bug#70435: 30.0.50; cc-mode: <> are sometimes not reconized as parentheses

[Herman]

Hello Alan,

Alan Mackenzie <acm@muc.de> writes:

> You've been a little less than fully explicit, but I think 
> you're
> executing these commands in the *scratch* buffer.  The first two 
> lines,
> which are commented out in emacs-lisp-mode, are no longer 
> commented out
> in C++ Mode.  There is a whole line of garbage after the last 
> end of
> statement marker, the (double) semicolon on line 2.
>
> On using ig<TAB> to insert the snippet, it is hardly surprising 
> that CC
> Mode's syntactic analysis gets confused.  If you first comment 
> out those
> first two lines (put the region around them and do C-c C-c), 
> then the
> inserted snippet appears to get the correct syntax on its 
> template
> markers.
>
> I don't think there's a bug here.  If you could show ig<TAB> 
> producing
> the effect when typed inside a syntactically correct context, 
> things
> might be different.  Can you reproduce the effect in correct C++ 
> code?

You're right, it seems that the example I provided wasn't the best 
(this issue happens with me in real code, I tried to create a 
minimal reproducible example).

If you delete the garbage from the scratch buffer, the bug doesn't 
reproduce indeed.  But, if you run (setq 
font-lock-maximum-decoration 2) before switching to c++-mode, the 
issue reproduces with an empty scratch buffer.  I use this setting 
because font-lock runs much faster this way, and I rely on the LSP 
server to do the "full" highlighting.

Sorry about the bad example, here are the fixed repro steps:

Repro:
- put the yasnippet file (included below) into
<emacs-config-dir>/snippets/c++-mode/something
- install yasnippet
- start emacs, scratch buffer appears
- delete the contents of the scratch buffer
- M-: (setq font-lock-maximum-decoration 2)
- M-x c++-mode
- M-x yas-minor-mode
- load snippets with "M-x yas-reload-all"
- write "ig", then press TAB to "yas-expand" the snippet
- move the cursor on the opening "<", and execute "M-x 
 describe-char"
- notice that it will say "syntax: . which means: punctuation"
- if you edit the buffer (like add a space somewhere), and execute
describe-char again, Emacs will say "syntax: > which means: open,
matches >", so the syntax class becomes correct.

Geza