Index: include/clang/Frontend/DependencyOutputOptions.h
===================================================================
--- include/clang/Frontend/DependencyOutputOptions.h
+++ include/clang/Frontend/DependencyOutputOptions.h
@@ -15,6 +15,9 @@
 
 namespace clang {
 
+/// ShowIncludesDestination - Destination for /showIncludes output.
+enum class ShowIncludesDestination { None, Stdout, Stderr };
+
 /// DependencyOutputFormat - Format for the compiler dependency file.
 enum class DependencyOutputFormat { Make, NMake };
 
@@ -28,9 +31,11 @@
                                      /// dependency, which can avoid some 'make'
                                      /// problems.
   unsigned AddMissingHeaderDeps : 1; ///< Add missing headers to dependency list
-  unsigned PrintShowIncludes : 1; ///< Print cl.exe style /showIncludes info.
   unsigned IncludeModuleFiles : 1; ///< Include module file dependencies.
 
+  /// Destination of cl.exe style /showIncludes info.
+  ShowIncludesDestination ShowIncludesDestination;
+
   /// The format for the dependency file.
   DependencyOutputFormat OutputFormat;
 
@@ -65,8 +70,8 @@
     ShowHeaderIncludes = 0;
     UsePhonyTargets = 0;
     AddMissingHeaderDeps = 0;
-    PrintShowIncludes = 0;
     IncludeModuleFiles = 0;
+    ShowIncludesDestination = ShowIncludesDestination::None;
     OutputFormat = DependencyOutputFormat::Make;
   }
 };
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -5077,13 +5077,8 @@
     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,
Index: lib/Frontend/CompilerInstance.cpp
===================================================================
--- lib/Frontend/CompilerInstance.cpp
+++ lib/Frontend/CompilerInstance.cpp
@@ -462,7 +462,7 @@
                            /*ShowDepth=*/false);
   }
 
-  if (DepOpts.PrintShowIncludes) {
+  if (DepOpts.ShowIncludesDestination != ShowIncludesDestination::None) {
     AttachHeaderIncludeGen(*PP, DepOpts,
                            /*ShowAllHeaders=*/true, /*OutputPath=*/"",
                            /*ShowDepth=*/true, /*MSStyle=*/true);
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1130,7 +1130,17 @@
   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.ShowIncludesDestination = ShowIncludesDestination::Stderr;
+    else
+      Opts.ShowIncludesDestination = ShowIncludesDestination::Stdout;
+  } else {
+    Opts.ShowIncludesDestination = ShowIncludesDestination::None;
+  }
   Opts.DOTOutputFile = Args.getLastArgValue(OPT_dependency_dot);
   Opts.ModuleDependencyOutputDir =
       Args.getLastArgValue(OPT_module_dependency_dir);
Index: lib/Frontend/HeaderIncludeGen.cpp
===================================================================
--- lib/Frontend/HeaderIncludeGen.cpp
+++ lib/Frontend/HeaderIncludeGen.cpp
@@ -80,9 +80,23 @@
                                    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.ShowIncludesDestination) {
+    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;
Index: test/Driver/cl-options.c
===================================================================
--- test/Driver/cl-options.c
+++ test/Driver/cl-options.c
@@ -198,10 +198,8 @@
 // RUN: %clang_cl /E /showIncludes -### -- %s 2>&1 | FileCheck -check-prefix=showIncludes_E %s
 // RUN: %clang_cl /EP /showIncludes -### -- %s 2>&1 | FileCheck -check-prefix=showIncludes_E %s
 // RUN: %clang_cl /E /EP /showIncludes -### -- %s 2>&1 | FileCheck -check-prefix=showIncludes_E %s
-// showIncludes_E: warning: argument unused during compilation: '--show-includes'
-
-// RUN: %clang_cl /EP /P /showIncludes -### -- %s 2>&1 | FileCheck -check-prefix=showIncludes_E_And_P %s
-// showIncludes_E_And_P-NOT: warning: argument unused during compilation: '--show-includes'
+// RUN: %clang_cl /EP /P /showIncludes -### -- %s 2>&1 | FileCheck -check-prefix=showIncludes_E %s
+// showIncludes_E-NOT: warning: argument unused during compilation: '--show-includes'
 
 // /source-charset: should warn on everything except UTF-8.
 // RUN: %clang_cl /source-charset:utf-16 -### -- %s 2>&1 | FileCheck -check-prefix=source-charset-utf-16 %s