Subject: 24.5; (newline 0) fails when point not after newline
From: Dan Torop <[email-masked]>
To: [email-masked]
Date: Fri, 29 Jan 2016 13:46:47 -0500
List-Id: <debbugs-submit.debbugs.gnu.org>

The function "newline" with an argument of 0 will fail unless called
when the character before the point is a newline. To test, start a new
Emacs session, type space, then evaluate "(newline 0)". It should
fail with a backtrace:

Debugger entered--Lisp error: (cl-assertion-failed (eq 10
(char-before)))
  signal(cl-assertion-failed ((eq 10 (char-before))))
  #[0
  "`Sf\304=\204^N^@\305\306\307C\"\210^K\203^[^@\310`\311\300!Z`\"\210\212\302b\210\312
  \210\313\314!\2034^@\315 \316V\2034^@`\317
  |\210)\301\206=^@\320\321\322\"\207" [0 nil 193 use-hard-newlines 10
  signal cl-assertion-failed (eq 10 (char-before))
  set-hard-newline-properties prefix-numeric-value beginning-of-line
  looking-at "[     ]$" current-left-margin 0 line-end-position
  move-to-left-margin nil t] 4 "\n\n(fn)"]()
  self-insert-command(0)
  newline(0)
  eval((newline 0) nil)
  eval-expression((newline 0) nil)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

If one instead types a newline then (newline 0), the call will be a
NOP, as expected.

This appears to be the result of commit
62ee8b1a59d5343197cb59d9f0a52e07a23af1d8, intended to catch
bug # 18913, in particular the (cl-assert (eq ?\n (char-before)))
in post-self-insert-hook.

While it may seem absurd to execute (newline 0), this can have a
purpose, e.g. when programmatically inserting an arbitrary number of
newlines at the start of an empty buffer.

In GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.18.6)
 of 2016-01-22 on binet, modified by Debian
Windowing system distributor `The X.Org Foundation', version
11.0.11703000
System Description:     Debian GNU/Linux testing (stretch)

Configured using:
 `configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
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 --build x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes
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Major mode: Lisp Interaction

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Memory information:
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