bug#27340: 26.0.50; byte-compile-delete-errors value changes unexpectedly

[Katsumi Yamaoka <yamaoka@jpl.org>]

Hi,

When loading a certain Lisp module that requires eieio, the value
of `byte-compile-delete-errors' is changed to t (the default nil)
unexpectedly.  For instance:

emacs -batch -Q -l auth-source -eval '(message "%s" byte-compile-delete-errors)'
=> t

Because of this, the byte compiler gets oddly quiet when compiling
a Lisp source on a running Emacs than `batch-byte-compile'.  What
changes the value of `byte-compile-delete-errors' is the following
section of eieio-core.el:

--8<---------------cut here---------------start------------->8---
(cl-defstruct (eieio--class
               (:constructor nil)
               (:constructor eieio--class-make (name))
               (:include cl--class)
               (:copier nil))
  children
  initarg-tuples                  ;; initarg tuples list
  (class-slots nil :type eieio--slot)
  class-allocation-values         ;; class allocated value vector
  default-object-cache ;; what a newly created object would look like.
                       ; This will speed up instantiation time as
                       ; only a `copy-sequence' will be needed, instead of
                       ; looping over all the values and setting them from
                       ; the default.
  options ;; storage location of tagged class option
          ; Stored outright without modifications or stripping
  )
--8<---------------cut here---------------end--------------->8---

Though I don't know why it changes `byte-compile-delete-errors',
I think it's a bug.

Thanks.

In GNU Emacs 26.0.50 (build 1, i686-pc-cygwin, GTK+ Version 3.18.9)
 of 2017-06-12 built on localhost
Windowing system distributor 'The Cygwin/X Project', version 11.0.11900000