Source

File: wp-admin/includes/class-wp-filesystem-base.php

public function abspath() {
		$folder = $this->find_folder( ABSPATH );

		// Perhaps the FTP folder is rooted at the WordPress install.
		// Check for wp-includes folder in root. Could have some false positives, but rare.
		if ( ! $folder && $this->is_dir( '/' . WPINC ) ) {
			$folder = '/';
		}

		return $folder;
	}