Code compiled with older versions of
cl-defstruct that doesn’t use records may run into problems when used in a new Emacs. To alleviate this, Emacs detects when an old
cl-defstruct is used, and enables a mode in which
type-of handles old struct objects as if they were records.
function cl-old-struct-compat-mode arg
arg is positive, enable backward compatibility with old-style structs.