llvm: enable clang and rtti by default, simplifying deps

[apjanke]

This allows all core dependencies on llvm to be on the default llvm, instead of using 'with-rtti' or 'with-clang', which cause a 1-hour+ build from source and timeouts on the CI. Follows discussion in #47269.

Affected formulae:

[/usr/local/Library/Formula on ⇄ master]
$ grep llvm * | grep depends_on | grep 'clang\|rtti'
creduce.rb:  depends_on "llvm" => "with-clang"
doxygen.rb:  depends_on "llvm" => "with-clang" if build.with? "libclang"
emacs-clang-complete-async.rb:  depends_on "llvm" => "with-clang"
ponyc.rb:  depends_on "llvm" => "with-rtti"
rtags.rb:  depends_on "llvm" => "with-clang"

I'm omitting emacs-clang-complete-async from this PR because it lacks a test do and will cause the audit checks to fail, and I'd like this to go green. I'll fix it up separately if and when this is merged.

I don't think this requires a revision bump to any of the dependent formulae, because they use llvm as an external command and don't require a rebuild. (Except maybe doxygen?)

[apjanke]

9 tests failed? <iostream> not found? What?!?

        Testing llvm
==> Using the sandbox
/usr/bin/sandbox-exec -f /tmp/homebrew20160125-62368-1ifikcv.sb /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby -W0 -I /usr/local/Library/Homebrew -- /usr/local/Library/Homebrew/test.rb /usr/local/Library/Formula/llvm.rb --verbose
==> /usr/local/Cellar/llvm/3.6.2_1/bin/llvm-config --prefix
==> /usr/local/Cellar/llvm/3.6.2_1/bin/clang++ test.cpp -o test
test.cpp:1:10: fatal error: 'iostream' file not found
#include <iostream>
         ^
1 error generated.
Error: llvm: failed
Failed executing: /usr/local/Cellar/llvm/3.6.2_1/bin/clang++ test.cpp -o test

Is this because --with-libcxx is not enabled by default, and clang++ wants to draw <iostream> from its own C++ library? i.e. this C++ "Hello world" test is conditioned on build.with? "clang", but it really wants build.with? "clang" && build.with? "libcxx"?

  test do
    ...

    if build.with? "clang"
      (testpath/"test.cpp").write <<-EOS.undent
     ...
      EOS
      system "#{bin}/clang++", "test.cpp", "-o", "test"
      system "./test"
    end

If so, thoughts on making --with-libcxx the default? Given how much C++ there is out there, seems like one would want it by default if you're going to build clang in the first place. The clang++ built with --with-clang --without-libcxx seems effectively broken. Or is there a way to point it at a different C++ library (without requiring the user to fiddle with it at invocation time)?

[MikeMcQuaid]

If so, thoughts on making --with-libcxx the default? Given how much C++ there is out there, seems like one would want it by default if you're going to build clang in the first place. The clang++ built with --with-clang --without-libcxx seems effectively broken.

Seems fine 👍

[apjanke]

Amended to make --with-libcxx the default, so the test can run overnight. Still open to feedback from other maintainers on this.

[apjanke]

Failures again. The CI is a timeout, but Jenkins has real failures.

ponyc failed to build. From the el cap failure:

Undefined symbols for architecture x86_64:
  "std::terminate()", referenced from:
      ___clang_call_terminate in libponyc.a(genopt.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [build/release/ponyc] Error 1

The rtags failure is just a download failure, looks unrelated.

failed: brew fetch --retry rtags --build-bottle --force

Stacktrace

        ==> Cloning https://github.com/Andersbakken/rtags.git
Cloning into '/Library/Caches/Homebrew/rtags--git'...
warning: Could not find remote branch v2.0 to clone.
fatal: Remote branch v2.0 not found in upstream origin
==> Retrying download

I'll look in to them.

[apjanke]

The rtags failure looks like it's just that upstream removed the tag for that release, and no longer has any named releases. See #42045 and upstream Andersbakken/rtags#582. Maybe a boneyard candidate if they don't do releases.

[apjanke]

Could be the issue is that the ponyc formula, or brew's build process, isn't detecting all the C++ switches it needs to do with llvm.

From the llvm formula's caveats:

==> Caveats
LLVM executables are installed in /usr/local/opt/llvm/bin.
Extra tools are installed in /usr/local/opt/llvm/share/llvm.
To use the bundled libc++ please add the following LDFLAGS:
  LDFLAGS="-L/usr/local/opt/llvm/lib -lc++abi"

Looks like this is a known issue: #47149, #47858. Going to read through those and see if I can understand it better.

Related, putting in a PR to enable verbose builds for ponyc so we can see the compiler commands, #48578.

[apjanke]

Closing this out as a rejection/withdrawal. Seems to me that there are some nontrivial issues here with the C++ standards support and C++ library builds here, which need someone who's really familiar with them to sort out.

And that's not me: even if I could get everything to build here, working on this PR has given me enough familiarity with LLVM and C++ libs to know that I'm not qualified to give a thumbs-up on a change like this.

[apjanke]

Looks like this might be the default in formula for newer versions of llvm, based on the homebrew-versions formulae for 3.7 (Homebrew/homebrew-versions#955) and 3.8 (Homebrew/homebrew-versions#1183).

The core PR for 3.8 leaves them as options though: #47824