clang: Gate the use of libc++ to Mac

[theopolis]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

---

[theopolis]

libc++ provides the system C++ implementation for Darwin platforms, based on LLVM. For most Linux systems the C++ implementation is provided by GCC. When folks override their compiler to clang it should default to GCC for C++.

[rwhogg]

Agree with this change (not tested yet).

According to LLVM's documentation, Linux is not fully supported by libc++ yet anyways. The most recent results show 33 failures.

@theopolis, thanks for contributing 😄

[theopolis]

Now realizing I used the GCC library name in the commit and PR title. Please excuse me :P

I wasn't able to run brew test locally due to some odd permission bits I need to investigate-- but I've tried setting clang as my compiler and installing various packages and everything seems good to go!

[rwhogg]

I wasn't able to run brew test locally due to some odd permission bits I need to investigate

Try umask 022 before running the tests. That happens to me a lot too.

[theopolis]

For some context on why I'm playing with clang:

On some distributions there's a GCC 5.1+ installed so my brew-installed libraries have the cxx11 ABI version injected into the relative symbols. If a Formula explicitly sets clang then there might be trouble. It's nice to have the --cc=clang option around for these odd-ball scenarios. 💃

[theopolis]

Try umask 022 before running the tests. That happens to me a lot too.

Epic, thanks @rwhogg!

[sjackman]

Makes sense to me, but I haven't used clang on Linux. Does it work equally well with libstdc++ on Linux? I guess from what you're saying, Bob, it works better with libstdc++ than libc++ on Linux.

@DoomHammer Any comments on this patch, Piotr?

[theopolis]

On Linux:

libstdc++ = 👍
libc++ = 👎

[sjackman]

😄 ✅

[sjackman]

Once Piotr has responded, I'm happy with merging this PR.

[DoomHammer]

Hello, missed that one, sorry.

My experiences (somehow limited, mind you) are that libc++ on Linux rather can't, so this patch should be good for us.

A bit similar was https://github.com/Linuxbrew/homebrew-core/commit/c44e038cd7ff2fdda467a4c8cbaf13834f75247f

[sjackman]

Merged. Thanks for your contribution, Teddy!