Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -1386,7 +1386,7 @@
   PosFlag<SetTrue, [CC1Option], "Enable EH Asynchronous exceptions">, NegFlag<SetFalse>>;
 defm cxx_modules : BoolFOption<"cxx-modules",
   LangOpts<"CPlusPlusModules">, Default<cpp20.KeyPath>,
-  NegFlag<SetFalse, [CC1Option], "Disable">, PosFlag<SetTrue, [], "Enable">,
+  NegFlag<SetFalse, [CC1Option], "Disable">, PosFlag<SetTrue, [CC1Option], "Enable">,
   BothFlags<[NoXarchOption], " modules for C++">>,
   ShouldParseIf<cplusplus.KeyPath>;
 def fdebug_pass_arguments : Flag<["-"], "fdebug-pass-arguments">, Group<f_Group>;
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -3590,6 +3590,12 @@
     HaveModules = true;
   }
 
+  if (Args.hasFlag(options::OPT_fcxx_modules,
+                   options::OPT_fno_cxx_modules, false)) {
+    CmdArgs.push_back("-fcxx-modules");
+    HaveModules = true;
+  }
+
   // -fmodule-maps enables implicit reading of module map files. By default,
   // this is enabled if we are using Clang's flavor of precompiled modules.
   if (Args.hasFlag(options::OPT_fimplicit_module_maps,
Index: clang/test/Driver/modules.cpp
===================================================================
--- clang/test/Driver/modules.cpp
+++ clang/test/Driver/modules.cpp
@@ -73,3 +73,8 @@
 // CHECK-HEADER-UNIT-USE: BAR;
 FOO;
 #endif
+
+// Check the independent use of -fcxx-modules
+//
+// RUN: %clang -fcxx-modules -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-CXX-MODULES
+// CHECK-CXX-MODULES: "-fcxx-modules"
Index: clang/test/Modules/cxx-modules.cppm
===================================================================
--- /dev/null
+++ clang/test/Modules/cxx-modules.cppm
@@ -0,0 +1,4 @@
+// This tests that we could use C++20 modules standalone.
+// RUN: %clang -fcxx-modules -fsyntax-only -Xclang -verify %s
+// expected-no-diagnostics
+export module M;