From 0c90971a987b8ebf44021f073837e85fdcf534fb Mon Sep 17 00:00:00 2001
From: Fabien Potencier <fabien@potencier.org>
Date: Sat, 26 Sep 2026 10:54:52 +0200
Subject: [PATCH] Fix the loop variable of a for tag being undefined in the
 sequence of a nested loop

---
 CHANGELOG                                     |  1 +
 src/NodeVisitor/OptimizerNodeVisitor.php      | 21 +++++++++++++++++--
 .../tags/for/loop_in_nested_sequence.test     | 10 +++++++++
 tests/NodeVisitor/OptimizerTest.php           |  4 ++++
 4 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 tests/Fixtures/tags/for/loop_in_nested_sequence.test

diff --git a/CHANGELOG b/CHANGELOG
index d40bf0913..5c5c32b4b 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,6 @@
 # 3.30.1 (2026-XX-XX)
 
+ * Fix the `loop` variable being undefined when used in the sequence of a nested `for` tag
  * 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
diff --git a/src/NodeVisitor/OptimizerNodeVisitor.php b/src/NodeVisitor/OptimizerNodeVisitor.php
index b778ba40e..dc33f0443 100644
--- a/src/NodeVisitor/OptimizerNodeVisitor.php
+++ b/src/NodeVisitor/OptimizerNodeVisitor.php
@@ -47,6 +47,8 @@ final class OptimizerNodeVisitor implements NodeVisitorInterface
 
     private $loops = [];
     private $loopsTargets = [];
+    /** @var list<array{Node, ForNode, string, string}> */
+    private array $suspendedLoops = [];
 
     /**
      * @param int $optimizers The optimizer mode
@@ -129,7 +131,16 @@ final class OptimizerNodeVisitor implements NodeVisitorInterface
             array_unshift($this->loops, $node);
             array_unshift($this->loopsTargets, $node->getNode('value_target')->getAttribute('name'));
             array_unshift($this->loopsTargets, $node->getNode('key_target')->getAttribute('name'));
-        } elseif (!$this->loops) {
+
+            return;
+        }
+
+        // the sequence of a loop is evaluated in the scope enclosing the loop
+        if ($this->loops && $node === $this->loops[0]->getNode('seq')) {
+            array_unshift($this->suspendedLoops, [$node, array_shift($this->loops), array_shift($this->loopsTargets), array_shift($this->loopsTargets)]);
+        }
+
+        if (!$this->loops) {
             // we are outside a loop
             return;
         }
@@ -137,7 +148,7 @@ final class OptimizerNodeVisitor implements NodeVisitorInterface
         // when do we need to add the loop variable back?
 
         // the loop variable is referenced for the current loop
-        elseif ($node instanceof ContextVariable && 'loop' === $node->getAttribute('name')) {
+        if ($node instanceof ContextVariable && 'loop' === $node->getAttribute('name')) {
             $node->setAttribute('always_defined', true);
             $this->addLoopToCurrent();
         }
@@ -187,6 +198,12 @@ final class OptimizerNodeVisitor implements NodeVisitorInterface
      */
     private function leaveOptimizeFor(Node $node): void
     {
+        if ($this->suspendedLoops && $node === $this->suspendedLoops[0][0]) {
+            [, $loop, $keyTarget, $valueTarget] = array_shift($this->suspendedLoops);
+            array_unshift($this->loops, $loop);
+            array_unshift($this->loopsTargets, $keyTarget, $valueTarget);
+        }
+
         if ($node instanceof ForNode) {
             array_shift($this->loops);
             array_shift($this->loopsTargets);
diff --git a/tests/Fixtures/tags/for/loop_in_nested_sequence.test b/tests/Fixtures/tags/for/loop_in_nested_sequence.test
new file mode 100644
index 000000000..c3ab985d3
--- /dev/null
+++ b/tests/Fixtures/tags/for/loop_in_nested_sequence.test
@@ -0,0 +1,10 @@
+--TEST--
+"for" tag gives access to the loop variable of the enclosing loop in the sequence of a nested loop
+--TEMPLATE--
+{% for i in ['a', 'b'] %}{% for j in [loop.index] %}{{ i }}{{ j }}{% endfor %}{% endfor %}
+--DATA--
+return []
+--CONFIG--
+return ['strict_variables' => true]
+--EXPECT--
+a1b2
diff --git a/tests/NodeVisitor/OptimizerTest.php b/tests/NodeVisitor/OptimizerTest.php
index 5a7799049..4596aaea2 100644
--- a/tests/NodeVisitor/OptimizerTest.php
+++ b/tests/NodeVisitor/OptimizerTest.php
@@ -124,6 +124,10 @@ class OptimizerTest extends TestCase
 
             ['{% for i in foo %}{% for j in foo %}{{ loop.parent.loop.index }}{% endfor %}{% endfor %}', ['i' => true, 'j' => true]],
 
+            ['{% for i in foo %}{% for j in [loop.index] %}{% endfor %}{% endfor %}', ['i' => true, 'j' => false]],
+
+            ['{% for i in foo %}{% for j in loop %}{% endfor %}{% endfor %}', ['i' => true, 'j' => false]],
+
             ['{% for i in foo %}{% set l = loop %}{% for j in foo %}{{ l.index }}{% endfor %}{% endfor %}', ['i' => true, 'j' => false]],
 
             ['{% for i in foo %}{% for j in foo %}{{ foo.parent.loop.index }}{% endfor %}{% endfor %}', ['i' => false, 'j' => false]],