4 files changed, 29 insertions, 10 deletions
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 25ce68bffc0..ef76817c362 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5082,13 +5082,8 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
     CmdArgs.push_back("--dependent-lib=oldnames");
   }
 
-  // Both /showIncludes and /E (and /EP) write to stdout. Allowing both
-  // would produce interleaved output, so ignore /showIncludes in such cases.
-  if ((!Args.hasArg(options::OPT_E) && !Args.hasArg(options::OPT__SLASH_EP)) ||
-      (Args.hasArg(options::OPT__SLASH_P) &&
-       Args.hasArg(options::OPT__SLASH_EP) && !Args.hasArg(options::OPT_E)))
-    if (Arg *A = Args.getLastArg(options::OPT_show_includes))
-      A->render(Args, CmdArgs);
+  if (Arg *A = Args.getLastArg(options::OPT_show_includes))
+    A->render(Args, CmdArgs);
 
   // This controls whether or not we emit RTTI data for polymorphic types.
   if (Args.hasFlag(options::OPT__SLASH_GR_, options::OPT__SLASH_GR,
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;