> I think it is clearer to catch the error than to check the property by
 > hand.  But the question is where to do this?  Where you did it, in
 > delete-selection-pre-hook, or where I did it, in delete-active-region?

Honestly, mine was a simple-minded attempt to handle read-only buffers
and read-only text analogously.

 > The question is, if you type a self-inserting character, is it right
 > to skip just the deletion done by delete-active-region and do the
 > insertion, or should it skip both?  My patch does the former, yours
 > does the latter.

I suppose your patch has the flaw that when `overwrite-mode' is on, the
following part

		 (if (and overwrite-mode (eq this-command 'self-insert-command))
		   (let ((overwrite-mode nil))
		     (self-insert-command (prefix-numeric-value current-prefix-arg))
		     (setq this-command 'ignore)))))

will still cause an error and clear `pre-command-hook'.

Maybe just add buffer-read-only and text-read-only to the handler in
`delete-selection-pre-hook' as in the attached patch?