Fix the loop variable of a for tag being undefined in the sequence of a nested loop

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

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);

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

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]],