base: fix ABSL_ATTRIBUTE_WARN_UNUSED with gcc <= 12

[jolivain]

Gcc <= 12 does not support mixing standard C++ attributes with GNU attributes. See [1].

This can lead to build failures such as [2] [3] and [4]. In those situations, the compilation fails with error such as:

/usr/include/absl/base/attributes.h:1076:36: error: expected identifier before '[' token

Gcc maintainers mentioned in [1] comment 9 that this bugfix will not be backported in Gcc 12. Gcc 12 is still used in LTS distributions. For example, it is included in Debian 12 (Bookworm), which is still supported until 2028. See [5].

This commit adds a workaround for gcc <= 12 which uses attribute in that case, which fixes the compilation failure.

[1] https://gcc.gnu.org/PR69585
[2] protocolbuffers/protobuf#26383
[3] https://autobuild.buildroot.org/results/33f6cfd37cb48c15a53b3e7123d5ce8388a0f2ab/build-end.log
[4] https://gitlab.com/buildroot.org/buildroot/-/jobs/13904066346
[5] https://www.debian.org/releases/bookworm/

[derekmauro]

The proposed patch happens to solve this particular problem, but it is not a general solution.

Consider what would happen if we had flipped the definition syntax so that ABSL_ATTRIBUTE_WARN_UNUSED uses GNU syntax and PROTOBUF_EXPORT uses C++ attributes:

#define ABSL_ATTRIBUTE_WARN_UNUSED __attribute__((warn_unused))
#define PROTOBUF_EXPORT [[gnu::visibility("default")]]

We are left with the same problem, and by making this change, we might actually introduce the same problem in a new location.

It is almost as if we need an all-or-none approach to how we declare these attributes.

[jolivain]

I see.

Since gcc-12 is phasing out and this issue seems a specific case on class declaration only, we could simply define an empty macro, as if the compile was not supporting it.

#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 13
#define ABSL_ATTRIBUTE_WARN_UNUSED
#else
#define ABSL_ATTRIBUTE_WARN_UNUSED [[gnu::warn_unused]]
#endif
#else
#define ABSL_ATTRIBUTE_WARN_UNUSED
#endif

What do you think?