Index: SemaDecl.cpp
===================================================================
--- SemaDecl.cpp
+++ 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