temp: fail build on gcc 6

.travis.yml

@@ -58,6 +58,20 @@ matrix:
           packages:
             - g++-8
 
+      # ARM g++ 6 on Linux with C++14
+    - env: COMPILER=g++-6 BUILD=Debug STANDARD=14
+      compiler: gcc
+      arch: arm64
+      os: linux
+      dist: xenial
+      addons:
+        apt:
+          update: true
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-6
+
       # Apple clang on OS X with C++14
     - env: BUILD=Debug STANDARD=14
       compiler: clang

test/CMakeLists.txt

@@ -83,6 +83,9 @@ function(add_fmt_test name)
   if (FMT_WERROR)
     target_compile_options(${name} PRIVATE ${WERROR_FLAG})
   endif ()
+  if (CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+    target_compile_options(${name} PRIVATE -fvisibility=hidden -Werror=attributes)
+  endif()
   target_include_directories(${name} SYSTEM PUBLIC gtest gmock)
   add_test(NAME ${name} COMMAND ${name})
 endfunction()