[clang][DependencyFileGenerator] Fix missing -MT option handling

[jkorous]

Summary

Some ten years ago the burden of providing target for what later became today's DependencyFileGenerator shifted to Driver in this patch:

52e96cc9329 2009-03-30:  Improve dependency file support.  - Rip out various bits of logic from clang-cc's dependency file gen,    force driver to provide instead.

DependencyFileGenerator later got decoupled from Driver in the sense that it can be used from a program that doesn't even support -MT option (we're using it for example in clang-scan-deps).

I believe we should move the error diagnostics to make sure it's always properly raised - currently it's essentially in one specific method of an implementation detail. It seems there's no reason to not do it right when processing command line arguments.

Also, it seems there's no technical reason why DependencyFileGenerator couldn't work with empty Targets. Afterall it's just used as a prefix for dependency output in void DependencyFileGenerator::outputDependencyFile and it would lead to line starting with the semicolon meant to separate target and it's dependencies this:

: foo.c bar.c baz.c

However, we might still consider that a bug and report the error in some way to clients of DependencyFileGenerator (maybe add some encapsulation and check in constructors?) - I'm open to suggestions.

[arphaman]

Thanks! I think that's a better layering. LGTM. I don't think there's a particular need to report error to clients of just the DependencyFileGenerator.

clang/lib/Frontend/CompilerInvocation.cpp

else
       Diags.Report(diag::err_drv_unknown_argument_with_suggestion)
           << ArgString << Nearest;
     Success = false;
   }
 
   Success &= ParseAnalyzerArgs(*Res.getAnalyzerOpts(), Args, Diags);
   Success &= ParseMigratorArgs(Res.getMigratorOpts(), Args);
   ParseDependencyOutputArgs(Res.getDependencyOutputOpts(), Args);
+  if (!Res.getDependencyOutputOpts().OutputFile.empty() &&
+      Res.getDependencyOutputOpts().Targets.empty()) {
+    Diags.Report(diag::err_fe_dependency_file_requires_MT);
+    Success = false;
+  }
   Success &=
       ParseDiagnosticArgs(Res.getDiagnosticOpts(), Args, &Diags,
                           false /*DefaultDiagColor*/, false /*DefaultShowOpt*/);
   ParseCommentArgs(LangOpts.CommentOpts, Args);
   ParseFileSystemArgs(Res.getFileSystemOpts(), Args);
   // FIXME: We shouldn't have to pass the DashX option around here
   InputKind DashX = ParseFrontendArgs(Res.getFrontendOpts(), Args, Diags,
                                       LangOpts.IsHeaderFile);

clang/lib/Frontend/DependencyFile.cpp

: OutputFile(Opts.OutputFile), Targets(Opts.Targets),
       OutputFormat(Opts.OutputFormat), InputFileIndex(0) {
   for (const auto &ExtraDep : Opts.ExtraDeps) {
     if (addDependency(ExtraDep))
       ++InputFileIndex;
   }
 }
 
 void DependencyFileGenerator::attachToPreprocessor(Preprocessor &PP) {
-  if (Targets.empty()) {
-    PP.getDiagnostics().Report(diag::err_fe_dependency_file_requires_MT);
-    return;
-  }
-
   // Disable the "file not found" diagnostic if the -MG option was given.
   if (AddMissingHeaderDeps)
     PP.SetSuppressIncludeNotFoundError(true);
 
   DependencyCollector::attachToPreprocessor(PP);
 }
 
 bool DependencyFileGenerator::sawDependency(StringRef Filename, bool FromModule,