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Re: Avoid clang warnings regarding [[__nodiscard__]]

From: Bruno Haible
Subject: Re: Avoid clang warnings regarding [[__nodiscard__]]
Date: Sat, 28 Jan 2023 20:22:33 +0100

Paul Eggert wrote:
> > +  /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]] 
> > produces
> > +     a warning.  */
> > +#  if __clang_major__ >= 1000
> I found this a little hard to follow

Indeed, I wondered whether I should add a comment. Guess I should have :)

> How about something like the attached, to simplify this for Clang non-experts?

I wanted the code to be prepared for the day when clang++ actually adds
support for this syntax [[__nodiscard__]]. The code you are proposing
optimizes the current needs, but not the future needs.

I'm adding this comment instead. I hope that's clear enough.

2023-01-28  Bruno Haible  <bruno@clisp.org>

        Clarify _GL_ATTRIBUTE_NODISCARD code.
        Reported by Paul Eggert.
        * m4/gnulib-common.m4 (gl_COMMON_BODY): Add comment regarding
        _GL_ATTRIBUTE_NODISCARD definition.

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index d632819864..c0181abdc5 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -381,7 +381,10 @@ AC_DEFUN([gl_COMMON_BODY], [
 # if defined __clang__ && defined __cplusplus
   /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]] 
-     a warning.  */
+     a warning.
+     The 1000 below means a yet unknown threshold.  When clang++ version X
+     starts supporting [[__nodiscard__]] without warning about it, you can
+     replace the 1000 with X.  */
 #  if __clang_major__ >= 1000
 #   define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
 #  endif

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