In an indirect buffer `buffer-file-name' is nil, and consequently M-x
diff-buffer-with-file fails with "diff-no-select: Wrong type argument:
stringp, nil"

I think it would be sensible to use the base buffer's file name in this
situation.

Patch attached.


-Phil




In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll
bars)
 of 2018-06-19 built on shodan
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description:	Ubuntu 18.04 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Scanning for dabbrevs...done
user-error: No dynamic expansion for ‘diff-bu’ found

Configured using:
 'configure --prefix=/home/phil/emacs/26.1/usr/local
 --with-x-toolkit=lucid --without-sound'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK DBUS GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 THREADS
LCMS2

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

Major mode: Dired by name

Minor modes in effect:
  show-paren-mode: t
  minibuffer-depth-indicate-mode: t
  winner-mode: t
  global-hl-line-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr dabbrev emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib format-spec rfc822 mml
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 dired-x easymenu paren mb-depth winner
ring hl-line dired dired-loaddefs advice elec-pair time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd 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 dbusbind inotify lcms2 dynamic-setting system-font-setting
font-render-setting x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 101859 10773)
 (symbols 48 20824 1)
 (miscs 40 108 168)
 (strings 32 30401 971)
 (string-bytes 1 796958)
 (vectors 16 15482)
 (vector-slots 8 504854 6666)
 (floats 8 60 57)
 (intervals 56 279 0)
 (buffers 992 13))