experiment with -Wimplicit-fallthrough

base/compiler_specific.h

Index: base/compiler_specific.h
diff --git a/base/compiler_specific.h b/base/compiler_specific.h
index 09b4e7033269c7e9279c37b9c267a5560e7ca06c..a527df755a37272ccc32c56e028ca37d6214148f 100644
--- a/base/compiler_specific.h
+++ b/base/compiler_specific.h
@@ -149,6 +149,16 @@
 #define OVERRIDE
 #endif
 
+// XXX amazing comment
+#if defined(__clang__) && defined(__has_warning)
+#if __has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough")
+#define FALLTHROUGH_INTENDED [[clang::fallthrough]]  // NOLINT
+#endif
+#endif
+
+#ifndef FALLTHROUGH_INTENDED
+#define FALLTHROUGH_INTENDED do { } while (0)
+
 // Annotate a virtual method indicating that subclasses must not override it,
 // or annotate a class to indicate that it cannot be subclassed.
 // Use like:
@@ -167,6 +177,18 @@
 #define FINAL
 #endif
 
+// Annotate a virtual method indicating it must be overriding a virtual
+// method in the parent class.
+// Use like:
+//   virtual void foo() OVERRIDE;
+#if defined(COMPILER_MSVC)
+#define OVERRIDE override
+#elif defined(__clang__)
+#define OVERRIDE override
+#else
+#define OVERRIDE
+#endif
+
 // Annotate a function indicating the caller must examine the return value.
 // Use like:
 //   int foo() WARN_UNUSED_RESULT;
@@ -196,7 +218,6 @@
 // If available, it would look like:
 //   __attribute__((format(wprintf, format_param, dots_param)))
 
-
 // MemorySanitizer annotations.
 #ifdef MEMORY_SANITIZER
 extern "C" {