bug #4915 Fix the default filter fallback reusing a null-safe temporary variable (lazerg, fabpot)

This PR was merged into the 3.x branch.

Discussion
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Fix the default filter fallback reusing a null-safe temporary variable

The `default` filter reuses the fallback argument node in both branches of the conditional it compiles to. Because `GetAttrExpression` records the temporary variable it allocated for a null-safe chain on the node itself, the second compilation skipped the assignment and emitted a bare `$_vN` reference, so `{{ item?.label|default(item?.name) }}` warned about an undefined variable whenever `item` was null.

The fallback node is now cloned, like the node used for the defined test already is, so each branch compiles its own temporary.

Fixes #4914

Commits
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2d0c30b075 Remove redundant default filter cases
0b4199c522 Strengthen the default filter regression test
86c830ef45 Fix the default filter fallback reusing a null-safe temporary variable

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

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 {

tests/ExpressionParserTest.php

@@ -445,6 +445,22 @@ 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],
+                '',
+            ],
+            [
+                '{% for foo in foos %}[{{ foo?.bar|default(foo?.baz) }}]{% endfor %}',
+                ['foos' => [(object) ['bar' => 'corge', 'baz' => 'grault'], (object) ['baz' => 'qux']]],
+                '[corge][qux]',
+            ],
         ];
     }