Index: lib/ubsan/ubsan_handlers.cc
===================================================================
--- lib/ubsan/ubsan_handlers.cc
+++ lib/ubsan/ubsan_handlers.cc
@@ -566,8 +566,14 @@
 
   ScopedReport R(Opts, Loc, ET);
 
-  Diag(Loc, DL_Error, "pointer index expression with base %0 overflowed to %1")
-    << (void *)Base << (void*)Result;
+  if ((sptr(Base) > 0) == (sptr(Result) > 0))
+    Diag(Loc, DL_Error, "unsigned pointer index expression result is %0, "
+                        "preceding its base %1")
+        << (void *)Result << (void *)Base;
+  else
+    Diag(Loc, DL_Error,
+         "pointer index expression with base %0 overflowed to %1")
+        << (void *)Base << (void *)Result;
 }
 
 void __ubsan::__ubsan_handle_pointer_overflow(PointerOverflowData *Data,
Index: test/ubsan/TestCases/Pointer/unsigned-index-expression.cpp
===================================================================
--- /dev/null
+++ test/ubsan/TestCases/Pointer/unsigned-index-expression.cpp
@@ -0,0 +1,12 @@
+// RUN: %clangxx -fsanitize=pointer-overflow %s -o %t
+// RUN: %t 2>&1 | FileCheck %s
+
+int main(int argc, char *argv[]) {
+  unsigned long long offset = -1;
+  char *p = (char *)7;
+
+  // CHECK: runtime error: unsigned pointer index expression result is 0x{{0+}}6, preceding its base 0x{{0+}}7
+  char *q = p + offset;
+
+  return 0;
+}