Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp	(revision 209303)
+++ lib/Sema/SemaDecl.cpp	(working copy)
@@ -2748,18 +2748,13 @@
       Diag(New->getLocation(), diag::warn_redecl_library_builtin) << New;
       Diag(OldLocation, diag::note_previous_builtin_declaration)
         << Old << Old->getType();
+      // New user declared function is overriding the built-in. Not only
+      // forget about its "builtin-ness", but also make the impliclitly
+      // declared built-in function invalid; otherwise, We have a built-in
+      // declaration AST whose Identifier is Builtin::NotBuiltin.
+      New->getIdentifier()->setBuiltinID(Builtin::NotBuiltin);
+      Old->setInvalidDecl();
 
-      // If this is a global redeclaration, just forget hereafter
-      // about the "builtin-ness" of the function.
-      //
-      // Doing this for local extern declarations is problematic.  If
-      // the builtin declaration remains visible, a second invalid
-      // local declaration will produce a hard error; if it doesn't
-      // remain visible, a single bogus local redeclaration (which is
-      // actually only a warning) could break all the downstream code.
-      if (!New->getLexicalDeclContext()->isFunctionOrMethod())
-        New->getIdentifier()->setBuiltinID(Builtin::NotBuiltin);
-
       return false;
     }
 
Index: test/CodeGen/locally-redeclared-builtin-call.c
===================================================================
--- test/CodeGen/locally-redeclared-builtin-call.c	(revision 0)
+++ test/CodeGen/locally-redeclared-builtin-call.c	(working copy)
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+// rdar://16897451
+
+int Test()
+{
+  int a,b;
+  int pow (int,int);
+  return pow(a,b);
+}
+// CHECK-NOT: call i32 @llvm.pow.i32
+// CHECK: [[CALL:%.*]]  = call i32 @pow(i32 %0, i32 %1)
+// ret i32 [[CALL]]
+
+int pow(int c,int d)
+{
+  return c*d;
+}
Index: test/Sema/extern-redecl.c
===================================================================
--- test/Sema/extern-redecl.c	(revision 209303)
+++ test/Sema/extern-redecl.c	(working copy)
@@ -49,12 +49,12 @@
 extern float *calloc(); // expected-warning {{incompatible redeclaration of library function}} expected-note {{is a builtin}} expected-note 2 {{previous declaration is here}}
 void test5a() {
   int abort(); // expected-error {{conflicting types}}
-  float *malloc(); // expected-warning {{incompatible redeclaration of library function}} expected-note 2 {{is a builtin}}
+  float *malloc(); // expected-warning {{incompatible redeclaration of library function}} expected-note {{is a builtin}}
   int *calloc(); // expected-error {{conflicting types}}
 }
 void test5b() {
   int abort(); // expected-error {{conflicting types}}
-  float *malloc(); // expected-warning {{incompatible redeclaration of library function}}
+  float *malloc();
   int *calloc(); // expected-error {{conflicting types}}
 }
 void test5c() {