Not sure what the reference to circular structures means in this context. As I said, all versions of Emacs less than 26 it worked fine with these same Lisp Objects.

As I said: what changed? Are you trying to say something in 26.1 is now "fixed" i.e. it should never have worked in versions less than 26? and what I have been doing for years should have always failed?

As for the file reference, try

and look for else-structs.el

On Wed, Nov 21, 2018 at 8:02 AM Noam Postavsky <npostavs@gmail.com> wrote:

On Tue, 20 Nov 2018 at 15:57, Paul Eggert <eggert@cs.ucla.edu> wrote:

> The syntax error occurs at the "#5" here:
>
> ... #s(#5 "" "" "" nil nil nil nil nil) (:custom-groups nil
> :documentation "Placeholder Class base." ...
>
> and you might want to investigate what is outputting "#5".

Standard Emacs printing functions will produce that for circular
structures if print-circle is nil (otherwise they use #5#).