Index: lib/StaticAnalyzer/Core/PathDiagnostic.cpp
===================================================================
--- lib/StaticAnalyzer/Core/PathDiagnostic.cpp
+++ lib/StaticAnalyzer/Core/PathDiagnostic.cpp
@@ -211,6 +211,10 @@
     const SourceManager &SMgr = D->path.front()->getLocation().getManager();
     SmallVector<const PathPieces *, 5> WorkList;
     WorkList.push_back(&D->path);
+    SmallString<128> buf;
+    llvm::raw_svector_ostream warning(buf);
+    warning << "warning: Path diagnostic report is not generated. " << getName()
+            << " does not support diagnostics that cross file boundaries.\n";
 
     while (!WorkList.empty()) {
       const PathPieces &path = *WorkList.pop_back_val();
@@ -222,19 +226,25 @@
 
         if (FID.isInvalid()) {
           FID = SMgr.getFileID(L);
-        } else if (SMgr.getFileID(L) != FID)
-          return; // FIXME: Emit a warning?
+        } else if (SMgr.getFileID(L) != FID) {
+          llvm::errs() << warning.str();
+          return;
+        }
 
         // Check the source ranges.
         ArrayRef<SourceRange> Ranges = piece->getRanges();
         for (ArrayRef<SourceRange>::iterator I = Ranges.begin(),
                                              E = Ranges.end(); I != E; ++I) {
           SourceLocation L = SMgr.getExpansionLoc(I->getBegin());
-          if (!L.isFileID() || SMgr.getFileID(L) != FID)
-            return; // FIXME: Emit a warning?
+          if (!L.isFileID() || SMgr.getFileID(L) != FID) {
+            llvm::errs() << warning.str();
+            return;
+          }
           L = SMgr.getExpansionLoc(I->getEnd());
-          if (!L.isFileID() || SMgr.getFileID(L) != FID)
-            return; // FIXME: Emit a warning?
+          if (!L.isFileID() || SMgr.getFileID(L) != FID) {
+            llvm::errs() << warning.str();
+            return;
+          }
         }
 
         if (const PathDiagnosticCallPiece *call =
Index: test/Analysis/PR12421.h
===================================================================
--- test/Analysis/PR12421.h
+++ test/Analysis/PR12421.h
@@ -0,0 +1,4 @@
+static void f() {
+  int *p = 0;
+  *p = 1;       // expected-warning{{Dereference of null pointer}}
+}
Index: test/Analysis/PR12421.c
===================================================================
--- test/Analysis/PR12421.c
+++ test/Analysis/PR12421.c
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -analyze -analyzer-checker=core -verify %s
+// RUN: %clang_cc1 -analyze -analyzer-checker=core -analyzer-output=html -o PR12421.html %s 2>&1 | FileCheck %s
+
+#include "PR12421.h"
+
+int main(){
+  f();
+  return 0;
+}
+
+// CHECK: warning: Path diagnostic report is not generated. HTMLDiagnostics does not support diagnostics that cross file boundaries.