bug#48670: 27.2; cc-mode and constexpr if

bug#48670: 27.2; cc-mode and constexpr if

[John Eivind Helset]

cc-mode has problems with constexpr-if-statements whose true-statement
is not a compound statement.

From 'emacs -Q' open a buffer in cc-mode and compare the formatting of:

void f(){
  if(a == 4)
    return 4;
  else if(a == 3)
    return 3;
  else
    return 2;
}

void f_constexpr(){
  if constexpr(a == 4)
                return 4;
  else if constexpr(a == 3)
         return 3;
  else
    return 2;
}


In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo version 1.17.4)
 of 2021-03-26 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Arch Linux

Recent messages:
Loading /home/jeh/.config/emacs/custom.el (source)...done
Turning on magit-auto-revert-mode...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Package cl is deprecated

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-- 
John Eivind Helset

bug#48670: 27.2; cc-mode and constexpr if

[Alan Mackenzie]

Hello, John.

On Wed, May 26, 2021 at 05:08:03 +0200, John Eivind Helset wrote:

> cc-mode has problems with constexpr-if-statements whose true-statement
> is not a compound statement.

> >From 'emacs -Q' open a buffer in cc-mode and compare the formatting of:

> void f(){
>   if(a == 4)
>     return 4;
>   else if(a == 3)
>     return 3;
>   else
>     return 2;
> }

> void f_constexpr(){
>   if constexpr(a == 4)
>                 return 4;
>   else if constexpr(a == 3)
>          return 3;
>   else
>     return 2;
> }

Thanks for taking the trouble to report this bug, and thanks even more
for creating a minimal, easy to work with, test case.

The bug has in fact already been fixed in the development version of
Emacs, and should be getting released with Emacs 28.1, whenever that
will happen.

In the mean time, please apply the following patch to
..../emacs/lisp/progmodes/cc-engine.el, and then byte compile that file.
Should you want any help with the patching or byte compilation, feel
free to send me private email.



--- cc-engine.el.orig	2021-05-30 16:08:48.855885223 +0000
+++ cc-engine.el	2021-05-30 16:16:47.785903083 +0000
@@ -1181,6 +1181,15 @@
 			  ;; suitable error.
 			  (setq pre-stmt-found t)
 			  (throw 'loop nil))
+			;; Handle C++'s `constexpr', etc.
+			(if (save-excursion
+			      (and (looking-at c-block-stmt-hangon-key)
+				   (progn
+				     (c-backward-syntactic-ws lim)
+				     (c-safe (c-backward-sexp) t))
+				   (looking-at c-block-stmt-2-key)
+				   (setq pos (point))))
+			    (goto-char pos))
 			(cond
 			 ;; Have we moved into a macro?
 			 ((and (not macro-start)


> In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo version 1.17.4)
>  of 2021-03-26 built on juergen
> Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
> System Description: Arch Linux

[ .... ]

> -- 
> John Eivind Helset

-- 
Alan Mackenzie (Nuremberg, Germany).

bug#48670: 27.2; cc-mode and constexpr if

[Alan Mackenzie]

tags 48670 fixed 28.1
close 48670
quit

On Wed, May 26, 2021 at 05:08:03 +0200, John Eivind Helset wrote:

> cc-mode has problems with constexpr-if-statements whose true-statement
> is not a compound statement.

[ .... ]

-- 
Alan Mackenzie (Nuremberg, Germany).