>
> 1. emacs -Q
>
> 2. Go to *scratch* (but it is happening anywhere)
>
> 2. Evaluate (number-to-string nil) and get error, do not press Q in 
> *Backtrace*, switch back to *scratch*
>
> 3. Try evaluating any defun with C-M-x it will give error: 
> elisp--eval-defun: Symbol’s function definition is void: nil
>
> 4. You can evaluate with C-x C-e on the end of the defun, but not in the 
> middle with C-M-x
>
> 5. Go to *Backtrace*, click q or erase *Backtrace*, come back and it 
> works again.
>

IIUC, this is a kind chicken-and-egg problem: eval-defun at that point 
does not work without a (require 'edebug), which executes 
(edebug-install-read-eval-functions), which puts an advice around 
eval-defun...

Here's a patch, which I'm sure is not TRT, but which at least fixes that 
bug.  Cc'ing Stefan, who wrote bae2cfe63c, who might have a better idea on 
how this should be done.