serapeum library regression: Internal error: Cannot print object readably.

(This is similar to issue #754 (closed) although it is a separate bug: in a recent commit, ECL stopped being able to build serapeum. The same disclaimer applies as for that bug report: I apologize in advance for the lack of a small reproducible bit of lisp code ; I'm not the library author, merely a downstream package repository maintainer.)

I have found a regression in package serapeum in ECL revision 2e732782:

compiling the library now yields this error:

;;;
;;;
;;; Compiling /private/tmp/nix-build-system-serapeum.drv-0/source/vector=.lisp.
;;; OPTIMIZE levels: Safety=2, Space=0, Speed=3, Debug=0
;;;
;;; Internal error:
;;;   ** Cannot print object  readably.An error occurred during initialization:
COMPILE-FILE-ERROR while
compiling #<cl-source-file "serapeum" "level2" "vector=">.

I'm not sure why this change leads to this bug, but according to git bisect, this is where it started happening. Before 2e732782, ECL was able to build serapeum.

The offending file is https://github.com/ruricolist/serapeum/blob/af8e411f2cd33a59336803ff520a59e486ea883e/vector%3D.lisp though I don't know exactly which form in that file is causing the bug. And of course I don't know if it's a serapeum bug or an ECL bug.

If you have some flags / vars I can pass to ECL on build which help with debugging I'm happy to do so and report back with more details.