Subject: 27.0.50; `frame-title-format' doesn't save match data

From: Philipp <[email-masked]>
Date: Thu, 24 May 2018 23:55:57 +0200

emacs -Q -eval "(setq frame-title-format '(\"\" (:eval (string-match \".\" \"a\"))))"

Then go to the first character in the scratch buffer (M-<), and run

M-% a RET b RET

The first time you attempt to replace something, Emacs will signal an
error:

perform-replace: Args out of range: #<buffer *scratch*>, 0, 1

Backtrace:

Debugger entered--Lisp error: (args-out-of-range #<buffer *scratch*> 0 1)
  buffer-substring-no-properties(0 1)
  perform-replace("a" "b" t nil nil nil nil nil nil nil nil)
  query-replace("a" "b" nil nil nil nil nil)
  funcall-interactively(query-replace "a" "b" nil nil nil nil nil)
  call-interactively(query-replace nil nil)
  command-execute(query-replace)

Apparently the `string-match' in `frame-title-format' has overwritten
the match data.  This worked in Emacs 24.5.  It breaks the informal
contract for the match data, namely that any code is free to change it,
and code that relies on the match data staying intact needs to protect
it.

In GNU Emacs 27.0.50 (build 69, x86_64-apple-darwin17.4.0, NS appkit-1561.20 Version 10.13.3 (Build 17D102))
 of 2018-05-22 built on p
Repository revision: 02f303d75f876517b7802f787413cbb418203315
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.3

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --with-modules --without-pop --with-mailutils
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 --enable-check-lisp-object-type 'CFLAGS=-ggdb3 -O0''

Configured features:
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JSON

Important settings:
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Major mode: Lisp Interaction

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Load-path shadows:
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Memory information:
((conses 16 204577 6502)
 (symbols 48 20009 1)
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