bug#30219: 27.0.50; (should (equal ...)) bug for string equality

[Philipp]

Define the following ERT test:

(ert-deftest foo ()
  (should (equal "a\xFF" "a\u00FF")))

Then run M-x ert.  The test triggers an assertion.  This is because ERT
incorrectly assumes that two arrays are equal if their lengths and
elements are equal, but that's not the case when comparing unibyte and
multibyte strings.


In GNU Emacs 27.0.50 (build 10, x86_64-apple-darwin17.3.0, NS appkit-1561.20 
Version 10.13.2 (Build 17C205))
 of 2018-01-22 built on p
Repository revision: 3558d96b60393893a346f4382b813ca0738f9d9b
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.2

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --without-threads --with-modules --without-pop
 --with-mailutils --enable-gcc-warnings=yes --enable-checking
 --enable-check-lisp-object-type 'CFLAGS=-ggdb3 -O0''

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES JSON

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/ns-win ns-win ucs-normalize mule-util term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core term/tty-colors frame cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote kqueue cocoa ns
multi-tty make-network-process emacs)

Memory information:
((conses 16 204871 8960)
 (symbols 48 20154 1)
 (miscs 40 57 145)
 (strings 32 28911 1996)
 (string-bytes 1 771735)
 (vectors 16 35241)
 (vector-slots 8 721940 14772)
 (floats 8 52 64)
 (intervals 56 208 0)
 (buffers 992 11))