Hello,

If you follow the official directions for clang, build failures will ensue because the G++ headers will get used (rather than clang's):

/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/complex:539:18: error: no matching member function for call to 'real'
    { return __z.real(); }

While trying to figure this out, I noticed nice little undocumented gem at the beginning of my build logs:

$ export CXX=clang++
$ export CC=clang
$ clang --version
clang version 5.0.0 (tags/RELEASE_500/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/clang-5.0.0/bin

This is definitely coming from Travis, AKA the relevant tools are already installed. So unless clang@5.0.0 is a problem, virtually all of the Clang specific instructions are incorrect / incomplete / outdated, since Travis appears to have a full llvm installation here.

In other words, by me saying compiler: clang, I already have clang@5.0.0 directly available. Wherever the code is that sets the above CXX and CC, all you need to do is this instead:

export CXX="/usr/local/clang-5.0.0/bin/clang++"
export CC="/usr/local/clang-5.0.0/bin/clang"
export CPATH="/usr/local/clang-5.0.0/include/c++/v1"
export LD_LIBRARY_PATH="/usr/local/clang-5.0.0/lib"

By simply putting this in my before_install, all build failures are gone with clang. Namely, I do have access to libc++ and libc++-abi, and the clang headers are used rather than the system G++ headers.

I do not need to install any extra packages, just need the right dev environment flags setup.

I am happy to help update the clang documentation, but wanted to see what other users thought here. For reference, this is a

• CMake based project, but none of the above are CMake specific
• sudo: false on a dist: trusty worker

I feel that if the above four variables are set correctly, the load on travis for users wanting to test with clang will be significantly reduced -- the image already has everything we need! I tried to find out where this stuff is set and open a PR...but got really turned around...