[Driver] Enable to use C++20 standalne by -fcxx-modules

This patch allows user to use C++20 module by -fcxx-modules. Previously,
we could only use it under -std=c++20. Given that user could use C++20
coroutine standalonel by -fcoroutines-ts. It makes sense to offer an
option to use C++20 modules without enabling C++20.

Reviewed By: iains, MaskRay

Differential Revision: https://reviews.llvm.org/D120540

clang/include/clang/Driver/Options.td

@@ -1410,7 +1410,7 @@ defm async_exceptions: BoolFOption<"async-exceptions",
   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>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3627,6 +3627,12 @@ static void RenderModulesOptions(Compilation &C, const Driver &D,
     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,

clang/test/Driver/modules.cpp

@@ -73,3 +73,14 @@ import "foo.h";
 // CHECK-HEADER-UNIT-USE: BAR;
 FOO;
 #endif
+
+// Check the independent use of -fcxx-modules
+//
+// RUN: %clang++ -fcxx-modules -std=c++17 -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-CXX17-MODULES
+// CHECK-CXX17-MODULES: "-fcxx-modules"
+// RUN: %clang++ -fcxx-modules -std=c++14 -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-CXX14-MODULES
+// CHECK-CXX14-MODULES: "-fcxx-modules"
+// RUN: %clang++ -fcxx-modules -std=c++11 -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-CXX11-MODULES
+// CHECK-CXX11-MODULES: "-fcxx-modules"
+// RUN: %clang++ -fcxx-modules -std=c++03 -### %s 2>&1 | FileCheck %s --check-prefix=CHECK-CXX03-MODULES
+// CHECK-CXX03-MODULES: "-fcxx-modules"

clang/test/Modules/cxx-modules.cppm

@@ -0,0 +1,7 @@
+// This tests that we could use C++20 modules standalone.
+// RUN: %clang -std=c++03 -fcxx-modules -fsyntax-only -Xclang -verify %s
+// RUN: %clang -std=c++11 -fcxx-modules -fsyntax-only -Xclang -verify %s
+// RUN: %clang -std=c++14 -fcxx-modules -fsyntax-only -Xclang -verify %s
+// RUN: %clang -std=c++17 -fcxx-modules -fsyntax-only -Xclang -verify %s
+// expected-no-diagnostics
+export module M;