From 86c830ef45a9f1b06b3ed52c517147db567ef7ed Mon Sep 17 00:00:00 2001
From: Lazizbek Ergashev <lazerg2@gmail.com>
Date: Tue, 1 Sep 2026 14:06:43 +0500
Subject: [PATCH 1/3] Fix the default filter fallback reusing a null-safe
 temporary variable

---
 CHANGELOG                                    |  1 +
 src/Node/Expression/Filter/DefaultFilter.php |  2 +-
 tests/ExpressionParserTest.php               | 21 ++++++++++++++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG b/CHANGELOG
index 991e99dfc..47e3a7240 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,6 @@
 # 3.29.0 (2026-XX-XX)
 
+ * Fix the `default` filter fallback emitting an undefined variable warning when it uses the null-safe operator
  * Fix the `matches` operator silently treating PCRE execution errors as non-matches
  * Add the `HtmlExtension::htmlAttrValue()` method to resolve a single HTML attribute value the way the `html_attr` function renders it
  * Fix `html_attr` JSON encoding a `Stringable` value in a `data-*` attribute instead of using its string representation
diff --git a/src/Node/Expression/Filter/DefaultFilter.php b/src/Node/Expression/Filter/DefaultFilter.php
index fc64b88a9..cef374829 100644
--- a/src/Node/Expression/Filter/DefaultFilter.php
+++ b/src/Node/Expression/Filter/DefaultFilter.php
@@ -55,7 +55,7 @@ class DefaultFilter extends FilterExpression
 
         if ('default' === $name && ($node instanceof ContextVariable || $node instanceof GetAttrExpression)) {
             $test = new DefinedTest(clone $node, new TwigTest('defined', null, ['always_allowed_in_sandbox' => true]), new EmptyNode(), $node->getTemplateLine());
-            $false = \count($arguments) ? $arguments->getNode('0') : new ConstantExpression('', $node->getTemplateLine());
+            $false = \count($arguments) ? clone $arguments->getNode('0') : new ConstantExpression('', $node->getTemplateLine());
 
             $node = new ConditionalTernary($test, $default, $false, $node->getTemplateLine());
         } else {
diff --git a/tests/ExpressionParserTest.php b/tests/ExpressionParserTest.php
index 5754bf330..649e2b84c 100644
--- a/tests/ExpressionParserTest.php
+++ b/tests/ExpressionParserTest.php
@@ -445,6 +445,27 @@ class ExpressionParserTest extends TestCase
                 ['foo' => (object) ['bar' => (object) ['baz' => null]]],
                 '',
             ],
+            // default filter fallback
+            [
+                '{{ foo?.bar|default(foo?.baz) }}',
+                ['foo' => null],
+                '',
+            ],
+            [
+                '{{ foo?.bar|default(foo?.baz.qux) }}',
+                ['foo' => null],
+                '',
+            ],
+            [
+                '{{ foo?.bar|default(foo?.baz) }}',
+                ['foo' => (object) ['bar' => null, 'baz' => 'qux']],
+                'qux',
+            ],
+            [
+                '{{ foo?.bar|default(foo?.baz) }}',
+                ['foo' => (object) ['bar' => 'corge', 'baz' => 'qux']],
+                'corge',
+            ],
         ];
     }
 

From 0b4199c5224013efea6ae043742a1c511af64423 Mon Sep 17 00:00:00 2001
From: Fabien Potencier <fabien@potencier.org>
Date: Thu, 3 Sep 2026 08:32:22 +0200
Subject: [PATCH 2/3] Strengthen the default filter regression test

---
 tests/ExpressionParserTest.php | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tests/ExpressionParserTest.php b/tests/ExpressionParserTest.php
index 649e2b84c..3bdfea734 100644
--- a/tests/ExpressionParserTest.php
+++ b/tests/ExpressionParserTest.php
@@ -456,6 +456,11 @@ class ExpressionParserTest extends TestCase
                 ['foo' => null],
                 '',
             ],
+            [
+                '{% for foo in foos %}[{{ foo?.bar|default(foo?.baz) }}]{% endfor %}',
+                ['foos' => [(object) ['bar' => 'corge', 'baz' => 'grault'], (object) ['baz' => 'qux']]],
+                '[corge][qux]',
+            ],
             [
                 '{{ foo?.bar|default(foo?.baz) }}',
                 ['foo' => (object) ['bar' => null, 'baz' => 'qux']],

From 2d0c30b07528dee7a1409bf2be3c5a779225e041 Mon Sep 17 00:00:00 2001
From: Fabien Potencier <fabien@potencier.org>
Date: Thu, 3 Sep 2026 08:49:11 +0200
Subject: [PATCH 3/3] Remove redundant default filter cases

---
 tests/ExpressionParserTest.php | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/tests/ExpressionParserTest.php b/tests/ExpressionParserTest.php
index 3bdfea734..26ee469f2 100644
--- a/tests/ExpressionParserTest.php
+++ b/tests/ExpressionParserTest.php
@@ -461,16 +461,6 @@ class ExpressionParserTest extends TestCase
                 ['foos' => [(object) ['bar' => 'corge', 'baz' => 'grault'], (object) ['baz' => 'qux']]],
                 '[corge][qux]',
             ],
-            [
-                '{{ foo?.bar|default(foo?.baz) }}',
-                ['foo' => (object) ['bar' => null, 'baz' => 'qux']],
-                'qux',
-            ],
-            [
-                '{{ foo?.bar|default(foo?.baz) }}',
-                ['foo' => (object) ['bar' => 'corge', 'baz' => 'qux']],
-                'corge',
-            ],
         ];
     }