Move extends validation into correctness visitor

src/NodeVisitor/CorrectnessNodeVisitor.php

@@ -42,6 +42,7 @@ final class CorrectnessNodeVisitor implements NodeVisitorInterface
     private bool $hasParent = false;
     private int $blockDepth = 0;
     private int $macroDepth = 0;
+    private ?ConfigNode $extendsNode = null;
 
     public function enterNode(Node $node, Environment $env): Node
     {
@@ -52,6 +53,7 @@ final class CorrectnessNodeVisitor implements NodeVisitorInterface
             $this->tagStack = [];
             $this->blockDepth = 0;
             $this->macroDepth = 0;
+            $this->extendsNode = null;
 
             $body = $node->getNode('body')->getNode('0');
             // Parser::subparse() does not wrap the parsed nodes when there is only one,
@@ -77,15 +79,21 @@ final class CorrectnessNodeVisitor implements NodeVisitorInterface
             $this->tagStack[] = $node;
         }
 
-        if ($node instanceof ConfigNode && !isset($this->rootNodes[$node])) {
+        if ($node instanceof ConfigNode && 'extends' === $node->getNodeTag()) {
             // "extends" inside a "block" or a "macro" has always been a hard error; keep it
-            if ('extends' === $node->getNodeTag() && $this->blockDepth) {
+            if ($this->blockDepth) {
                 throw new SyntaxError('Cannot use "extend" in a block.', $node->getTemplateLine(), $node->getSourceContext());
             }
-            if ('extends' === $node->getNodeTag() && $this->macroDepth) {
+            if ($this->macroDepth) {
                 throw new SyntaxError('Cannot use "extend" in a macro.', $node->getTemplateLine(), $node->getSourceContext());
             }
+            if ($this->extendsNode) {
+                throw new SyntaxError('Multiple extends tags are forbidden.', $node->getTemplateLine(), $node->getSourceContext());
+            }
+            $this->extendsNode = $node;
+        }
 
+        if ($node instanceof ConfigNode && !isset($this->rootNodes[$node])) {
             trigger_deprecation('twig/twig', '3.27', 'Using the "%s" tag outside the root of a template is deprecated in %s at line %d.', $node->getNodeTag(), $node->getSourceContext()->getName(), $node->getTemplateLine());
         }
 
@@ -112,6 +120,7 @@ final class CorrectnessNodeVisitor implements NodeVisitorInterface
         if ($node instanceof ModuleNode) {
             $this->rootNodes = null;
             $this->hasParent = false;
+            $this->extendsNode = null;
 
             return $node;
         }

src/Parser.php

@@ -414,7 +414,7 @@ class Parser
         return $this->parent || 0 < \count($this->traits);
     }
 
-    public function setParent(?Node $parent): void
+    public function setParent(?Node $parent, bool $throwOnMultiple = true): void
     {
         if (null === $parent) {
             trigger_deprecation('twig/twig', '3.12', 'Passing "null" to "%s()" is deprecated.', __METHOD__);
@@ -425,6 +425,10 @@ class Parser
         }
 
         if (null !== $this->parent) {
+            if (!$throwOnMultiple) {
+                return;
+            }
+
             throw new SyntaxError('Multiple extends tags are forbidden.', $parent->getTemplateLine(), $parent->getSourceContext());
         }
 

src/TokenParser/ExtendsTokenParser.php

@@ -12,7 +12,6 @@
 
 namespace Twig\TokenParser;
 
-use Twig\Error\SyntaxError;
 use Twig\Node\ConfigNode;
 use Twig\Node\Node;
 use Twig\Token;
@@ -29,14 +28,7 @@ final class ExtendsTokenParser extends AbstractTokenParser
     public function parse(Token $token): Node
     {
         $stream = $this->parser->getStream();
-
-        if ($this->parser->peekBlockStack()) {
-            throw new SyntaxError('Cannot use "extend" in a block.', $token->getLine(), $stream->getSourceContext());
-        } elseif (!$this->parser->isMainScope()) {
-            throw new SyntaxError('Cannot use "extend" in a macro.', $token->getLine(), $stream->getSourceContext());
-        }
-
-        $this->parser->setParent($this->parser->parseExpression());
+        $this->parser->setParent($this->parser->parseExpression(), false);
         $stream->expect(Token::BLOCK_END_TYPE);
 
         return new ConfigNode($token->getLine());

tests/Fixtures/tags/inheritance/multiple_extends.test

@@ -0,0 +1,11 @@
+--TEST--
+Multiple "extends" tags are forbidden
+--TEMPLATE--
+{% extends "base.twig" %}
+{% extends "other.twig" %}
+--TEMPLATE(base.twig)--
+--TEMPLATE(other.twig)--
+--DATA--
+return []
+--EXCEPTION--
+Twig\Error\SyntaxError: Multiple extends tags are forbidden in "index.twig" at line 3.