Enforce more precise type on ListExpression

src/ExpressionParser/Prefix/GroupingExpressionParser.php

@@ -17,6 +17,7 @@ use Twig\ExpressionParser\ExpressionParserDescriptionInterface;
 use Twig\ExpressionParser\PrefixExpressionParserInterface;
 use Twig\Node\Expression\AbstractExpression;
 use Twig\Node\Expression\ListExpression;
+use Twig\Node\Expression\Variable\AssignContextVariable;
 use Twig\Node\Expression\Variable\ContextVariable;
 use Twig\Parser;
 use Twig\Token;
@@ -36,7 +37,7 @@ final class GroupingExpressionParser extends AbstractExpressionParser implements
                 return $expr->setExplicitParentheses();
             }
 
-            return new ListExpression([$expr], $token->getLine());
+            return new ListExpression([self::toAssignContextVariable($expr)], $token->getLine());
         }
 
         // determine if we are parsing an arrow function arguments
@@ -58,7 +59,16 @@ final class GroupingExpressionParser extends AbstractExpressionParser implements
             throw new SyntaxError('A list of variables must be followed by an arrow.', $stream->getCurrent()->getLine(), $stream->getSourceContext());
         }
 
-        return new ListExpression($names, $token->getLine());
+        return new ListExpression(array_map(self::toAssignContextVariable(...), $names), $token->getLine());
+    }
+
+    private static function toAssignContextVariable(AbstractExpression $expr): AssignContextVariable
+    {
+        if (!$expr instanceof ContextVariable) {
+            throw new SyntaxError('A list must only contain variables.', $expr->getTemplateLine(), $expr->getSourceContext());
+        }
+
+        return $expr instanceof AssignContextVariable ? $expr : new AssignContextVariable($expr->getAttribute('name'), $expr->getTemplateLine());
     }
 
     public function getName(): string

src/Node/Expression/ArrowFunctionExpression.php

@@ -26,14 +26,14 @@ class ArrowFunctionExpression extends AbstractExpression
 {
     public function __construct(AbstractExpression $expr, Node $names, $lineno)
     {
-        if (!$names instanceof ListExpression && !$names instanceof ContextVariable) {
-            throw new SyntaxError('The arrow function argument must be a list of variables or a single variable.', $names->getTemplateLine(), $names->getSourceContext());
-        }
-
         if ($names instanceof ContextVariable) {
             $names = new ListExpression([new AssignContextVariable($names->getAttribute('name'), $names->getTemplateLine())], $lineno);
         }
 
+        if (!$names instanceof ListExpression) {
+            throw new SyntaxError('The arrow function argument must be a list of variables or a single variable.', $names->getTemplateLine(), $names->getSourceContext());
+        }
+
         parent::__construct(['expr' => $expr, 'names' => $names], [], $lineno);
     }
 

src/Node/Expression/ListExpression.php

@@ -12,12 +12,12 @@
 namespace Twig\Node\Expression;
 
 use Twig\Compiler;
-use Twig\Node\Expression\Variable\ContextVariable;
+use Twig\Node\Expression\Variable\AssignContextVariable;
 
 class ListExpression extends AbstractExpression
 {
     /**
-     * @param array<ContextVariable> $items
+     * @param array<AssignContextVariable> $items
      */
     public function __construct(array $items, int $lineno)
     {