bug#20307: 25.0.50; (regexp-opt nil ...) returns ""

[David Kastrup]

Both

M-: (regexp-opt nil) RET

and

M-: (regexp-opt nil t) RET

return "".  However, they should return a regexp matching _nothing_
rather than everything, and the second invocation should also count as
one \(\) pairing.

So something like "[b-a]" and "\([b-a]\)" or
"[^[:unibyte:][:multibyte:]]" or something similarly contorted.



In GNU Emacs 25.0.50.1 (i686-pc-linux-gnu, GTK+ Version 3.12.2)
 of 2015-03-04 on lola
Repository revision: ca2b0e220ee6b2cab538e84703559696ce477e71
Windowing system distributor `The X.Org Foundation', version 11.0.11600000
System Description:     Ubuntu 14.10

Configured using:
 `configure --without-toolkit-scroll-bars'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB

Important settings:
  value of $LC_MONETARY: en_US.UTF-8
  value of $LC_NUMERIC: en_US.UTF-8
  value of $LC_TIME: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Group

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Recent messages:
nnml: Reading incoming mail (no new mail)...done
Reading active file from private via nnml...done
Reading active file from archive via nnfolder...done
Reading active file via nndraft...done
Reading active file from /home/dak3/Mail/family via nndir...done
Checking new news...done
Opening nnml server on private...done
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"" [2 times]
Auto-saving...
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Load-path shadows:
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Memory information:
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-- 
David Kastrup