bug#47551: 28.0.50; hang in c++ font lock

[Eli Zaretskii]

> Date: Thu, 1 Apr 2021 10:11:03 -0700
> From:  Matt Liberty via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> If I open the file below and goto-line to line 54 ("X X()") then emacs hangs.
> I sent a SIGUSR2 and I see:
> Debugger entered--entering a function:
> * #f(compiled-function () #<bytecode 0x1c83247378ca389b>)()
>   font-lock-default-fontify-region(1217 1227 nil)
>   c-font-lock-fontify-region(1217 1227 nil)
>   font-lock-fontify-region(1217 1227)
>   #f(compiled-function (fun) #<bytecode 
> 0x1f37dc2e6f385ebe>)(font-lock-fontify-region)
>   run-hook-wrapped(#f(compiled-function (fun) #<bytecode 0x1f37dc2e6f385ebe>) 
> font-lock-fontify-region)
>   jit-lock--run-functions(1217 1227)
>   jit-lock-fontify-now(1217 1717)
>   jit-lock-function(1217)
>   redisplay_internal\ \(C\ function\)()

Your build is from a relatively old repository, so please update and
try again.  I think this could have been fixed recently.