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

[Paul Eggert]

(Moving this from <https://savannah.gnu.org/support/?111072> to 
<mailto:bug-gnulib@gnu.org> because it's Gnulib, not Autoconf.]

> 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__]]
>        |                                     ^

It looks like -Wc23-extensions is the problem. If you stop using it (or 
if you also use -std=gnu23), you shouldn't get those warnings. At least, 
that's the behavior I see with clang 18.1.6 (a bit later than your 
clang) on Fedora.

-Wc23-extensions is more trouble than it's worth when programs (as is 
commonly the case) use C23 extensions only when available.


> 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__?

In practice __STDC_VERSION__ has been less reliable than the 
more-detailed checks.


> 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
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