId((string) $value);
+            $targetDefinition = $this->getDefinition($targetId);
 
             $this->graph->connect(
                 $this->currentId,
                 $this->currentDefinition,
-                $this->getDefinitionId((string) $value),
+                $targetId,
                 $targetDefinition,
                 $value,
                 $this->lazy || ($targetDefinition && $targetDefinition->isLazy()),
@@ -125,10 +127,8 @@ class AnalyzeServiceReferencesPass extends AbstractRecursivePass implements Repe
         return $value;
     }
 
-    private function getDefinition(string $id): ?Definition
+    private function getDefinition(?string $id): ?Definition
     {
-        $id = $this->getDefinitionId($id);
-
         return null === $id ? null : $this->container->getDefinition($id);
     }
 
@@ -156,11 +156,12 @@ class AnalyzeServiceReferencesPass extends AbstractRecursivePass implements Repe
             $this->expressionLanguage = new ExpressionLanguage(null, $providers, function ($arg) {
                 if ('""' === substr_replace($arg, '', 1, -1)) {
                     $id = stripcslashes(substr($arg, 1, -1));
+                    $id = $this->getDefinitionId($id);
 
                     $this->graph->connect(
                         $this->currentId,
                         $this->currentDefinition,
-                        $this->getDefinitionId($id),
+                        $id,
                         $this->getDefinition($id)
                     );
                 }
diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php
index cb05f90143..d60272b276 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInvalidReferencesPass.php
@@ -90,9 +90,7 @@ class ResolveInvalidReferencesPass implements CompilerPassInterface
                 $value = array_values($value);
             }
         } elseif ($value instanceof Reference) {
-            $id = (string) $value;
-
-            if ($this->container->has($id)) {
+            if ($this->container->has($value)) {
                 return $value;
             }
             $invalidBehavior = $value->getInvalidBehavior();
diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index 8d7011c1b0..1d49b2af18 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -592,7 +592,7 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
      *
      * @throws BadMethodCallException When this ContainerBuilder is compiled
      */
-    public function merge(ContainerBuilder $container)
+    public function merge(self $container)
     {
         if ($this->isCompiled()) {
             throw new BadMethodCallException('Cannot merge on a compiled container.');
@@ -1324,16 +1324,16 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
             $value = $bag->resolveValue($value);
         }
 
-        if (is_array($value)) {
+        if (\is_array($value)) {
             $result = array();
             foreach ($value as $k => $v) {
-                $result[$this->resolveEnvPlaceholders($k, $format, $usedEnvs)] = $this->resolveEnvPlaceholders($v, $format, $usedEnvs);
+                $result[\is_string($k) ? $this->resolveEnvPlaceholders($k, $format, $usedEnvs) : $k] = $this->resolveEnvPlaceholders($v, $format, $usedEnvs);
             }
 
             return $result;
         }
 
-        if (!is_string($value)) {
+        if (!\is_string($value) || 38 > \strlen($value)) {
             return $value;
         }
         $envPlaceholders = $bag instanceof EnvPlaceholderParameterBag ? $bag->getEnvPlaceholders() : $this->envPlaceholders;
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
index 4e475fe0b8..55537643d3 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
@@ -1552,11 +1552,12 @@ EOF;
         } elseif ($value instanceof Variable) {
             return '$'.$value;
         } elseif ($value instanceof Reference) {
-            if (null !== $this->referenceVariables && isset($this->referenceVariables[$id = (string) $value])) {
+            $id = (string) $value;
+            if (null !== $this->referenceVariables && isset($this->referenceVariables[$id])) {
                 return $this->dumpValue($this->referenceVariables[$id], $interpolate);
             }
 
-            return $this->getServiceCall((string) $value, $value);
+            return $this->getServiceCall($id, $value);
         } elseif ($value instanceof Expression) {
             return $this->getExpressionLanguage()->compile((string) $value, array('this' => 'container'));
         } elseif ($value instanceof Parameter) {
diff --git a/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php b/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php
index da6a21b837..dc5a4be292 100644
--- a/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php
+++ b/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php
@@ -171,16 +171,16 @@ class ParameterBag implements ParameterBagInterface
      */
     public function resolveValue($value, array $resolving = array())
     {
-        if (is_array($value)) {
+        if (\is_array($value)) {
             $args = array();
             foreach ($value as $k => $v) {
-                $args[$this->resolveValue($k, $resolving)] = $this->resolveValue($v, $resolving);
+                $args[\is_string($k) ? $this->resolveValue($k, $resolving) : $k] = $this->resolveValue($v, $resolving);
             }
 
             return $args;
         }
 
-        if (!is_string($value)) {
+        if (!\is_string($value) || 2 > \strlen($value)) {
             return $value;
         }