diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index abd6319..32791f9 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -793,13 +793,13 @@
 
   case ASTContext::GE_Missing_stdio:
     if (ForRedeclaration)
-      Diag(Loc, diag::err_implicit_decl_requires_stdio)
+      Diag(Loc, diag::warn_implicit_decl_requires_stdio)
         << Context.BuiltinInfo.GetName(BID);
     return 0;
 
   case ASTContext::GE_Missing_setjmp:
     if (ForRedeclaration)
-      Diag(Loc, diag::err_implicit_decl_requires_setjmp)
+      Diag(Loc, diag::warn_implicit_decl_requires_setjmp)
         << Context.BuiltinInfo.GetName(BID);
     return 0;
   }
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index 6972536..5ed973b 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -483,12 +483,21 @@
             S.Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID))
           return false;
         
-        NamedDecl *D = S.LazilyCreateBuiltin((IdentifierInfo *)II, BuiltinID,
-                                             S.TUScope, R.isForRedeclaration(),
-                                             R.getNameLoc());
-        if (D) 
+        if (NamedDecl *D = S.LazilyCreateBuiltin((IdentifierInfo *)II, 
+                                                 BuiltinID, S.TUScope, 
+                                                 R.isForRedeclaration(),
+                                                 R.getNameLoc())) {
           R.addDecl(D);
-        return (D != NULL);
+          return true;
+        }
+
+        if (R.isForRedeclaration()) {
+          // If we're redeclaring this function anyway, forget that
+          // this was a builtin at all.
+          S.Context.BuiltinInfo.ForgetBuiltin(BuiltinID, S.Context.Idents);
+        }
+
+        return false;
       }
     }
   }