Index: lib/Sema/SemaExceptionSpec.cpp
===================================================================
--- lib/Sema/SemaExceptionSpec.cpp
+++ lib/Sema/SemaExceptionSpec.cpp
@@ -254,9 +254,15 @@
   // to many libc functions as an optimization. Unfortunately, that
   // optimization isn't permitted by the C++ standard, so we're forced
   // to work around it here.
-  if (MissingEmptyExceptionSpecification && NewProto &&
-      (Old->getLocation().isInvalid() ||
-       Context.getSourceManager().isInSystemHeader(Old->getLocation())) &&
+  bool IsFromSystemHeader = false;
+  for (const FunctionDecl *Prev = Old; Prev; Prev = Prev->getPreviousDecl())
+    if (Prev->getLocation().isInvalid() ||
+        Context.getSourceManager().isInSystemHeader(Prev->getLocation())) {
+      IsFromSystemHeader = true;
+      break;
+    }
+
+  if (MissingEmptyExceptionSpecification && NewProto && IsFromSystemHeader &&
       Old->isExternC()) {
     New->setType(Context.getFunctionType(
         NewProto->getReturnType(), NewProto->getParamTypes(),
Index: test/SemaCXX/Inputs/malloc.h
===================================================================
--- test/SemaCXX/Inputs/malloc.h
+++ test/SemaCXX/Inputs/malloc.h
@@ -1,3 +1,5 @@
 extern "C" {
 extern void *malloc (__SIZE_TYPE__ __size) throw () __attribute__ ((__malloc__)) ;
+
+void MyFunc() throw();
 }
Index: test/SemaCXX/builtin-exception-spec.cpp
===================================================================
--- test/SemaCXX/builtin-exception-spec.cpp
+++ test/SemaCXX/builtin-exception-spec.cpp
@@ -4,4 +4,7 @@
 
 extern "C" {
 void *malloc(__SIZE_TYPE__);
+
+void MyFunc() __attribute__((__weak__));
+void MyFunc() { return; }
 }