c++-compile action should use clang++ instead of clang.

[sfc-gh-mhazy]

Hi,
Thank you for the work on the project so far. It makes cc toolchain configuration much easier.

I've noticed that some builds fail with linker errors, for instance, compiling cc_test with com_google_google_test fails with

ld.lld: error: undefined reference due to --no-allow-shlib-undefined: nextafter

This is related to google/googletest#3878

I think the problem is as described in the issue above - compiling c++ should use clang++.

This might be, however, problematic as it goes down to bazel implementation of default actions that is using gcc instead of g++.

It is possible to override the action configs in cc_common.create_cc_toolchain_config_info, but unix_cc_toolchain_config does not expose it.

I did some rough experiment and replaced clang with clang++ in https://github.com/bazel-contrib/toolchains_llvm/blob/master/toolchain/cc_wrapper.sh.tpl and that did the trick. This is not a generic solution though, as it would fail c compilation.

On the bright side, the workaround is to add -lm to cc_test linkopts or link_flags of cc toolchain configuration.

Perhaps we should start with exposing action_configs in unix_cc_toolchain_config and then override the cpp actions in toolchains_llvm. Alternative would be to inline unix_cc_toolchain_config into toolchains_llvm.

Looking forward to your feedback.

[fmeum]

With all these small inconsistencies between Bazel and the rest of the world, I wonder whether we shouldn't just make Bazel's default toolchain use clang for C and clang++ for C++. Bazel 8 could be a good time for that as it may not be doable in a fully backwards compatible way.

[jsharpe]

+1! This would make rules_foreign_cc a bit easier to work with too

[sfc-gh-mhazy]

Thank you for responses. Long term definitely changing the defaults is the best choice.

But would be good to have some improvement short / middle term.

1. I don't know how feasible it is to add -lm by default for linux compilation.
2. Maybe lets start a "Known issues" section in readme to increase visibility? It takes some time to debug such issues so that might help future strugglers.

Side note: The original post was about linux, on macos I had to add "-lstdc++" to link flags.