[sr #111072] "[[]] attributes are a C23 extension warning" with clang

URL:
  <https://savannah.gnu.org/support/?111072>

                 Summary: "[[]] attributes are a C23 extension warning" with
clang
                   Group: Autoconf
               Submitter: None
               Submitted: Fri 07 Jun 2024 11:49:01 PM UTC
                Priority: 5 - Unprioritized
                Severity: 3 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: [email protected]
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: *BSD


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Follow-up Comments:


-------------------------------------------------------
Date: Fri 07 Jun 2024 11:49:01 PM UTC By: Anonymous
Hello —

Recently, I started getting warnings like:


./slist.h:170:1: warning: [[]] attributes are a C23 extension
[-Wc23-extensions]
  170 | NODISCARD
      | ^
../lib/attribute.h:143:19: note: expanded from macro 'NODISCARD'
  143 | #define NODISCARD _GL_ATTRIBUTE_NODISCARD
      |                   ^
./config.h:1392:37: note: expanded from macro '_GL_ATTRIBUTE_NODISCARD'
 1392 | #    define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
      |                                     ^


Specifically, the value defined for NODISCARD defined in config.h as generated
by configure as generated by autoconf causes the warning.

The relevant section from config.h is:


#ifndef _GL_ATTRIBUTE_NODISCARD
# ifndef _GL_BRACKET_BEFORE_ATTRIBUTE
#  if defined __clang__ && defined __cplusplus
  /* With clang up to 15.0.6 (at least), in C++ mode, [[__nodiscard__]]
produces
     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
#  elif _GL_HAVE___HAS_C_ATTRIBUTE
#   if __has_c_attribute (__nodiscard__)
#    define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
#   endif
#  endif
# endif
# if !defined _GL_ATTRIBUTE_NODISCARD && _GL_HAS_ATTRIBUTE
(warn_unused_result)
#  define _GL_ATTRIBUTE_NODISCARD __attribute__ ((__warn_unused_result__))
# endif
# ifndef _GL_ATTRIBUTE_NODISCARD
#  define _GL_ATTRIBUTE_NODISCARD
# endif
#endif


I think the problem is this line:


#  if defined __clang__ && defined __cplusplus


My project is a C project, not C++, so __cplusplus is not defined so that #if
fails which means this part happens:


#  elif _GL_HAVE___HAS_C_ATTRIBUTE
#   if __has_c_attribute (__nodiscard__)
#    define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
#   endif


In this block, no check for __clang_major__ is done, so it always defines
_GL_ATTRIBUTE_NODISCARD to [[__nodiscard__]] and clang warns.

I don't think the && __cplusplus should be there at all. Attributes using [[
are supported in C23 or later or C++11 or later. Why not use
__STDC_VERSION__?

FYI:


$ clang --version
FreeBSD clang version 18.1.5 (https://github.com/llvm/llvm-project.git
llvmorg-18.1.5-0-g617a15a9eac9)
Target: x86_64-unknown-freebsd14.1
Thread model: posix
InstalledDir: /usr/bin

$ autoconf --version
autoconf (GNU Autoconf) 2.72
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+/Autoconf: GNU GPL version 3 or later
<https://gnu.org/licenses/gpl.html>,
<https://gnu.org/licenses/exceptions.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by David J. MacKenzie and Akim Demaille.


— Paul








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