[C++20] [Modules] [Driver] Emit unused argument warning if we use '-fmodule-output' with non-module input

We required the file name of an 'importable module unit' should end
with .cppm (or .ccm, .cxxm, .c++m).

But the driver can accept '-fmodule-output' for files with normal
suffixes (e.g., .cpp). This is somewhat inconsistency.

In this patch, we only claim the option `-fmodule-output` is used if
the type of the input file is modules related. Then now the compiler
will emit 'unused argument' warnings if the input file is not modules
related.

Author: ChuanqiXu9

clang/docs/ReleaseNotes.rst

@@ -328,6 +328,9 @@ Improvements to Clang's diagnostics
 - New ``-Wformat-signedness`` diagnostic that warn if the format string requires an
   unsigned argument and the argument is signed and vice versa.
 
+- Clang now emits ``unused argument`` warning when the -fmodule-output flag is used
+  with an input that is not of type c++-module.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4045,9 +4045,18 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
   // module fragment.
   CmdArgs.push_back("-fskip-odr-check-in-gmf");
 
-  // Claim `-fmodule-output` and `-fmodule-output=` to avoid unused warnings.
-  Args.ClaimAllArgs(options::OPT_fmodule_output);
-  Args.ClaimAllArgs(options::OPT_fmodule_output_EQ);
+  // We need to include the case the input file is a module file here.
+  // Since the default compilation model for C++ module interface unit will
+  // create temporary module file and compile the temporary module file
+  // to get the object file. Then the `-fmodule-output` flag will be
+  // brought to the second compilation process. So we have to claim it for
+  // the case too.
+  if (Input.getType() == driver::types::TY_CXXModule ||
+      Input.getType() == driver::types::TY_PP_CXXModule ||
+      Input.getType() == driver::types::TY_ModuleFile) {
+    Args.ClaimAllArgs(options::OPT_fmodule_output);
+    Args.ClaimAllArgs(options::OPT_fmodule_output_EQ);
+  }
 
   return HaveModules;
 }

clang/test/Driver/module-output.cppm

@@ -33,6 +33,9 @@
 // RUN: %clang -std=c++20 %t/Hello.cppm -fmodule-output=%t/Hello.pcm -fmodule-output -c -fsyntax-only \
 // RUN:   -### 2>&1 | FileCheck %t/Hello.cppm --check-prefix=CHECK-NOT-USED
 
+// Test that we can emit a warning if the type of the input file is not a module interface unit.
+// RUN: %clang -std=c++20 %t/a.cpp -fmodule-output -c -o %t/a.o -### 2>&1 | FileCheck %t/a.cpp
+
 //--- Hello.cppm
 export module Hello;
 
@@ -55,3 +58,8 @@ export module AnotherModule;
 // CHECK: "-emit-obj" {{.*}}"-main-file-name" "Hello.cppm" {{.*}}"-o" "{{.*}}/Hello-{{.*}}.o" "-x" "pcm" "{{.*}}/Hello.pcm"
 // CHECK: "-emit-module-interface" {{.*}}"-main-file-name" "AnotherModule.cppm" {{.*}}"-o" "{{.*}}/AnotherModule.pcm" "-x" "c++" "{{.*}}/AnotherModule.cppm"
 // CHECK: "-emit-obj" {{.*}}"-main-file-name" "AnotherModule.cppm" {{.*}}"-o" "{{.*}}/AnotherModule-{{.*}}.o" "-x" "pcm" "{{.*}}/AnotherModule.pcm"
+
+//--- a.cpp
+export module a;
+
+// CHECK: warning: argument unused during compilation: '-fmodule-output'