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7.2 Backward Compatibility

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

If arg is positive, enable backward compatibility with old-style structs.