-Wattributes visibility warning with some GCC versions

.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

include/fmt/format.h

@@ -3198,17 +3198,24 @@ FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
   return {s.data(), s.size()};
 }
 
-#define FMT_STRING_IMPL(s, base)                                  \
-  [] {                                                            \
-    /* Use a macro-like name to avoid shadowing warnings. */      \
-    struct FMT_COMPILE_STRING : base {                            \
-      using char_type = fmt::remove_cvref_t<decltype(s[0])>;      \
-      FMT_MAYBE_UNUSED FMT_CONSTEXPR                              \
-      operator fmt::basic_string_view<char_type>() const {        \
-        return fmt::detail::compile_string_to_view<char_type>(s); \
-      }                                                           \
-    };                                                            \
-    return FMT_COMPILE_STRING();                                  \
+#if defined(FMT_GCC_VERSION) && FMT_GCC_VERSION >= 400
+#  define FMT_COMPILE_STRING_VISIBILITY_ATTRIBUTE \
+    __attribute__((visibility("hidden")))
+#else
+#  define FMT_COMPILE_STRING_VISIBILITY_ATTRIBUTE
+#endif
+
+#define FMT_STRING_IMPL(s, base)                                               \
+  [] {                                                                         \
+    /* Use a macro-like name to avoid shadowing warnings. */                   \
+    struct FMT_COMPILE_STRING_VISIBILITY_ATTRIBUTE FMT_COMPILE_STRING : base { \
+      using char_type = fmt::remove_cvref_t<decltype(s[0])>;                   \
+      FMT_MAYBE_UNUSED FMT_CONSTEXPR                                           \
+      operator fmt::basic_string_view<char_type>() const {                     \
+        return fmt::detail::compile_string_to_view<char_type>(s);              \
+      }                                                                        \
+    };                                                                         \
+    return FMT_COMPILE_STRING();                                               \
   }()
 
 /**

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()