[Driver] [C++20] [Modules] Warning for the surprising useless case for reduced BMI

Found in downstream. I didn't realize the output file for precompile and
reduced BMI refers to the same location. Then the generating process of
reduced BMI is basically a waste of time.

Author: ChuanqiXu9

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -563,6 +563,11 @@ def err_test_module_file_extension_format : Error<
 def err_drv_module_output_with_multiple_arch : Error<
   "option '-fmodule-output' cannot be used with multiple arch options">;
 
+def err_drv_reduced_module_output_overrided : Warning<
+  "the implicit output of reduced BMI may be overrided by the output file specified by '--precompile'. "
+  "please consider use '-fmodule-output=' to specify the output file for reduced BMI explicitly">,
+  InGroup<DiagGroup<"reduced-bmi-output-overrided">>;
+
 def warn_drv_delayed_template_parsing_after_cxx20 : Warning<
   "-fdelayed-template-parsing is deprecated after C++20">,
   InGroup<DiagGroup<"delayed-template-parsing-in-cxx20">>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4250,10 +4250,18 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
 
     if (Args.hasArg(options::OPT_fmodule_output_EQ))
       Args.AddLastArg(CmdArgs, options::OPT_fmodule_output_EQ);
-    else
+    else {
+      if (Args.hasArg(options::OPT__precompile) &&
+          (!Args.hasArg(options::OPT_o) ||
+           Args.getLastArg(options::OPT_o)->getValue() ==
+               getCXX20NamedModuleOutputPath(Args, Input.getBaseInput()))) {
+        D.Diag(diag::err_drv_reduced_module_output_overrided);
+      }
+
       CmdArgs.push_back(Args.MakeArgString(
           "-fmodule-output=" +
           getCXX20NamedModuleOutputPath(Args, Input.getBaseInput())));
+    }
   }
 
   // Noop if we see '-fmodules-reduced-bmi' with other translation

clang/test/Driver/module-fgen-reduced-bmi.cppm

@@ -48,6 +48,14 @@
 // RUN: %clang -std=c++20 Hello.cppm --precompile -fmodules-reduced-bmi \
 // RUN:     -o Hello.full.pcm -### 2>&1 | FileCheck Hello.cppm \
 // RUN:     --check-prefix=CHECK-EMIT-MODULE-INTERFACE
+
+// RUN: %clang -std=c++20 Hello.cppm --precompile -fmodules-reduced-bmi \
+// RUN:     -### 2>&1 | FileCheck Hello.cppm \
+// RUN:     --check-prefix=CHECK-OVERRIDE-WARN
+
+// RUN: %clang -std=c++20 Hello.cppm --precompile -fmodules-reduced-bmi \
+// RUN:     -o Hello.pcm -### 2>&1 | FileCheck Hello.cppm \
+// RUN:    --check-prefix=CHECK-OVERRIDE-WARN
 //
 // RUN: %clang -std=c++20 Hello.cc -fmodules-reduced-bmi -Wall -Werror \
 // RUN:     -c -o Hello.o -### 2>&1 | FileCheck Hello.cc
@@ -74,6 +82,8 @@ export module Hello;
 // flag.
 // CHECK-EMIT-MODULE-INTERFACE: -emit-module-interface
 
+// CHECK-OVERRIDE-WARN: warning: the implicit output of reduced BMI may be overrided by the output file specified by '--precompile'. {{.*}}-Wreduced-bmi-output-overrided
+
 // NO_WARN-NOT: warning
 
 //--- Hello.cc