bug#31915: 26.0.50; highlight-regexp does not highlight regexp

[Markus Triska <triska@metalevel.at>]

Please invoke Emacs via:

    $ emacs -Q --eval "(let ((x \"x\"))                 \
                         (find-file x)                  \
                         (insert x)                     \
                         (highlight-regexp x 'hi-green) \
                         (when font-lock-mode (insert x)))"

The docstring of highlight-regexp states:

    Use Font lock mode, if enabled, to highlight REGEXP.  Otherwise,
    use overlays for highlighting.  If overlays are used, the
    highlighting will not update as you type.

Since font-lock-mode is clearly enabled when the second "x" is inserted,
I expect it to be highlighted. However, only the first "x" is highlighted.

Could you please adapt the docstring or, alternatively, highlight-regexp
to match what is documented with what is observed? Personally, I would
prefer that also the second "x" be highlighted in this case. Thank you!


In GNU Emacs 26.0.50.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2016-10-21 built on my computer
Repository revision: 0929e061fb544c0f4c10ff0412a41cf7e8148270
System Description:	Debian GNU/Linux 9.3 (stretch)


Configured using:
 'configure --with-gif=no --with-tiff=no --with-gnutls=no'

Configured features:
XPM JPEG PNG SOUND NOTIFY FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS LUCID
X11