system/Router/RouteCollection.php
public function setDefaultNamespace(string $value): RouteCollectionInterface
	{
		$this->defaultNamespace = filter_var($value, FILTER_SANITIZE_STRING);
		$this->defaultNamespace = rtrim($this->defaultNamespace, '\\') . '\\';

		return $this;
	}
public function setDefaultNamespace(string $value): RouteCollectionInterface
	{
		$this->defaultNamespace = filter_var($value, FILTER_SANITIZE_STRING);
		$this->defaultNamespace = rtrim($this->defaultNamespace, '\\') . '\\';

		return $this;
	}
Sets the default namespace to use for Controllers when no other
namespace has been specified.

@param $value

@return \CodeIgniter\Router\RouteCollectionInterface