Error using gcc 8.1.0 on Ubuntu 14.04

[bravecorvus]

• What is the issue you have?
  Using the default Ubuntu build environment for cpp on Travis-CI, I am getting a #unsupported GCC version despite installing gcc-8 from the ubuntu-toolchain-r-test apt repository and setting the cmake C++ compiler to use g++ 8.1.0 (by setting it in the command line options via cmake -D -CMAKE_CXX_COMPILER="/usr/bin/cpp-8").

The result of cpp-8 --version is:

cpp-8 (Ubuntu 8.1.0-5ubuntu1~14.04) 8.1.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Furthermore, swapping "/usr/bin/cpp-8" with "/usr/bin/gcc-8" gives me the same error.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

1. Create a new Github repository and hook it up to Travis CI.
2. Define the environment variables VIAPIKEY and VIAPITOKEN in the repository's environment variables by signing up for a free account on my company's website voiceit.io API version 2.
3. In your repository, do the following:

wget https://raw.githubusercontent.com/voiceittech/voiceit2-cpp/master/Test.cpp
wget https://raw.githubusercontent.com/voiceittech/voiceit2-cpp/master/VoiceIt2.hpp
wget https://raw.githubusercontent.com/voiceittech/voiceit2-cpp/master/json.hpp

4. Add a .travis.yml as follows:

language: cpp
compiler: gcc
addons:
  apt:
    sources:
      - ubuntu-toolchain-r-test
    packages:
      - gcc-8
      - libssl-dev
      - cmake
before_script:
  - git submodule add https://github.com/whoshuu/cpr.git
  - git submodule update --init --recursive
  - echo "set (CMAKE_CXX_STANDARD 11)" > CMakeLists.txt
  - echo "project (Tests)" >> CMakeLists.txt
  - echo "cmake_minimum_required(VERSION 2.8.12)" >> CMakeLists.txt
  - echo "add_subdirectory(cpr)" >> CMakeLists.txt
  - echo "include_directories(\${CPR_INCLUDE_DIRS})" >> CMakeLists.txt
  - echo "add_executable(Tests Test.cpp)" >> CMakeLists.txt
  - echo "target_link_libraries(Tests \${CPR_LIBRARIES})" >> CMakeLists.txt
  - gcc-8 --version
  - cpp-8 --version
  - cmake -D -CMAKE_CXX_COMPILER="/usr/bin/gcc-8"
  - make
script: "./Tests"

5. git add . && git commit -m "message" && git push origin master
   Note: The reason you aren't just going to fork the project is because Travis is kind of a pain to set up on forked repos.

• What is the expected behavior?
  No compiler error since we are using gcc 8.1.0

• And what is the actual behavior instead?
  error: #error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"

• Which compiler and operating system are you using? Is it a supported compiler?

cpp-8 (Ubuntu 8.1.0-5ubuntu1~14.04) 8.1.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

on Ubuntu 14.04 LTS.

Also tried setting it to use gcc-8 instead of cpp-8 which is version:

gcc-8 (Ubuntu 8.1.0-5ubuntu1~14.04) 8.1.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

which should be in the range of gcc 4.9 ~ gcc 8.2

• Did you use a released version of the library or the version from the develop branch?

No I specifically used version 3.1.2 since the new 3.2.0 version increased the number of dependencies for my project (json_fwd.hpp).

• If you experience a compilation error: can you compile and run the unit tests?

check out the logs for my Travis CI build. Specifically, search the page for "gcc-8 --version" which will show the gcc version. The error might not actually show up unless you check out the raw logs.

[theodelrieu]

FYI json_fwd.hpp was introduced in 3.1.0, there is no plan to drop the single header support, you can upgrade to newer versions without adding any other files besides json.hpp

[bravecorvus]

I am using json.hpp as a single header file (which works well minimizing dependencies for the project I am working on). Using the latest development json.hpp version 3.2.0 from here causes the following error:

/home/travis/build/voiceittech/voiceit2-cpp/json.hpp:49:33: fatal error: nlohmann/json_fwd.hpp: No such file or directory
 #include <nlohmann/json_fwd.hpp>

Logs for the above problem is here.

I would much rather using the single file header format if possible (rather than adding more stuff to the cmake build chain).

Update:
I also looked at nlohmann/json's Travis build script for gcc-8, and modified my script to install g++-8 rather than gcc-8 to which produces the same error.

The logs to using g++-8 on nlohmann/json version 3.1.2 is here.

[bravecorvus]

Ahh, but using the json.hpp version 3.2.0 from the Release section (rather than the development branch) doesn't give me the json_fwd.hpp error. However, it still is giving me the #error "unsupported GCC version error.

logs

[theodelrieu]

There might have been a versionning scheme change from GCC which makes the compiler detection fail... I cannot check that at the moment unfortunately.

[theodelrieu]

Could you compile a test program printing the following macro values?

__GNUC__, __GNUC_MINOR__, and __GNUC_PATCHLEVEL__

[bravecorvus]

Ahh, I just had to override the CC and CXX environment variables to point to use gcc-8 and g++-8 respectively before running make. Sorry for the confusion. I assumed I would be able to pass these via command line variables. Thanks for the help!