[C++20] [Modules] Deprecate `-fmodules-ts` flag

[ChuanqiXu]

Summary

Close https://github.com/llvm/llvm-project/issues/60060

See https://github.com/llvm/llvm-project/issues/60060 for details.

[h-vetinari]

clang/include/clang/Basic/DiagnosticDriverKinds.td

635

Shouldn't this be something like:

[ChuanqiXu]

Address comments.

[ChuanqiXu]

clang/include/clang/Basic/DiagnosticDriverKinds.td

635

Yeah it is better. (we may assume -std=c++20 will be enabled if -std=c++23 is specified.)

[aaron.ballman]

LGTM, thank you!

[ChuanqiXu]

Committed in https://reviews.llvm.org/rG98f36ac4fa86d7f7a3f05b70f792c416fbb23a4a
(I forgot to write the correct phab link)

clang/docs/ReleaseNotes.rst

.. code-block:: c
     };
 
 Deprecated Compiler Flags
 -------------------------
 - ``-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang``
   has been deprecated. The flag will be removed in Clang 18.
   ``-ftrivial-auto-var-init=zero`` is now available unconditionally, to be
   compatible with GCC.
+- ``-fmodules-ts`` has been deprecated. The flag will be removed in Clang 17.
+  Please use ``-std=c++20`` or higher to use standard C++ modules instead.
 
 Modified Compiler Flags
 -----------------------
 - Clang now permits specifying ``--config=`` multiple times, to load multiple
   configuration files.
 
 Removed Compiler Flags
 -------------------------

clang/include/clang/Basic/DiagnosticDriverKinds.td

def warn_drv_msp430_hwmult_no_device : Warning<
   "to set the hardware multiply type explicitly">,
   InGroup<InvalidCommandLineArgument>;
 
 def warn_drv_libstdcxx_not_found : Warning<
   "include path for libstdc++ headers not found; pass '-stdlib=libc++' on the "
   "command line to use the libc++ standard library instead">,
   InGroup<DiagGroup<"stdlibcxx-not-found">>;
 
+def warn_deprecated_fmodules_ts_flag : Warning<
+  "the '-fmodules-ts' flag is deprecated and it will be removed in Clang 17; "
+  "use '-std=c++20' or higher to use standard C++ modules instead">,
+  InGroup<DiagGroup<"deprecated-module-ts">>;
+
 def err_drv_cannot_mix_options : Error<"cannot specify '%1' along with '%0'">;
 
 def err_drv_invalid_object_mode : Error<
   "OBJECT_MODE setting %0 is not recognized and is not a valid setting">;
 
 def err_aix_unsupported_tls_model : Error<"TLS model '%0' is not yet supported on AIX">;
 
 def err_invalid_cxx_abi : Error<"invalid C++ ABI name '%0'">;

clang/lib/Driver/ToolChains/Clang.cpp

if (Args.hasFlag(options::OPT_fmodules, options::OPT_fno_modules, false)) {
     if (AllowedInCXX || !IsCXX) {
       CmdArgs.push_back("-fmodules");
       HaveClangModules = true;
     }
   }
 
   HaveModules |= HaveClangModules;
   if (Args.hasArg(options::OPT_fmodules_ts)) {
+    D.Diag(diag::warn_deprecated_fmodules_ts_flag);
     CmdArgs.push_back("-fmodules-ts");
     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,
                    options::OPT_fno_implicit_module_maps, HaveClangModules))

clang/test/Driver/modules-ts.cpp

 // Check compiling a module interface to a .pcm file.
 //
 // RUN: %clang -fmodules-ts -x c++-module --precompile %s -o %t.pcm -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE
 //
+// CHECK-PRECOMPILE:  warning: the '-fmodules-ts' flag is deprecated and it will be removed in Clang 17; use '-std=c++20' or higher to use standard C++ modules instead [-Wdeprecated-module-ts]
 // CHECK-PRECOMPILE: -cc1 {{.*}} -emit-module-interface
 // CHECK-PRECOMPILE-SAME: -o {{.*}}.pcm
 // CHECK-PRECOMPILE-SAME: -x c++
 // CHECK-PRECOMPILE-SAME: modules-ts.cpp
 
 // Check compiling a .pcm file to a .o file.
 //
-// RUN: %clang -fmodules-ts -fintegrated-as %t.pcm -c -o %t.pcm.o -v 2>&1 | FileCheck %s --check-prefix=CHECK-COMPILE
+// RUN: %clang -fmodules-ts -fintegrated-as %t.pcm -c -o %t.pcm.o -v 2>&1 | FileCheck %s --check-prefix=CHECK-COMPILE --check-prefix=CHECK-WARN
 //
+// CHECK-WARN:  warning: the '-fmodules-ts' flag is deprecated and it will be removed in Clang 17; use '-std=c++20' or higher to use standard C++ modules instead [-Wdeprecated-module-ts]
 // CHECK-COMPILE: -cc1 {{.*}} -emit-obj
 // CHECK-COMPILE-SAME: -o {{.*}}.pcm.o
 // CHECK-COMPILE-SAME: -x pcm
 // CHECK-COMPILE-SAME: {{.*}}.pcm
 
 // Check use of a .pcm file in another compilation.
 //
 // RUN: %clang -fmodules-ts -fmodule-file=%t.pcm -fintegrated-as -Dexport= %s -c -o %t.o -v 2>&1 | FileCheck %s --check-prefix=CHECK-USE
 //
+// CHECK-USE:  warning: the '-fmodules-ts' flag is deprecated and it will be removed in Clang 17; use '-std=c++20' or higher to use standard C++ modules instead [-Wdeprecated-module-ts]
 // CHECK-USE: -cc1
 // CHECK-USE-SAME: -emit-obj
 // CHECK-USE-SAME: -fmodule-file={{.*}}.pcm
 // CHECK-USE-SAME: -o {{.*}}.o{{"?}} {{.*}}-x c++
 // CHECK-USE-SAME: modules-ts.cpp
 
 // Check combining precompile and compile steps works.
 //
 // RUN: %clang -fmodules-ts -fintegrated-as -x c++-module %s -c -o %t.pcm.o -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE --check-prefix=CHECK-COMPILE
 
 // Check that .cppm is treated as a module implicitly.
 // RUN: cp %s %t.cppm
 // RUN: %clang -fmodules-ts --precompile %t.cppm -o %t.pcm -v 2>&1 | FileCheck %s --check-prefix=CHECK-PRECOMPILE
 
 // Note, we use -Dexport= to make this a module implementation unit when building the implementation.
 export module foo;