Index: lib/Driver/ToolChains/MinGW.cpp
===================================================================
--- lib/Driver/ToolChains/MinGW.cpp
+++ lib/Driver/ToolChains/MinGW.cpp
@@ -453,11 +453,14 @@
       DriverArgs.hasArg(options::OPT_nostdincxx))
     return;
 
+  StringRef Slash = llvm::sys::path::get_separator();
+
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx:
+    addSystemInclude(DriverArgs, CC1Args, Base + Arch + Slash + "include" +
+                                              Slash + "c++" + Slash + "v1");
     addSystemInclude(DriverArgs, CC1Args,
-                     Base + "include" + llvm::sys::path::get_separator() +
-                         "c++" + llvm::sys::path::get_separator() + "v1");
+                     Base + "include" + Slash + "c++" + Slash + "v1");
     break;
 
   case ToolChain::CST_Libstdcxx:
@@ -472,7 +475,7 @@
     llvm::sys::path::append(CppIncludeBases[3], "include", "c++");
     for (auto &CppIncludeBase : CppIncludeBases) {
       addSystemInclude(DriverArgs, CC1Args, CppIncludeBase);
-      CppIncludeBase += llvm::sys::path::get_separator();
+      CppIncludeBase += Slash;
       addSystemInclude(DriverArgs, CC1Args, CppIncludeBase + Arch);
       addSystemInclude(DriverArgs, CC1Args, CppIncludeBase + "backward");
     }
Index: test/Driver/mingw.cpp
===================================================================
--- test/Driver/mingw.cpp
+++ test/Driver/mingw.cpp
@@ -3,6 +3,10 @@
 // CHECK_MINGW_CLANG_TREE: "{{.*}}/Inputs/mingw_clang_tree/mingw32{{/|\\\\}}include"
 
 
+// RUN: %clang -target i686-windows-gnu -rtlib=platform -stdlib=libc++ -c -### --sysroot=%S/Inputs/mingw_clang_tree/mingw32 %s 2>&1 | FileCheck -check-prefix=CHECK_MINGW_CLANG_TREE_LIBCXX %s
+// CHECK_MINGW_CLANG_TREE_LIBCXX: "{{.*}}/Inputs/mingw_clang_tree/mingw32{{/|\\\\}}i686-w64-mingw32{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}v1"
+
+
 // RUN: %clang -target i686-pc-windows-gnu -rtlib=platform -stdlib=libstdc++ -c -### --sysroot=%S/Inputs/mingw_mingw_org_tree/mingw %s 2>&1 | FileCheck -check-prefix=CHECK_MINGW_ORG_TREE %s
 // CHECK_MINGW_ORG_TREE: "{{.*}}/Inputs/mingw_mingw_org_tree/mingw{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}mingw32{{/|\\\\}}4.8.1{{/|\\\\}}include{{/|\\\\}}c++"
 // CHECK_MINGW_ORG_TREE: "{{.*}}/Inputs/mingw_mingw_org_tree/mingw{{/|\\\\}}lib{{/|\\\\}}gcc{{/|\\\\}}mingw32{{/|\\\\}}4.8.1{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}mingw32"