diff --git a/extra/cache-extra/TokenParser/CacheTokenParser.php b/extra/cache-extra/TokenParser/CacheTokenParser.php
index 5590ab27441..323096dd1ae 100644
--- a/extra/cache-extra/TokenParser/CacheTokenParser.php
+++ b/extra/cache-extra/TokenParser/CacheTokenParser.php
@@ -40,13 +40,13 @@ public function parse(Token $token): Node
                     if (1 !== \count($args)) {
                         throw new SyntaxError(sprintf('The "ttl" modifier takes exactly one argument (%d given).', \count($args)), $stream->getCurrent()->getLine(), $stream->getSourceContext());
                     }
-                    $ttl = $args->getNode(0);
+                    $ttl = $args->getNode('0');
                     break;
                 case 'tags':
                     if (1 !== \count($args)) {
                         throw new SyntaxError(sprintf('The "tags" modifier takes exactly one argument (%d given).', \count($args)), $stream->getCurrent()->getLine(), $stream->getSourceContext());
                     }
-                    $tags = $args->getNode(0);
+                    $tags = $args->getNode('0');
                     break;
                 default:
                     throw new SyntaxError(sprintf('Unknown "%s" configuration.', $k), $stream->getCurrent()->getLine(), $stream->getSourceContext());
diff --git a/src/Environment.php b/src/Environment.php
index f9e0086c60a..1a575f6c68a 100644
--- a/src/Environment.php
+++ b/src/Environment.php
@@ -437,7 +437,7 @@ public function resolveTemplate($names): TemplateWrapper
         $count = \count($names);
         foreach ($names as $name) {
             if ($name instanceof Template) {
-                return $name;
+                return new TemplateWrapper($this, $name);
             }
             if ($name instanceof TemplateWrapper) {
                 return $name;
@@ -535,7 +535,7 @@ public function getLoader(): LoaderInterface
 
     public function setCharset(string $charset)
     {
-        if ('UTF8' === $charset = null === $charset ? null : strtoupper($charset)) {
+        if ('UTF8' === $charset = strtoupper($charset ?: '')) {
             // iconv on Windows requires "UTF-8" instead of "UTF8"
             $charset = 'UTF-8';
         }
diff --git a/src/ExpressionParser.php b/src/ExpressionParser.php
index 912d0a930a5..6839bc93204 100644
--- a/src/ExpressionParser.php
+++ b/src/ExpressionParser.php
@@ -452,14 +452,14 @@ public function getFunctionNode($name, $line)
                     throw new SyntaxError('The "block" function takes one argument (the block name).', $line, $this->parser->getStream()->getSourceContext());
                 }
 
-                return new BlockReferenceExpression($args->getNode(0), \count($args) > 1 ? $args->getNode(1) : null, $line);
+                return new BlockReferenceExpression($args->getNode('0'), \count($args) > 1 ? $args->getNode('1') : null, $line);
             case 'attribute':
                 $args = $this->parseArguments();
                 if (\count($args) < 2) {
                     throw new SyntaxError('The "attribute" function takes at least two arguments (the variable and the attributes).', $line, $this->parser->getStream()->getSourceContext());
                 }
 
-                return new GetAttrExpression($args->getNode(0), $args->getNode(1), \count($args) > 2 ? $args->getNode(2) : null, Template::ANY_CALL, $line);
+                return new GetAttrExpression($args->getNode('0'), $args->getNode('1'), \count($args) > 2 ? $args->getNode('2') : null, Template::ANY_CALL, $line);
             default:
                 if (null !== $alias = $this->parser->getImportedSymbol('function', $name)) {
                     $arguments = new ArrayExpression([], $line);
diff --git a/src/Extension/CoreExtension.php b/src/Extension/CoreExtension.php
index c4e0bdab228..598caca4536 100644
--- a/src/Extension/CoreExtension.php
+++ b/src/Extension/CoreExtension.php
@@ -1124,7 +1124,7 @@ public static function nl2br($string)
     /**
      * Removes whitespaces between HTML tags.
      *
-     * @param string|null $string
+     * @param string|null $content
      *
      * @return string
      *
@@ -1241,11 +1241,7 @@ public static function striptags($string, $allowable_tags = null)
      */
     public static function titleStringFilter(Environment $env, $string)
     {
-        if (null !== $charset = $env->getCharset()) {
-            return mb_convert_case($string ?? '', \MB_CASE_TITLE, $charset);
-        }
-
-        return ucwords(strtolower($string ?? ''));
+        return mb_convert_case($string ?? '', \MB_CASE_TITLE, $env->getCharset());
     }
 
     /**
@@ -1436,6 +1432,8 @@ public static function source(Environment $env, $name, $ignoreMissing = false)
             if (!$ignoreMissing) {
                 throw $e;
             }
+
+            return '';
         }
     }
 
@@ -1731,9 +1729,9 @@ public static function getAttribute(Environment $env, Source $source, $object, $
      *  {# fruits now contains ['apple', 'orange'] #}
      * </pre>
      *
-     * @param array|Traversable $array An array
-     * @param mixed             $name  The column name
-     * @param mixed             $index The column to use as the index/keys for the returned array
+     * @param array|\Traversable $array An array
+     * @param mixed              $name  The column name
+     * @param mixed              $index The column to use as the index/keys for the returned array
      *
      * @return array The array of values
      *
diff --git a/src/Extension/EscaperExtension.php b/src/Extension/EscaperExtension.php
index 9a8c66c9047..dee0f79fe8c 100644
--- a/src/Extension/EscaperExtension.php
+++ b/src/Extension/EscaperExtension.php
@@ -175,7 +175,7 @@ public static function escapeFilterIsSafe(Node $filterArgs)
      * @param string $charset    The charset
      * @param bool   $autoescape Whether the function is called by the auto-escaping feature (true) or by the developer (false)
      *
-     * @return string
+     * @return string|Markup
      *
      * @internal
      */
diff --git a/src/Node/Expression/Filter/DefaultFilter.php b/src/Node/Expression/Filter/DefaultFilter.php
index 6a572d48848..e8eae20ecd3 100644
--- a/src/Node/Expression/Filter/DefaultFilter.php
+++ b/src/Node/Expression/Filter/DefaultFilter.php
@@ -35,7 +35,7 @@ public function __construct(Node $node, ConstantExpression $filterName, Node $ar
 
         if ('default' === $filterName->getAttribute('value') && ($node instanceof NameExpression || $node instanceof GetAttrExpression)) {
             $test = new DefinedTest(clone $node, 'defined', new Node(), $node->getTemplateLine());
-            $false = \count($arguments) ? $arguments->getNode(0) : new ConstantExpression('', $node->getTemplateLine());
+            $false = \count($arguments) ? $arguments->getNode('0') : new ConstantExpression('', $node->getTemplateLine());
 
             $node = new ConditionalExpression($test, $default, $false, $node->getTemplateLine());
         } else {
diff --git a/src/Node/Expression/Test/ConstantTest.php b/src/Node/Expression/Test/ConstantTest.php
index 57e9319d574..867fd09517c 100644
--- a/src/Node/Expression/Test/ConstantTest.php
+++ b/src/Node/Expression/Test/ConstantTest.php
@@ -33,16 +33,16 @@ public function compile(Compiler $compiler): void
             ->raw(' === constant(')
         ;
 
-        if ($this->getNode('arguments')->hasNode(1)) {
+        if ($this->getNode('arguments')->hasNode('1')) {
             $compiler
                 ->raw('get_class(')
-                ->subcompile($this->getNode('arguments')->getNode(1))
+                ->subcompile($this->getNode('arguments')->getNode('1'))
                 ->raw(')."::".')
             ;
         }
 
         $compiler
-            ->subcompile($this->getNode('arguments')->getNode(0))
+            ->subcompile($this->getNode('arguments')->getNode('0'))
             ->raw('))')
         ;
     }
diff --git a/src/Node/Expression/Test/DivisiblebyTest.php b/src/Node/Expression/Test/DivisiblebyTest.php
index 4cb3ee09692..90d58a49a16 100644
--- a/src/Node/Expression/Test/DivisiblebyTest.php
+++ b/src/Node/Expression/Test/DivisiblebyTest.php
@@ -29,7 +29,7 @@ public function compile(Compiler $compiler): void
             ->raw('(0 == ')
             ->subcompile($this->getNode('node'))
             ->raw(' % ')
-            ->subcompile($this->getNode('arguments')->getNode(0))
+            ->subcompile($this->getNode('arguments')->getNode('0'))
             ->raw(')')
         ;
     }
diff --git a/src/Node/Expression/Test/SameasTest.php b/src/Node/Expression/Test/SameasTest.php
index c96d2bc01a3..f1e24db6f7e 100644
--- a/src/Node/Expression/Test/SameasTest.php
+++ b/src/Node/Expression/Test/SameasTest.php
@@ -27,7 +27,7 @@ public function compile(Compiler $compiler): void
             ->raw('(')
             ->subcompile($this->getNode('node'))
             ->raw(' === ')
-            ->subcompile($this->getNode('arguments')->getNode(0))
+            ->subcompile($this->getNode('arguments')->getNode('0'))
             ->raw(')')
         ;
     }
diff --git a/src/Node/IfNode.php b/src/Node/IfNode.php
index 569ab7950e0..b41ee828d43 100644
--- a/src/Node/IfNode.php
+++ b/src/Node/IfNode.php
@@ -47,13 +47,13 @@ public function compile(Compiler $compiler): void
             }
 
             $compiler
-                ->subcompile($this->getNode('tests')->getNode($i))
+                ->subcompile($this->getNode('tests')->getNode((string) $i))
                 ->raw(") {\n")
                 ->indent()
             ;
             // The node might not exists if the content is empty
-            if ($this->getNode('tests')->hasNode($i + 1)) {
-                $compiler->subcompile($this->getNode('tests')->getNode($i + 1));
+            if ($this->getNode('tests')->hasNode((string) ($i + 1))) {
+                $compiler->subcompile($this->getNode('tests')->getNode((string) ($i + 1)));
             }
         }
 
diff --git a/src/Node/ModuleNode.php b/src/Node/ModuleNode.php
index dce335c63f5..44186dc5d00 100644
--- a/src/Node/ModuleNode.php
+++ b/src/Node/ModuleNode.php
@@ -381,7 +381,7 @@ protected function compileIsTraitable(Compiler $compiler)
         $traitable = !$this->hasNode('parent') && 0 === \count($this->getNode('macros'));
         if ($traitable) {
             if ($this->getNode('body') instanceof BodyNode) {
-                $nodes = $this->getNode('body')->getNode(0);
+                $nodes = $this->getNode('body')->getNode('0');
             } else {
                 $nodes = $this->getNode('body');
             }
@@ -418,7 +418,7 @@ protected function compileIsTraitable(Compiler $compiler)
             ->write(" */\n")
             ->write("public function isTraitable()\n", "{\n")
             ->indent()
-            ->write(sprintf("return %s;\n", $traitable ? 'true' : 'false'))
+            ->write("return false;\n")
             ->outdent()
             ->write("}\n\n")
         ;
diff --git a/src/NodeVisitor/EscaperNodeVisitor.php b/src/NodeVisitor/EscaperNodeVisitor.php
index c390d7cc71b..91e2ea89392 100644
--- a/src/NodeVisitor/EscaperNodeVisitor.php
+++ b/src/NodeVisitor/EscaperNodeVisitor.php
@@ -131,10 +131,6 @@ private function escapeInlinePrintNode(InlinePrint $node, Environment $env, stri
 
     private function escapePrintNode(PrintNode $node, Environment $env, string $type): Node
     {
-        if (false === $type) {
-            return $node;
-        }
-
         $expression = $node->getNode('expr');
 
         if ($this->isSafeFor($type, $expression, $env)) {
diff --git a/src/Parser.php b/src/Parser.php
index 4016a5f39ab..0c7629b5572 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -101,6 +101,9 @@ public function parse(TokenStream $stream, $test = null, bool $dropNeedle = fals
 
         $traverser = new NodeTraverser($this->env, $this->visitors);
 
+        /**
+         * @var ModuleNode $node
+         */
         $node = $traverser->traverse($node);
 
         // restore previous stack so previous parse() call can resume working
diff --git a/src/Test/IntegrationTestCase.php b/src/Test/IntegrationTestCase.php
index 90ff39aadaf..570c378bbe0 100644
--- a/src/Test/IntegrationTestCase.php
+++ b/src/Test/IntegrationTestCase.php
@@ -149,6 +149,7 @@ protected function doIntegrationTest($file, $message, $condition, $templates, $e
         }
 
         if ($condition) {
+            $ret = '';
             eval('$ret = '.$condition.';');
             if (!$ret) {
                 $this->markTestSkipped($condition);
diff --git a/src/TokenParser/ForTokenParser.php b/src/TokenParser/ForTokenParser.php
index bac8ba2dae8..1af6da8fde5 100644
--- a/src/TokenParser/ForTokenParser.php
+++ b/src/TokenParser/ForTokenParser.php
@@ -49,12 +49,12 @@ public function parse(Token $token): Node
         $stream->expect(/* Token::BLOCK_END_TYPE */ 3);
 
         if (\count($targets) > 1) {
-            $keyTarget = $targets->getNode(0);
+            $keyTarget = $targets->getNode('0');
             $keyTarget = new AssignNameExpression($keyTarget->getAttribute('name'), $keyTarget->getTemplateLine());
-            $valueTarget = $targets->getNode(1);
+            $valueTarget = $targets->getNode('1');
         } else {
             $keyTarget = new AssignNameExpression('_key', $lineno);
-            $valueTarget = $targets->getNode(0);
+            $valueTarget = $targets->getNode('0');
         }
         $valueTarget = new AssignNameExpression($valueTarget->getAttribute('name'), $valueTarget->getTemplateLine());
 
diff --git a/src/TokenStream.php b/src/TokenStream.php
index 1eac11a02d6..cb578e4fcb9 100644
--- a/src/TokenStream.php
+++ b/src/TokenStream.php
@@ -60,9 +60,7 @@ public function next(): Token
      */
     public function nextIf($primary, $secondary = null)
     {
-        if ($this->tokens[$this->current]->test($primary, $secondary)) {
-            return $this->next();
-        }
+        return $this->tokens[$this->current]->test($primary, $secondary) ? $this->next() : null;
     }
 
     /**
diff --git a/tests/ExpressionParserTest.php b/tests/ExpressionParserTest.php
index afd1884d8ee..bb000225ae5 100644
--- a/tests/ExpressionParserTest.php
+++ b/tests/ExpressionParserTest.php
@@ -64,7 +64,7 @@ public function testArrayExpression($template, $expected)
         $parser = new Parser($env);
         $expected->setSourceContext($source);
 
-        $this->assertEquals($expected, $parser->parse($stream)->getNode('body')->getNode(0)->getNode('expr'));
+        $this->assertEquals($expected, $parser->parse($stream)->getNode('body')->getNode('0')->getNode('expr'));
     }
 
     /**
@@ -217,7 +217,7 @@ public function testStringExpression($template, $expected)
         $parser = new Parser($env);
         $expected->setSourceContext($source);
 
-        $this->assertEquals($expected, $parser->parse($stream)->getNode('body')->getNode(0)->getNode('expr'));
+        $this->assertEquals($expected, $parser->parse($stream)->getNode('body')->getNode('0')->getNode('expr'));
     }
 
     public function getTestsForString()
diff --git a/tests/Node/Expression/ArrayTest.php b/tests/Node/Expression/ArrayTest.php
index cfd9c67f3cb..f72eeab757b 100644
--- a/tests/Node/Expression/ArrayTest.php
+++ b/tests/Node/Expression/ArrayTest.php
@@ -22,7 +22,7 @@ public function testConstructor()
         $elements = [new ConstantExpression('foo', 1), $foo = new ConstantExpression('bar', 1)];
         $node = new ArrayExpression($elements, 1);
 
-        $this->assertEquals($foo, $node->getNode(1));
+        $this->assertEquals($foo, $node->getNode('1'));
     }
 
     public function getTests()
diff --git a/tests/Node/ForTest.php b/tests/Node/ForTest.php
index dbfac3269b2..42c34f8031c 100644
--- a/tests/Node/ForTest.php
+++ b/tests/Node/ForTest.php
@@ -33,7 +33,7 @@ public function testConstructor()
         $this->assertEquals($keyTarget, $node->getNode('key_target'));
         $this->assertEquals($valueTarget, $node->getNode('value_target'));
         $this->assertEquals($seq, $node->getNode('seq'));
-        $this->assertEquals($body, $node->getNode('body')->getNode(0));
+        $this->assertEquals($body, $node->getNode('body')->getNode('0'));
         $this->assertFalse($node->hasNode('else'));
 
         $else = new PrintNode(new NameExpression('foo', 1), 1);
diff --git a/tests/NodeVisitor/OptimizerTest.php b/tests/NodeVisitor/OptimizerTest.php
index 8d02f6c7e2a..b685f32248a 100644
--- a/tests/NodeVisitor/OptimizerTest.php
+++ b/tests/NodeVisitor/OptimizerTest.php
@@ -28,7 +28,7 @@ public function testRenderBlockOptimizer()
 
         $stream = $env->parse($env->tokenize(new Source('{{ block("foo") }}', 'index')));
 
-        $node = $stream->getNode('body')->getNode(0);
+        $node = $stream->getNode('body')->getNode('0');
 
         $this->assertInstanceOf(BlockReferenceExpression::class, $node);
         $this->assertTrue($node->getAttribute('output'));
@@ -40,7 +40,7 @@ public function testRenderParentBlockOptimizer()
 
         $stream = $env->parse($env->tokenize(new Source('{% extends "foo" %}{% block content %}{{ parent() }}{% endblock %}', 'index')));
 
-        $node = $stream->getNode('blocks')->getNode('content')->getNode(0)->getNode('body');
+        $node = $stream->getNode('blocks')->getNode('content')->getNode('0')->getNode('body');
 
         $this->assertInstanceOf(ParentExpression::class, $node);
         $this->assertTrue($node->getAttribute('output'));