Use plain symbols for eieio type descriptors (Bug#29220)
Since Emacs 26, eieio objects use a class record (with circular references) as the type descriptor of the object record. This causes problems when reading back an object from a string, because the class record is not `eq' to the canonical one (which means that read objects don't satisfy the foo-p predicate). * lisp/emacs-lisp/eieio.el (make-instance): As a (partial) fix, set the record's type descriptor to a plain symbol for the type descriptor when eieio-backward-compatibility is non-nil (the default). * lisp/emacs-lisp/eieio-core.el (eieio--object-class): Call eieio--class-object on the type tag when eieio-backward-compatibility is non-nil. (eieio-object-p): Use eieio--object-class instead of eieio--object-class-tag. * test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el (eieio-test-persist-hash-and-vector) (eieio-test-persist-interior-lists): Make into functions. (eieio-persist-hash-and-vector-backward-compatibility) (eieio-persist-hash-and-vector-no-backward-compatibility) (eieio-test-persist-interior-lists-backward-compatibility) (eieio-test-persist-interior-lists-no-backward-compatibility): New tests which call them, eieio-backward-compatibility let-bound.
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