Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -2033,6 +2033,12 @@
   if (getLangOpts().Modules || getLangOpts().C11)
     return;
   
+  // Added isImplicit() check, because implicit TypeDecl::getLocation()
+  // returns 0. The're many implicit typedefs in OpenCL, e.g. atomic_flag.
+  if (Old->isImplicit() || New->isImplicit()) {
+	  return;
+  }
+
   // If we have a redefinition of a typedef in C, emit a warning.  This warning
   // is normally mapped to an error, but can be controlled with
   // -Wtypedef-redefinition.  If either the original or the redefinition is
Index: test/SemaOpenCL/implicit-typedef.cl
===================================================================
--- test/SemaOpenCL/implicit-typedef.cl
+++ test/SemaOpenCL/implicit-typedef.cl
@@ -0,0 +1,3 @@
+// RUN: %clang_cc1 -main-file-name opencl_math.cl -cl-std=CL2.0 -fsyntax-only -x cl %s
+// expected-no-diagnostics
+typedef atomic_int atomic_flag;