Setup Clang on Travis CI for C++17

Question

I have a small C++17 project for which I want to setup Travis CI. Since it is C++17 it requires modern compilers; I settled for gcc-7 and clang-6. While the gcc build compiles and links just fine, I can't, for the life of me, figure out how to setup clang properly. It looks like it always uses the standard library implementation of the very outdated pre-installed gcc instead of its own.

The travis log shows the following lines on cmake --build . -- VERBOSE=1:

/usr/bin/clang-6.0 -I/home/travis/build/myuser/perlin/include -g -std=gnu++1z -o CMakeFiles/perlin.dir/main.cpp.o -c /home/travis/build/myuser/perlin/main.cpp

In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/algorithm:62: In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/stl_algo.h:66: In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/random:38: In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/cmath:44: /home/travis/build/myuser/perlin/include/math.h:48:28: error: no template named 'is_arithmetic_v' in namespace 'std'; did you mean 'is_arithmetic'?

Again, it builds fine on gcc. I also double checked that all required headers are included.

My .travis.yml looks like this:

language: cpp
dist: trusty

matrix:
  include:
  - os: linux
    addons:
      apt:
        sources:
        - ubuntu-toolchain-r-test
        packages:
        - g++-7
        - libpng-dev
    env:
    - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
    - LINKER_FLAGS=""

  - os: linux
    addons:
      apt:
        sources:
        - ubuntu-toolchain-r-test
        - llvm-toolchain-trusty-6.0
        - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-6.0 main'
          key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
        packages:
        - clang-6.0
        - libstdc++6
        - libpng-dev
    env:
    - MATRIX_EVAL="CC=clang-6.0 && CXX=clang-6.0"
    - LINKER_FLAGS="-stdlib=libc++ -lc++abi"

before_install:
- eval "${MATRIX_EVAL}"

script:
- cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXE_LINKER_FLAGS=${LINKER_FLAGS} .
- cmake --build . -- VERBOSE=1

The CMakeLists.txt:

cmake_minimum_required(VERSION 3.9)
project(perlin)

set(CMAKE_CXX_STANDARD 17)

if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic")
endif()

add_library(noise INTERFACE)
target_sources(noise INTERFACE
        ${PROJECT_SOURCE_DIR}/include/seamless_noise_generator_2d.h
        ${PROJECT_SOURCE_DIR}/include/fractal_noise_generator.h
        ${PROJECT_SOURCE_DIR}/include/perlin_noise_generator.h
        ${PROJECT_SOURCE_DIR}/include/point.h
        ${PROJECT_SOURCE_DIR}/include/vector.h
        ${PROJECT_SOURCE_DIR}/include/math.h)
target_include_directories(noise INTERFACE include)

find_package(PNG)
if (PNG_FOUND)
    add_executable(noise_test main.cpp)
    target_link_libraries(noise_test ${PNG_LIBRARY} noise m)
else()
    message(info "Did not find libpng. Not building test executable.")
endif ()

If anyone knows what I'm doing wrong here, help would be very appreciated.

Answer 1

I could notice these issues:

• CXX should point to the C++ driver, that is: clang++-6.0.
• You should specify -stdlib=libc++ in the CXXFLAGS as well.

The latter seems to be the culprit.

If not specified otherwise, clang++ defaults to stdlib++ for standard C++ headers.
The build output suggests that headers are indeed picked from gcc-4.9, which is most likely the system gcc. That explains the build failure, since gcc-4.9 is quite old and doesn't support C++17 (changelog suggests that it has experimental C++14 support). is_arithmetic_v, which fails the build, is indeed new C++17 addition.