Rule source on GitHub

RemoveReturnTypeDeclarationFromCloneRector

Remove return type from __clone() method

 final class SomeClass
 {
-    public function __clone(): void
+    public function __clone()
     {
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\ClassMethod\RemoveReturnTypeDeclarationFromCloneRector;

return RectorConfig::configure()
    ->withRules([
        RemoveReturnTypeDeclarationFromCloneRector::class,
    ]);