From d5011f4654042f03d8986b5b3ba12a4d85f976f8 Mon Sep 17 00:00:00 2001
From: Fabien Potencier <fabien@potencier.org>
Date: Sat, 26 Sep 2026 09:07:40 +0200
Subject: [PATCH] Discard what the non-selected branch of a guard tag registers
 on the parser

---
 CHANGELOG                                         |  1 +
 src/Parser.php                                    |  5 +++++
 src/TokenParser/GuardTokenParser.php              |  6 +++++-
 tests/Fixtures/tags/guard/discarded_branch.test   |  9 +++++++++
 .../tags/guard/discarded_parent.legacy.test       | 15 +++++++++++++++
 5 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 tests/Fixtures/tags/guard/discarded_branch.test
 create mode 100644 tests/Fixtures/tags/guard/discarded_parent.legacy.test

diff --git a/CHANGELOG b/CHANGELOG
index 8f423332e..d40bf0913 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -2,6 +2,7 @@
 
  * Reject a deprecated `Template` instance created by another environment in `Environment::resolveTemplate()`
  * Fix a compilation error inside an `autoescape` tag leaking its escaping strategy into the next compiled template
+ * Fix the non-selected branch of a `guard` tag registering its parent, blocks, and macros
  * Speed up reading object attributes backed by getters or class constants
  * Fix the sandbox not reporting the line of a rejected `guard` tag
 
diff --git a/src/Parser.php b/src/Parser.php
index ddd7fe2cd..8a44deb83 100644
--- a/src/Parser.php
+++ b/src/Parser.php
@@ -159,14 +159,19 @@ class Parser
         return $this->ignoreUnknownTwigCallables;
     }
 
+    /**
+     * Parses a discarded part of the template: what it registers (parent, blocks, macros, ...) is discarded as well.
+     */
     public function subparseIgnoreUnknownTwigCallables($test, bool $dropNeedle = false): void
     {
         $previous = $this->ignoreUnknownTwigCallables;
+        $state = [$this->parent, $this->blocks, $this->documentationTargets, $this->macros, $this->importedSymbols, $this->traits, $this->embeddedTemplates];
         $this->ignoreUnknownTwigCallables = true;
         try {
             $this->subparse($test, $dropNeedle);
         } finally {
             $this->ignoreUnknownTwigCallables = $previous;
+            [$this->parent, $this->blocks, $this->documentationTargets, $this->macros, $this->importedSymbols, $this->traits, $this->embeddedTemplates] = $state;
         }
     }
 
diff --git a/src/TokenParser/GuardTokenParser.php b/src/TokenParser/GuardTokenParser.php
index 7aba0f0d9..e5d93ac75 100644
--- a/src/TokenParser/GuardTokenParser.php
+++ b/src/TokenParser/GuardTokenParser.php
@@ -55,7 +55,11 @@ final class GuardTokenParser extends AbstractTokenParser
         $else = new EmptyNode();
         if ('else' === $stream->next()->getValue()) {
             $stream->expect(Token::BLOCK_END_TYPE);
-            $else = $this->parser->subparse([$this, 'decideGuardEnd'], true);
+            if ($exists) {
+                $this->parser->subparseIgnoreUnknownTwigCallables([$this, 'decideGuardEnd'], true);
+            } else {
+                $else = $this->parser->subparse([$this, 'decideGuardEnd'], true);
+            }
         }
         $stream->expect(Token::BLOCK_END_TYPE);
 
diff --git a/tests/Fixtures/tags/guard/discarded_branch.test b/tests/Fixtures/tags/guard/discarded_branch.test
new file mode 100644
index 000000000..7ccfd5bdd
--- /dev/null
+++ b/tests/Fixtures/tags/guard/discarded_branch.test
@@ -0,0 +1,9 @@
+--TEST--
+"guard" ignores the blocks and macros defined in the discarded branch
+--TEMPLATE--
+{% guard function constant %}{% else %}{% block discarded %}{% endblock %}{% macro discarded() %}{% endmacro %}{% endguard %}
+{{ block('discarded') is defined ? 'NEVER' : 'no block' }} {{ _self.discarded is defined ? 'NEVER' : 'no macro' }}
+--DATA--
+return []
+--EXPECT--
+no block no macro
diff --git a/tests/Fixtures/tags/guard/discarded_parent.legacy.test b/tests/Fixtures/tags/guard/discarded_parent.legacy.test
new file mode 100644
index 000000000..52a29c555
--- /dev/null
+++ b/tests/Fixtures/tags/guard/discarded_parent.legacy.test
@@ -0,0 +1,15 @@
+--TEST--
+"guard" ignores the parent defined in the discarded branch
+--DEPRECATION--
+Since twig/twig 3.28: Using the "extends" tag outside the root of a template is deprecated in index.twig at line 2.
+--TEMPLATE--
+{% guard function foobar %}{% extends "never.twig" %}{% else %}{% extends "layout.twig" %}{% endguard %}
+{% block content %}content{% endblock %}
+--TEMPLATE(layout.twig)--
+layout {{ block('content') }}
+--TEMPLATE(never.twig)--
+NEVER
+--DATA--
+return []
+--EXPECT--
+layout content