diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index f10a73e25fd5b..cf8351f58e5f7 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -731,18 +731,18 @@ def AbiTag : Attr {
 }
 
 def CMSThreadSafe : InheritableAttr {
-  let Spellings = [ CXX11<"cms","thread_safe", 200809>, C2x<"cms", "thread_safe"> ];
+  let Spellings = [ CXX11<"cms","thread_safe", 200809>, C23<"cms", "thread_safe"> ];
   let Documentation = [Undocumented];
 }
 
 def CMSThreadGuard : InheritableAttr {
-  let Spellings = [ CXX11<"cms","thread_guard", 200809>, C2x<"cms", "thread_guard"> ];
+  let Spellings = [ CXX11<"cms","thread_guard", 200809>, C23<"cms", "thread_guard"> ];
   let Args = [StringArgument<"GuardVar">];
   let Documentation = [Undocumented];
 }
 
 def CMSSaAllow : Attr {
-  let Spellings = [ CXX11<"cms","sa_allow", 200809>, C2x<"cms", "sa_allow"> ];
+  let Spellings = [ CXX11<"cms","sa_allow", 200809>, C23<"cms", "sa_allow"> ];
   let Documentation = [Undocumented];
 }