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         ],
     ];
     
-    public function getCacheDirectory(): ?string
+    public function getCacheDirectory(): string
     {
         $dir = $this->config['cache']['directory'] ?? null;
         \assert(\is_string($dir) || $dir === null, 'Invalid cache directory.');

PHP snippet to change

<?php

final class DemoFile
{
    /**
     * @var array{
     *     cache: array{directory: null}
     * }
     */
    protected array $config = [
        'cache' => [
            'directory' => null,
        ],
    ];
    
    public function getCacheDirectory(): ?string
    {
        $dir = $this->config['cache']['directory'] ?? null;
        \assert(\is_string($dir) || $dir === null, 'Invalid cache directory.');

        return $dir;
    }
}

Applied Rules:

NarrowWideUnionReturnTypeRector

Config rector.php

<?php

declare(strict_types=1);

use Rector\Config\RectorConfig;
use Rector\DeadCode\Rector\FunctionLike\NarrowWideUnionReturnTypeRector;

return static function (RectorConfig $rectorConfig): void {
    $rectorConfig->rule(NarrowWideUnionReturnTypeRector::class);
};

Rector version: 1453e6 - released at 2026-09-07 10:59