Index: lib/Basic/DiagnosticIDs.cpp
===================================================================
--- lib/Basic/DiagnosticIDs.cpp	(revision 184534)
+++ lib/Basic/DiagnosticIDs.cpp	(working copy)
@@ -16,6 +16,7 @@
 #include "clang/Basic/DiagnosticCategories.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <map>
 using namespace clang;
@@ -71,6 +72,7 @@
   { diag::ENUM, DEFAULT_MAPPING, CLASS, SFINAE, ACCESS,           \
     NOWERROR, SHOWINSYSHEADER, CATEGORY, GROUP,                   \
     STR_SIZE(DESC, uint16_t), DESC },
+  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
 #include "clang/Basic/DiagnosticCommonKinds.inc"
 #include "clang/Basic/DiagnosticDriverKinds.inc"
 #include "clang/Basic/DiagnosticFrontendKinds.inc"
@@ -82,11 +84,9 @@
 #include "clang/Basic/DiagnosticSemaKinds.inc"
 #include "clang/Basic/DiagnosticAnalysisKinds.inc"
 #undef DIAG
-  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
 };
 
-static const unsigned StaticDiagInfoSize =
-  sizeof(StaticDiagInfo)/sizeof(StaticDiagInfo[0])-1;
+static const unsigned StaticDiagInfoSize = llvm::array_lengthof(StaticDiagInfo);
 
 /// GetDiagInfo - Return the StaticDiagInfoRec entry for the specified DiagID,
 /// or null if the ID is invalid.
@@ -122,7 +122,6 @@
   // memory at all.
   unsigned Offset = 0;
   unsigned ID = DiagID;
-#define DIAG_START_COMMON 0 // Sentinel value.
 #define CATEGORY(NAME, PREV) \
   if (DiagID > DIAG_START_##NAME) { \
     Offset += NUM_BUILTIN_##PREV##_DIAGNOSTICS - DIAG_START_##PREV - 1; \
@@ -138,7 +137,6 @@
 CATEGORY(SEMA, COMMENT)
 CATEGORY(ANALYSIS, SEMA)
 #undef CATEGORY
-#undef DIAG_START_COMMON
 
   // Avoid out of bounds reads.
   if (ID + Offset >= StaticDiagInfoSize)
Index: include/clang/Basic/Diagnostic.h
===================================================================
--- include/clang/Basic/Diagnostic.h	(revision 184534)
+++ include/clang/Basic/Diagnostic.h	(working copy)
@@ -1057,6 +1057,7 @@
 inline DiagnosticBuilder DiagnosticsEngine::Report(SourceLocation Loc,
                                             unsigned DiagID){
   assert(CurDiagID == ~0U && "Multiple diagnostics in flight at once!");
+  assert(DiagID != 0 && "Invalid diagnostic id given!");
   CurDiagLoc = Loc;
   CurDiagID = DiagID;
   return DiagnosticBuilder(this);
Index: include/clang/Basic/DiagnosticIDs.h
===================================================================
--- include/clang/Basic/DiagnosticIDs.h	(revision 184534)
+++ include/clang/Basic/DiagnosticIDs.h	(working copy)
@@ -28,7 +28,8 @@
   namespace diag {
     // Start position for diagnostics.
     enum {
-      DIAG_START_DRIVER        =                               300,
+      DIAG_START_COMMON        =                                 0,
+      DIAG_START_DRIVER        = DIAG_START_COMMON          +  300,
       DIAG_START_FRONTEND      = DIAG_START_DRIVER          +  100,
       DIAG_START_SERIALIZATION = DIAG_START_FRONTEND        +  100,
       DIAG_START_LEX           = DIAG_START_SERIALIZATION   +  120,
@@ -49,6 +50,7 @@
     enum {
 #define DIAG(ENUM,FLAGS,DEFAULT_MAPPING,DESC,GROUP,\
              SFINAE,ACCESS,CATEGORY,NOWERROR,SHOWINSYSHEADER) ENUM,
+#define COMMONSTART
 #include "clang/Basic/DiagnosticCommonKinds.inc"
       NUM_BUILTIN_COMMON_DIAGNOSTICS
 #undef DIAG