In the declaration of isearch--state in isearch.el, the default values
are omitted in the list of read-only slots. This causes edebug not to be
able to instrument the file, since the edebug specification for
cl-defstruct does not include this possibility.

To reproduce, starting with emacs -Q at the head of the emacs source tree:
1. C-x C-f lisp/isearch.el RET
2. M-x edebug-all-defs RET
3. M-x toggle-debug-on-error RET
4. M-x eval-buffer RET

Here's the resulting backtrace: