bug#19250: elisp-mode.el: completion-at-point fails at M-: for some buffers

[Ivan Shmakov <ivan@siamics.net>]

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Package:  emacs
Severity: minor

	With a recently-built Emacs (287740d6; 2014-11-29 13:56:59Z),
	the following sequence results in the backtrace MIMEd
	(redacted.)  I do not observe the issue with 24.3.1, so it
	appears to be a regression.

$ emacs -Q 
C-x b *xxx* RET
(#
M-: xx TAB

	This seems to happen because elisp--local-variables tries to
	gather variable names for completion from the contents of the
	buffer M-: is invoked from, while such buffer does not
	necessarily hold any (valid) Emacs Lisp code.

	My suggestion would be to either make elisp--local-variables
	fail more gracefully (say, using with-demoted-errors), or to
	introduce a (derived-mode-p 'emacs-lisp-mode) guard to the code.

	As a workaround, the following seems to help:

(setq elisp--local-variables-completion-table nil)

-- 
FSF associate member #7257  http://boycottsystemd.org/  … 3013 B6A0 230E 334A

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Debugger entered--Lisp error: (invalid-read-syntax "#")
  read-from-string("(#elisp--witness--lisp)")
  elisp--local-variables()
  #[257 …]("xx")
  #[771 …]("xx" nil nil)
  try-completion("xx" #[771 …] nil)
  #[257 …](#[771 …])
  mapcar(#[257 …] (#[771 …] (closure (t) (&rest args) (apply (quote completion-table-with-predicate) …))))
  #[771 …]("xx" nil nil)
  try-completion("xx" #[771 …] nil)
  completion-basic-try-completion("xx" #[771 …] nil 2)
  #[257 …](basic)
  completion--some(#[257 …] (basic partial-completion emacs22))
  completion--nth-completion(1 "xx" #[771 …] nil 2 (metadata))
  completion-try-completion("xx" #[771 …] nil 2 (metadata))
  completion--do-completion(#<marker at 7 in  *Minibuf-1*> 9)
  completion--in-region-1(#<marker at 7 in  *Minibuf-1*> 9)
  #[1028 …](#<marker at 7 in  *Minibuf-1*> … nil)
  apply(#[1028 …] (#<marker at 7 in  *Minibuf-1*> … nil))
  #[771 …](nil nil (#<marker at 7 in  *Minibuf-1*> … nil))
  completion--in-region(#<marker at 7 in  *Minibuf-1*> 9 #[771 …] nil)
  completion-in-region(#<marker at 7 in  *Minibuf-1*> 9 #[771 …] nil)
  completion-at-point()
  funcall-interactively(completion-at-point)
  call-interactively(completion-at-point nil nil)
  command-execute(completion-at-point)
  read-from-minibuffer("Eval: " nil (keymap …) t read-expression-history)
  read--expression("Eval: ")
  byte-code(… [current-prefix-arg read--expression "Eval: "] 2)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)