Index: libcxx/trunk/include/__config
===================================================================
--- libcxx/trunk/include/__config
+++ libcxx/trunk/include/__config
@@ -1248,8 +1248,12 @@
 #  define _LIBCPP_DIAGNOSE_ERROR(...)
 #endif
 
-#if __has_attribute(fallthough) || _GNUC_VER >= 700
 // Use a function like macro to imply that it must be followed by a semicolon
+#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
+#elif __has_attribute(fallthough) || _GNUC_VER >= 700
 #  define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
 #  define _LIBCPP_FALLTHROUGH() ((void)0)
Index: libcxxabi/trunk/src/demangle/ItaniumDemangle.h
===================================================================
--- libcxxabi/trunk/src/demangle/ItaniumDemangle.h
+++ libcxxabi/trunk/src/demangle/ItaniumDemangle.h
@@ -2812,6 +2812,7 @@
       SoFar = make<ExpandedSpecialSubstitution>(SSK);
       if (!SoFar)
         return nullptr;
+      break;
     default:
       break;
     }