3 files changed, 27 insertions, 3 deletions
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 4199cbe0f66..2ec8e8c1652 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -462,7 +462,7 @@ void CompilerInstance::createPreprocessor(TranslationUnitKind TUKind) {
                            /*ShowDepth=*/false);
   }
 
-  if (DepOpts.PrintShowIncludes) {
+  if (DepOpts.ShowIncludesDest != ShowIncludesDestination::None) {
     AttachHeaderIncludeGen(*PP, DepOpts,
                            /*ShowAllHeaders=*/true, /*OutputPath=*/"",
                            /*ShowDepth=*/true, /*MSStyle=*/true);
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index ddaa16477aa..94e3332fc4f 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1130,7 +1130,17 @@ static void ParseDependencyOutputArgs(DependencyOutputOptions &Opts,
   Opts.ShowHeaderIncludes = Args.hasArg(OPT_H);
   Opts.HeaderIncludeOutputFile = Args.getLastArgValue(OPT_header_include_file);
   Opts.AddMissingHeaderDeps = Args.hasArg(OPT_MG);
-  Opts.PrintShowIncludes = Args.hasArg(OPT_show_includes);
+  if (Args.hasArg(OPT_show_includes)) {
+    // Writing both /showIncludes and preprocessor output to stdout
+    // would produce interleaved output, so use stderr for /showIncludes.
+    // This behaves the same as cl.exe, when /E, /EP or /P are passed.
+    if (Args.hasArg(options::OPT_E) || Args.hasArg(options::OPT_P))
+      Opts.ShowIncludesDest = ShowIncludesDestination::Stderr;
+    else
+      Opts.ShowIncludesDest = ShowIncludesDestination::Stdout;
+  } else {
+    Opts.ShowIncludesDest = ShowIncludesDestination::None;
+  }
   Opts.DOTOutputFile = Args.getLastArgValue(OPT_dependency_dot);
   Opts.ModuleDependencyOutputDir =
       Args.getLastArgValue(OPT_module_dependency_dir);
diff --git a/clang/lib/Frontend/HeaderIncludeGen.cpp b/clang/lib/Frontend/HeaderIncludeGen.cpp
index 5bff4ecd0b4..9dc107c9d54 100644
--- a/clang/lib/Frontend/HeaderIncludeGen.cpp
+++ b/clang/lib/Frontend/HeaderIncludeGen.cpp
@@ -80,9 +80,23 @@ void clang::AttachHeaderIncludeGen(Preprocessor &PP,
                                    const DependencyOutputOptions &DepOpts,
                                    bool ShowAllHeaders, StringRef OutputPath,
                                    bool ShowDepth, bool MSStyle) {
-  raw_ostream *OutputFile = MSStyle ? &llvm::outs() : &llvm::errs();
+  raw_ostream *OutputFile = &llvm::errs();
   bool OwnsOutputFile = false;
 
+  // Choose output stream, when printing in cl.exe /showIncludes style.
+  if (MSStyle) {
+    switch (DepOpts.ShowIncludesDest) {
+    default:
+      llvm_unreachable("Invalid destination for /showIncludes output!");
+    case ShowIncludesDestination::Stderr:
+      OutputFile = &llvm::errs();
+      break;
+    case ShowIncludesDestination::Stdout:
+      OutputFile = &llvm::outs();
+      break;
+    }
+  }
+
   // Open the output file, if used.
   if (!OutputPath.empty()) {
     std::error_code EC;