diff --git a/include/clang/Basic/DiagnosticKinds.def b/include/clang/Basic/DiagnosticKinds.def
index e0023e3..d7698d2 100644
--- a/include/clang/Basic/DiagnosticKinds.def
+++ b/include/clang/Basic/DiagnosticKinds.def
@@ -739,6 +739,8 @@
      "non-static declaration of '%0' follows static declaration")
 DIAG(err_redefinition_different_kind, ERROR,
      "redefinition of '%0' as different kind of symbol")
+DIAG(err_redefinition_different_typedef, ERROR,
+     "typedef redefinition with different types ('%0' vs '%1')")
 DIAG(err_conflicting_types, ERROR,
      "conflicting types for '%0'")
 DIAG(err_nested_redefinition, ERROR,
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 4d726cf..610bf61 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -248,6 +248,18 @@
     return New;
   }
   
+  // If the typedef types are not identical, reject them in all languages and
+  // with any extensions enabled.
+  if (Old->getUnderlyingType() != New->getUnderlyingType() && 
+      Context.getCanonicalType(Old->getUnderlyingType()) != 
+      Context.getCanonicalType(New->getUnderlyingType())) {
+    Diag(New->getLocation(), diag::err_redefinition_different_typedef,
+         New->getUnderlyingType().getAsString(),
+         Old->getUnderlyingType().getAsString());
+    Diag(Old->getLocation(), diag::err_previous_definition);
+    return Old;
+  }
+  
   // Allow multiple definitions for ObjC built-in typedefs.
   // FIXME: Verify the underlying types are equivalent!
   if (getLangOptions().ObjC1 && isBuiltinObjCType(New))
diff --git a/test/Sema/typedef-redef.c b/test/Sema/typedef-redef.c
index 1b12ae4..d3904dd 100644
--- a/test/Sema/typedef-redef.c
+++ b/test/Sema/typedef-redef.c
@@ -1,4 +1,13 @@
 // RUN: clang < %s -fsyntax-only -verify
 
+// size_t coming from a system header.
 #include <stddef.h>
-typedef unsigned long size_t;
+typedef __SIZE_TYPE__ size_t;
+
+
+
+typedef const int x; // expected-error {{previous definition is here}}
+extern x a;
+typedef int x;  // expected-error {{typedef redefinition with different types}}
+extern x a;
+