ICU-13812 Define U_FALLTHROUGH for GCC 7+ for ICU4C. (#100)

Currently U_FALLTHROUGH is not defined for GCC, meaning that any switch case statements with fall-throughs will generate warnings when building ICU4C.

In GCC 7, they added support for: "_attribute_((fallthrough))" which we can now use for U_FALLTHROUGH, which eliminates many warnings when building with GCC.

diff --git a/icu4c/source/common/unicode/platform.h b/icu4c/source/common/unicode/platform.h
index db34613b614781e5b00fdecb709f596648168691..4c1603554fb8bfc1e1536039d21f53cf0dee35f1 100644 (file)
--- a/icu4c/source/common/unicode/platform.h
+++ b/icu4c/source/common/unicode/platform.h
@@ -517,6 +517,8 @@ namespace std {
             (__has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough"))
 #       define U_FALLTHROUGH [[clang::fallthrough]]
 #   endif
+#elif defined(__GNUC__) && (__GNUC__ >= 7)
+#   define U_FALLTHROUGH __attribute__((fallthrough))
 #endif
 
 #ifndef U_FALLTHROUGH