diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 312a2d1df1dbe..f1fd45d8394ab 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -846,6 +846,7 @@ def Xoffload_linker : JoinedAndSeparate<["-"], "Xoffload-linker">,
 def Xpreprocessor : Separate<["-"], "Xpreprocessor">, Group<Preprocessor_Group>,
   HelpText<"Pass <arg> to the preprocessor">, MetaVarName<"<arg>">;
 def X_Flag : Flag<["-"], "X">, Group<Link_Group>;
+def X_Joined : Joined<["-"], "X">, IgnoredGCCCompat;
 def Z_Flag : Flag<["-"], "Z">, Group<Link_Group>;
 def all__load : Flag<["-"], "all_load">;
 def allowable__client : Separate<["-"], "allowable_client">;
diff --git a/clang/test/Misc/warn-not-error-Xfoo.c b/clang/test/Misc/warn-not-error-Xfoo.c
new file mode 100644
index 0000000000000..49a594d4c2f29
--- /dev/null
+++ b/clang/test/Misc/warn-not-error-Xfoo.c
@@ -0,0 +1,14 @@
+// RUN: %clang -c -Xfoo %s 2>&1 | FileCheck --check-prefix=CHECK_STANDALONE_FOO %s
+// RUN: %clang -c -Xfoo=bar %s 2>&1 | FileCheck --check-prefix=CHECK_JOINED_FOO %s
+
+// This test ensures that we only warn on -X<unknown> and -X<unknown=value>
+// in case it is used downstream. If we error, we can't ignore it and some
+// use of these (ignored) flags are in legacy use.
+// TODO: Deprecate with timebox warning so consumers can respond.
+
+// CHECK_STANDALONE_FOO: warning: argument unused during compilation: '-Xfoo' [-Wunused-command-line-argument]
+// CHECK_JOINED_FOO: warning: argument unused during compilation: '-Xfoo=bar' [-Wunused-command-line-argument]
+
+// CHECK-NOT: clang{.*}: error: unknown argument:
+
+void f(void) {}