diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -423,6 +423,8 @@
 }
 
 bool FileManager::FixupRelativePath(SmallVectorImpl<char> &path) const {
+  using namespace llvm::sys::path;
+
   StringRef pathRef(path.data(), path.size());
 
   if (FileSystemOpts.WorkingDir.empty()
@@ -430,7 +432,19 @@
     return false;
 
   SmallString<128> NewPath(FileSystemOpts.WorkingDir);
-  llvm::sys::path::append(NewPath, pathRef);
+  Style sepStyle = Style::native;
+
+  for (char c : NewPath) {
+    if (is_separator(c, Style::windows)) {
+      sepStyle = Style::windows;
+      break;
+    } else if (is_separator(c, Style::posix)) {
+      sepStyle = Style::posix;
+      break;
+    }
+  }
+
+  append(NewPath, sepStyle, pathRef);
   path = NewPath;
   return true;
 }