diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 0047181..e43a318 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -8687,7 +8687,8 @@
 
   // Builtin functions cannot be defined.
   if (unsigned BuiltinID = FD->getBuiltinID()) {
-    if (!Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID)) {
+    if (!Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID) &&
+        !Context.BuiltinInfo.isPredefinedRuntimeFunction(BuiltinID)) {
       Diag(FD->getLocation(), diag::err_builtin_definition) << FD;
       FD->setInvalidDecl();
     }