folly: support FOLLY_FALLTHROUGH on GCC

Summary:
This is required to avoid new GCC 7 -Wimplict-fallthrough warnings.
We also update to use the C++17 [[fallthrough]] attribute if supported.

Reviewed By: yfeldblum

Differential Revision: D6367140

fbshipit-source-id: d5380983cb300f944df9c2885d0faa0155994be7

diff --git a/folly/CppAttributes.h b/folly/CppAttributes.h
index 466aab91a81144527e968415207f666674d52d46..2ebbae8cff35a164d3f778da3db64da12c91ead9 100644 (file)
--- a/folly/CppAttributes.h
+++ b/folly/CppAttributes.h
@@ -53,8 +53,12 @@
  *     FOLLY_FALLTHROUGH; // no warning: annotated fall-through
  * }
  */
-#if FOLLY_HAS_CPP_ATTRIBUTE(clang::fallthrough)
+#if FOLLY_HAS_CPP_ATTRIBUTE(fallthrough)
+#define FOLLY_FALLTHROUGH [[fallthrough]]
+#elif FOLLY_HAS_CPP_ATTRIBUTE(clang::fallthrough)
 #define FOLLY_FALLTHROUGH [[clang::fallthrough]]
+#elif FOLLY_HAS_CPP_ATTRIBUTE(gnu::fallthrough)
+#define FOLLY_FALLTHROUGH [[gnu::fallthrough]]
 #else
 #define FOLLY_FALLTHROUGH
 #endif