Fix inconsistent array access with a Stringable key

CHANGELOG

@@ -1,5 +1,6 @@
 # 3.27.1 (2026-XX-XX)
 
+ * Fix array access with a `Stringable` key to coerce the key to string consistently instead of throwing in the optimized path
  * Fix sandbox replacing `IteratorAggregate` arguments (e.g. Symfony's `FormView`) by a plain array
 
 # 3.27.0 (2026-05-27)

src/Node/Expression/GetAttrExpression.php

@@ -73,9 +73,9 @@ public function compile(Compiler $compiler): void
                     ->raw(' instanceof ArrayAccess ? (')
                     ->raw($var)
                     ->raw('[')
-                    ->subcompile($this->getNode('attribute'))
-                    ->raw('] ?? null) : null)')
                 ;
+                $this->compileArrayKey($compiler);
+                $compiler->raw('] ?? null) : null)');
 
                 return;
             }
@@ -90,9 +90,9 @@ public function compile(Compiler $compiler): void
                 ->raw(', CoreExtension::ARRAY_LIKE_CLASSES, true) ? (')
                 ->raw($var)
                 ->raw('[')
-                ->subcompile($this->getNode('attribute'))
-                ->raw('] ?? null) : ')
             ;
+            $this->compileArrayKey($compiler);
+            $compiler->raw('] ?? null) : ');
         }
 
         if ($this->getAttribute('ignore_strict_check')) {
@@ -160,8 +160,41 @@ public function compile(Compiler $compiler): void
 
     public function getStringCoercedChildNames(): array
     {
-        // for a method-like access, the host PHP method may coerce any of its arguments to string
-        return $this->hasNode('arguments') ? ['arguments'] : [];
+        $names = [];
+
+        // the host PHP method may coerce any argument to string
+        if ($this->hasNode('arguments')) {
+            $names[] = 'arguments';
+        }
+
+        // compileArrayKey() coerces a Stringable key; expose it so the sandbox checks __toString()
+        if (Template::ARRAY_CALL === $this->getAttribute('type')) {
+            $names[] = 'attribute';
+        }
+
+        return $names;
+    }
+
+    /**
+     * Coerces a Stringable array key to string so the optimized path matches
+     * CoreExtension::getAttribute(); scalars are left to PHP's native offset coercion.
+     */
+    private function compileArrayKey(Compiler $compiler): void
+    {
+        $attribute = $this->getNode('attribute');
+
+        if ($attribute instanceof ConstantExpression) {
+            $compiler->subcompile($attribute);
+
+            return;
+        }
+
+        $key = '$'.$compiler->getVarName();
+        $compiler
+            ->raw('(('.$key.' = ')
+            ->subcompile($attribute)
+            ->raw(') instanceof \Stringable ? (string) '.$key.' : '.$key.')')
+        ;
     }
 
     private function changeIgnoreStrictCheck(self $node): void

tests/TemplateTest.php

@@ -252,6 +252,58 @@ public function testGetAttributeOnArrayWithConfusableKey()
         $this->assertSame('EmptyString', CoreExtension::getAttribute($twig, $template->getSourceContext(), $array, null), 'null is treated as "" when accessing a sequence/mapping (equals PHP behavior)');
     }
 
+    /**
+     * @dataProvider getStrictVariablesModes
+     */
+    public function testArrayAccessWithStringableKeyIsConsistentAcrossStrictModes(bool $strict)
+    {
+        $twig = new Environment(new ArrayLoader(['index' => '{{ array[object] }}']), [
+            'strict_variables' => $strict,
+            'autoescape' => false,
+        ]);
+
+        $object = new class implements \Stringable {
+            public function __toString(): string
+            {
+                return 'string';
+            }
+        };
+
+        $this->assertSame('value', $twig->render('index', ['array' => ['string' => 'value'], 'object' => $object]));
+    }
+
+    public static function getStrictVariablesModes(): iterable
+    {
+        yield 'lax' => [false];
+        yield 'strict' => [true];
+    }
+
+    public function testArrayAccessWithStringableKeyIsCheckedBySandbox()
+    {
+        $object = new class implements \Stringable {
+            public function __toString(): string
+            {
+                return 'string';
+            }
+        };
+        $data = ['array' => ['string' => 'value'], 'object' => $object];
+
+        $twig = new Environment(new ArrayLoader(['index' => '{{ array[object] }}']), ['autoescape' => false]);
+        $twig->addExtension(new SandboxExtension(new SecurityPolicy([], [], [], [], []), true));
+
+        try {
+            $twig->render('index', $data);
+            $this->fail('The sandbox must reject the __toString() coercion of the array key.');
+        } catch (SecurityError $e) {
+            $this->assertStringContainsStringIgnoringCase('__toString', $e->getMessage());
+        }
+
+        $twig = new Environment(new ArrayLoader(['index' => '{{ array[object] }}']), ['autoescape' => false]);
+        $twig->addExtension(new SandboxExtension(new SecurityPolicy([], [], [$object::class => ['__toString']], [], []), true));
+
+        $this->assertSame('value', $twig->render('index', $data));
+    }
+
     /**
      * @dataProvider getGetAttributeTests
      */