bug#50640: 28.0.50; incorrect highlighting in C++ mode

[Vincent Lefevre <vincent@vinc17.net>]

On 2021-09-25 21:17:06 +0000, Alan Mackenzie wrote:
> Hello, Vincent and Stefan.
> 
> On Sat, Sep 25, 2021 at 11:17:07 -0700, Stefan Kangas wrote:
> > Vincent Lefevre <vincent@vinc17.net> writes:
> 
> > > Consider a test.cc file containing:
> 
> > >   if (xMin - xt < t3Font->glyphX ||
> > >       yMin - yt < t3Font->glyphY ||
> > >       xMax - xt > t3Font->glyphX + t3Font->glyphW ||
> > >       yMax - yt > t3Font->glyphY + t3Font->glyphH) {
> > >   }
> 
> > > (this comes from the xpdf source) and open it with "emacs -Q".
> 
> > > The first two "xt" and "yt" are highlighted in green instead of
> > > remaining in black.
> 
> Yes.  C++ Mode is recognising the < .. < .. > .. > as two nested
> templates.  The green is font-lock-type-face.  :-(
> 
> > > If I remove the last condition as follows and reopen the file:
> 
> > >   if (xMin - xt < t3Font->glyphX ||
> > >       yMin - yt < t3Font->glyphY ||
> > >       xMax - xt > t3Font->glyphX + t3Font->glyphW) {
> > >   }
> 
> > > then only the "yt" is highlighted incorrectly.
> 
> Yes, then only yt opens a "template", there being no closing > to balance
> the xt <.
> 
> There's really not much which can be done about this in CC Mode, sorry.
> CC Mode's analysis of ambiguous C++ constructs is not very deep, so it
> sometimes gets it wrong, as here.

Note that when < .. < .. > .. > comparisons occur, there is
probably || or && between them. Are C++ nested templates common
with || or && inside them? If not, || and && should be forbidden
in nested template matching.

Alternatively, whitespace characters could also give a hint:
AFAIK, for templates, this is usually no whitespace after '<'
and no whitespace before '>'.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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