This was discovered in `haskell-mode` [1]. The documentation of  define-obsolete-face-alias claims that `when` is optional, but the macro definition does not follow the docs. This patch fixes the problem.

In GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin19.2.0, NS appkit-1894.20 Version 10.15.2 (Build 19C57))
 of 2019-12-19 built on moomin.local
Repository revision: f38e8a86650e8a7f72293d81a7392289c298ae5a
Repository branch: fix_define-obsolete-face-alias_optional
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.2

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
user-error: The mark is not set now, so there is no region
Making completion list...

Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_CA.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 dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
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
tab-bar menu-bar rfn-eshadow isearch timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core term/tty-colors frame
minibuffer 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 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
threads kqueue cocoa ns lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 43939 6393)
 (symbols 48 5909 1)
 (strings 32 15245 1247)
 (string-bytes 1 502913)
 (vectors 16 9297)
 (vector-slots 8 118962 11572)
 (floats 8 20 25)
 (intervals 56 185 0)
 (buffers 1000 12))



[1] https://github.com/haskell/haskell-mode/issues/1691