Try Rector Online | Rector - Automated Way to Instantly Upgrade and Refactor any PHP code

Run Rector on your code to see what it can do for you:

 
     public function run()
     {
-        fn() => $this->executesCallback(fn() => $this->doNohing('a'));
+        fn(): mixed => $this->executesCallback(fn() => $this->doNohing('a'));
     }
 
     /**

PHP snippet to change

<?php

final class SkipVoidReturn
{
    public function doNohing(string $a): void
    {
        echo $a;
    }

    public function run()
    {
        fn() => $this->executesCallback(fn() => $this->doNohing('a'));
    }

    /**
     * @template T
     * @param callable(): T $callback
     * @return T
     */
    public function executesCallback(callable $callback): mixed
    {
        return $callback();
    }
}

Applied Rules:

AddArrowFunctionReturnTypeRector

Not a change you expect?

Create an issue on GitHub
...or to speed up fix – add a test fixture

Config rector.php

<?php

declare(strict_types=1);

use Rector\Config\RectorConfig;
use Rector\TypeDeclaration\Rector\ArrowFunction\AddArrowFunctionReturnTypeRector;
use Rector\ValueObject\PhpVersionFeature;

return static function (RectorConfig $rectorConfig): void {
    $rectorConfig->phpVersion(PhpVersionFeature::UNION_TYPES);
    $rectorConfig->rule(AddArrowFunctionReturnTypeRector::class);
};

Rector version: aeab56 - released at 2025-09-08 02:29