Re: [PATCH v2] compat/posix.h: enable UNUSED warning messages for Clang

[Patrick Steinhardt <ps@pks.im>]

On Fri, Jun 05, 2026 at 11:46:47AM +0200, Dominik Loidolt wrote:
> Use a dedicated Clang version check for the UNUSED macro.
> 
> Commit 7c07f36ad2 (git-compat-util.h: GCC deprecated message arg only in
> GCC 4.5+, 2022-10-05) restricted use of the deprecated attribute's
> message argument in the UNUSED macro to GCC 4.5 or newer.

Ah. I was briefly wondering about this because the UNUSED macro already
works. But the important part here is that it's really only about better
diagnostics via the attribute message.

> Clang identifies itself as GNUC 4.2.1 for compatibility, so
> GIT_GNUC_PREREQ(4, 5) does not detect whether Clang supports the
> deprecated("...") form. Add GIT_CLANG_PREREQ() macro and use it to
> enable the UNUSED warning message for Clang 2.9 and newer.

There's a second user of `GIT_GNUC_PREREQ` in "git-compat-util.h", but
that user checks for GCC 3.1. And as Clang identifies as a newer version
we don't have to adapt any other callsites.

> diff --git a/compat/posix.h b/compat/posix.h
> index faaae1b655..88ad29d74b 100644
> --- a/compat/posix.h
> +++ b/compat/posix.h
> @@ -22,6 +22,17 @@
>   #define GIT_GNUC_PREREQ(maj, min) 0
>  #endif
> 
> +/*
> + * Similar for Clang
> + */

Micronit, not worth rerolling over: this could have easily been a single
line: `/* Similar for Clang. */`

> +#if defined(__clang__) && defined(__clang_minor__) && defined(__clang_major__)
> +# define GIT_CLANG_PREREQ(maj, min) \
> +	((__clang_major__ > (maj)) || \
> +	 (__clang_major__ == (maj) && (__clang_minor__ >= (min))))
> +#else
> +# define GIT_CLANG_PREREQ(maj, min) 0
> +#endif
> +
>  /*
>   * UNUSED marks a function parameter that is always unused.  It also
>   * can be used to annotate a function, a variable, or a type that is
> @@ -35,7 +46,7 @@
>   * When a parameter may be used or unused, depending on conditional
>   * compilation, consider using MAYBE_UNUSED instead.
>   */
> -#if GIT_GNUC_PREREQ(4, 5)
> +#if GIT_GNUC_PREREQ(4, 5) || GIT_CLANG_PREREQ(2, 9)
>  #define UNUSED __attribute__((unused)) \
>  	__attribute__((deprecated ("parameter declared as UNUSED")))
>  #elif defined(__GNUC__)

Makes sense, thanks!

Patrick