[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.

1 files changed, 12 insertions, 3 deletions

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index b03ac60..7fd6ad6 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/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;
 }