bug#23078: 24.5; Query-replace case insensitive - do not prompt for do-nothing replacements

[Ed Avis]

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Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from `emacs -Q':

M-x query-replace is case insensitive by default.  You may sometimes
want to replace a string with another differing only in case, for
example changing 'aaa' to 'Aaa'.  But query-replace prompts for all
cases where a case-insensitive match for 'aaa' is found, even those that
already equal the destination string.

Here are the keypresses starting from 'emacs -Q' to reproduce:

a a a RET A a a RET ESC O A ESC O A ESC % a a a RET
A a a RET y

I suggest that if the search and replacement strings differ only in
case, then Emacs should prompt only where there is a real choice to be
made.  When the match of 'Aaa' is highlighted it makes no difference
whether you answer y or n; so Emacs should not waste the user's time
asking.

This does mean that the matches highlighted by query-replace differ from
those you would find with a normal case insensitive search, in these
specific circumstances where the search and replacement strings differ
only in case.  I think that is a fair tradeoff.


If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/share/emacs/24.5/etc/DEBUG.


In GNU Emacs 24.5.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.18.7)
 of 2016-02-03 on buildhw-05.phx2.fedoraproject.org
Configured using:
 `configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
 --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr
 --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
 --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
 --libexecdir=/usr/libexec --localstatedir=/var
 --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-dbus --with-gif --with-jpeg --with-png
 --with-rsvg --with-tiff --with-xft --with-xpm --with-x-toolkit=gtk3
 --with-gpm=no build_alias=x86_64-redhat-linux-gnu
 host_alias=x86_64-redhat-linux-gnu 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g
 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2
 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4
 -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
 -m64 -mtune=generic' LDFLAGS=-Wl,-z,relro'

Important settings:
  value of $LC_COLLATE: C
  value of $LC_CTYPE: en_GB.UTF-8
  value of $LC_MESSAGES: en_GB.UTF-8
  value of $LC_MONETARY: en_GB.UTF-8
  value of $LC_NUMERIC: en_GB.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-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
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

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Memory information:
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 (symbols 48 17645 0)
 (miscs 40 35 138)
 (strings 32 9016 4221)
 (string-bytes 1 247931)
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