system/Autoloader/Autoloader.php
protected function loadLegacy(string $class)
	{
		// If there is a namespace on this class, then
		// we cannot load it from traditional locations.
		if (strpos($class, '\\') !== false)
		{
			return false;
		}

		$paths = [
			APPPATH . 'Controllers/',
			APPPATH . 'Libraries/',
			APPPATH . 'Models/',
		];

		$class = str_replace('\\', '/', $class) . '.php';

		foreach ($paths as $path)
		{
			if ($file = $this->requireFile($path . $class))
			{
				return $file;
			}
		}

		return false;
	}
protected function loadLegacy(string $class)
	{
		// If there is a namespace on this class, then
		// we cannot load it from traditional locations.
		if (strpos($class, '\\') !== false)
		{
			return false;
		}

		$paths = [
			APPPATH . 'Controllers/',
			APPPATH . 'Libraries/',
			APPPATH . 'Models/',
		];

		$class = str_replace('\\', '/', $class) . '.php';

		foreach ($paths as $path)
		{
			if ($file = $this->requireFile($path . $class))
			{
				return $file;
			}
		}

		return false;
	}
Attempts to load the class from common locations in previous
version of CodeIgniter, namely 'app/Libraries', and
'app/Models'.

@param string $class The class name. This typically should NOT have a namespace.

@return mixed    The mapped file name on success, or boolean false on failure