# Steps to reproduce

1) emacs -Q
2) M-x set-variable RET
   async-shell-command-display-buffer RET
   nil RET
3) M-: (async-shell-command
        "ls ~" (generate-new-buffer "home")) RET

This results in the following error:

  error in process filter: Wrong type argument:
    stringp, #<buffer home>

I attach the relevant backtrace.

# Diagnosis

When async-shell-command-display-buffer is nil, the async
shell-command process filter passes output-buffer, which
could be a buffer object, to string=.

# Patch

I attach two patches to address this issue.

The first patch replaces the quoted lambda process filter
with a closure over the name of output-buffer.  By keeping
track of this name, a lot of the surrounding
buffer-name-determining code can be simplified via DRY.

The second patch is purely aesthetic and simplifies the cond
that is touched by my first patch into an equivalent pcase.

# Environment

In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2017-10-25 built on thunk
Repository revision: 090f4f157eea6f0d0d13963520f5e05706de142f
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
System Description:	Debian GNU/Linux testing (buster)

Recent messages:
Configuring package avy...done
Configuring package ace-window...done
Configuring package mm-decode...done
Configuring package dired...done
Loading /home/blc/.emacs.d/eudc-options...
Loading eudcb-bbdb...
Configuring package bbdb...done
Loading eudcb-bbdb...done
Loading /home/blc/.emacs.d/eudc-options...done
Configuring package message...done

Configured using:
 'configure --prefix=/home/blc/.local
 --enable-locallisppath= --with-mailutils --with-sound=yes
 --with-x-toolkit=lucid --with-xpm --with-jpeg --with-tiff
 --with-gif --with-png --with-rsvg --with-libsystemd
 --with-xml2 --with-imagemagick --with-xft --with-libotf
 --with-m17n-flt --with-toolkit-scroll-bars --with-xaw3d
 --with-xim --with-gpm --with-dbus --with-gsettings
 --with-selinux --with-gnutls --with-zlib --with-modules
 --with-threads --with-file-notification=yes --with-x
 --without-gconf --with-lcms2 'CFLAGS=-flto
 -fomit-frame-pointer -march=native -maes -mavx -mcrc32
 -mf16c -mfpmath=sse -mfsgsbase -mfxsr
 -minline-all-stringops -mmmx -mpclmul -mpopcnt -msahf
 -msse4.2 -mxsave -mxsaveopt -mvzeroupper -O3 -pipe'
 LDFLAGS=-flto'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS
GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE
M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11
MODULES LIBSYSTEMD LCMS2

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

Major mode: Lisp Interaction

Minor modes in effect:
  ace-window-display-mode: t
  shell-dirtrack-mode: t
  counsel-mode: t
  bug-reference-prog-mode: t
  fic-mode: t
  global-paren-face-mode: t
  paren-face-mode: t
  xterm-mouse-mode: t
  winner-mode: t
  global-whitespace-mode: t
  display-time-mode: t
  global-subword-mode: t
  subword-mode: t
  save-place-mode: t
  show-paren-mode: t
  global-hi-lock-mode: t
  hi-lock-mode: t
  engine-mode: t
  delete-selection-mode: t
  display-battery-mode: t
  override-global-mode: t
  blc-dropbox-mode: t
  blc-rainbow-mode: t
  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
  window-divider-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t
  auto-save-visited-mode: t

Features:
(shadow sort footnote face-remap mail-extr gnus-msg gnus-art
mm-uu mml2015 mm-view mml-smime smime dig mailcap gnus-sum
gnus-group gnus-undo gnus-start gnus-cloud nnimap nnmail
mail-source tls gnutls utf7 netrc nnoo parse-time gnus-spec
gnus-int gnus-range gnus-win gnus nnheader emacsbug
eudcb-bbdb bbdb-com crm bbdb bbdb-site timezone eudc
eudc-options-file cus-edit eudc-vars wid-edit message rmc
puny git-annex dired-x dired dired-loaddefs format-spec
rfc822 mml mml-sec epa derived epg gnus-util rmail
rmail-loaddefs xdg mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils ace-window
avy cl-print debug shell pcomplete comint ansi-color
cus-start cus-load colir color counsel jka-compr esh-util
etags xref project swiper ivy flx ivy-overlay ffap thingatpt
server bug-reference fic-mode fill-column-indicator
paren-face elec-pair xt-mouse winner ring disp-table
whitespace time cap-words superword subword saveplace paren
ibuf-macs highlight-escape-sequences hi-lock time-date
engine-mode delsel battery edmacro kmacro cl-extra help-mode
delight advice zenburn-theme browse-url use-package diminish
bind-key finder-inf tex-site info package easymenu
epg-config url-handlers url-parse auth-source cl-seq eieio
eieio-core cl-macs eieio-loaddefs password-cache url-vars
blc-macs blc-lib easy-mmode rx pcase thunk subr-x map seq
byte-opt gv bytecomp byte-compile cconv cl-loaddefs cl-lib
realpath 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 359441 89175)
 (symbols 48 41275 27)
 (miscs 40 1373 2435)
 (strings 32 101726 9595)
 (string-bytes 1 3144509)
 (vectors 16 36288)
 (vector-slots 8 830885 84664)
 (floats 8 420 172)
 (intervals 56 1018 82)
 (buffers 992 17))

Thanks,

-- 
Basil