Index: clang-tidy/ClangTidy.cpp
===================================================================
--- clang-tidy/ClangTidy.cpp
+++ clang-tidy/ClangTidy.cpp
@@ -87,10 +87,34 @@
   ClangTidyContext &Context;
 };
 
+void pushVfsOverlayFromFile(StringRef OverlayFile,
+                            DiagnosticsEngine &Diags,
+                            vfs::OverlayFileSystem &OverlayFS) {
+  if (OverlayFile.empty())
+    return;
+
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Buffer =
+      OverlayFS.getBufferForFile(OverlayFile);
+  if (!Buffer) {
+    Diags.Report(diag::err_missing_vfs_overlay_file) << OverlayFile;
+    return;
+  }
+
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = vfs::getVFSFromYAML(
+      std::move(Buffer.get()), /*DiagHandler*/ nullptr, OverlayFile);
+  if (!FS.get()) {
+    Diags.Report(diag::err_invalid_vfs_overlay) << OverlayFile;
+    return;
+  }
+  OverlayFS.pushOverlay(FS);
+}
+
 class ErrorReporter {
 public:
   ErrorReporter(ClangTidyContext &Context, bool ApplyFixes)
-      : Files(FileSystemOptions()), DiagOpts(new DiagnosticOptions()),
+      : OverlayFS(new vfs::OverlayFileSystem(vfs::getRealFileSystem())),
+        Files(FileSystemOptions(), OverlayFS), 
+        DiagOpts(new DiagnosticOptions()),
         DiagPrinter(new TextDiagnosticPrinter(llvm::outs(), &*DiagOpts)),
         Diags(IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs), &*DiagOpts,
               DiagPrinter),
@@ -98,6 +122,11 @@
         TotalFixes(0), AppliedFixes(0), WarningsAsErrors(0) {
     DiagOpts->ShowColors = llvm::sys::Process::StandardOutHasColors();
     DiagPrinter->BeginSourceFile(LangOpts);
+
+    if (Context.getOptions().VfsOverlay) {
+      pushVfsOverlayFromFile(*Context.getOptions().VfsOverlay, Diags,
+                             *OverlayFS);
+    }
   }
 
   SourceManager &getSourceManager() { return SourceMgr; }
@@ -249,6 +278,7 @@
         << Message.Message;
   }
 
+  IntrusiveRefCntPtr<vfs::OverlayFileSystem> OverlayFS;
   FileManager Files;
   LangOptions LangOpts; // FIXME: use langopts from each original file
   IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts;
@@ -475,7 +505,10 @@
 void runClangTidy(clang::tidy::ClangTidyContext &Context,
                   const CompilationDatabase &Compilations,
                   ArrayRef<std::string> InputFiles, ProfileData *Profile) {
-  ClangTool Tool(Compilations, InputFiles);
+  llvm::IntrusiveRefCntPtr<vfs::OverlayFileSystem> OverlayFS(
+      new vfs::OverlayFileSystem(vfs::getRealFileSystem()));
+  ClangTool Tool(Compilations, InputFiles,
+                 std::make_shared<PCHContainerOperations>(), OverlayFS);
 
   // Add extra arguments passed by the clang-tidy command-line.
   ArgumentsAdjuster PerFileExtraArgumentsInserter =
@@ -520,6 +553,11 @@
 
   Tool.setDiagnosticConsumer(&DiagConsumer);
 
+  if (Context.getOptions().VfsOverlay) {
+    pushVfsOverlayFromFile(*Context.getOptions().VfsOverlay,
+                           Context.getDiagnosticsEngine(), *OverlayFS);
+  }
+
   class ActionFactory : public FrontendActionFactory {
   public:
     ActionFactory(ClangTidyContext &Context) : ConsumerFactory(Context) {}
Index: clang-tidy/ClangTidyDiagnosticConsumer.h
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.h
+++ clang-tidy/ClangTidyDiagnosticConsumer.h
@@ -166,6 +166,9 @@
   /// \brief Returns all collected errors.
   ArrayRef<ClangTidyError> getErrors() const { return Errors; }
 
+  /// \brief Returns diagnostics engine
+  DiagnosticsEngine &getDiagnosticsEngine() const { return *DiagEngine; }
+
   /// \brief Clears collected errors.
   void clearErrors() { Errors.clear(); }
 
Index: clang-tidy/ClangTidyOptions.h
===================================================================
--- clang-tidy/ClangTidyOptions.h
+++ clang-tidy/ClangTidyOptions.h
@@ -108,6 +108,10 @@
 
   /// \brief Add extra compilation arguments to the start of the list.
   llvm::Optional<ArgList> ExtraArgsBefore;
+
+  /// \brief Overlay the virtual filesystem described by file 
+  ///        over the real file system
+  llvm::Optional<std::string> VfsOverlay;
 };
 
 /// \brief Abstract interface for retrieving various ClangTidy options.
Index: clang-tidy/ClangTidyOptions.cpp
===================================================================
--- clang-tidy/ClangTidyOptions.cpp
+++ clang-tidy/ClangTidyOptions.cpp
@@ -92,6 +92,7 @@
     IO.mapOptional("CheckOptions", NOpts->Options);
     IO.mapOptional("ExtraArgs", Options.ExtraArgs);
     IO.mapOptional("ExtraArgsBefore", Options.ExtraArgsBefore);
+    IO.mapOptional("VfsOverlay", Options.VfsOverlay);
   }
 };
 
@@ -152,6 +153,7 @@
   overrideValue(Result.User, Other.User);
   mergeVectors(Result.ExtraArgs, Other.ExtraArgs);
   mergeVectors(Result.ExtraArgsBefore, Other.ExtraArgsBefore);
+  overrideValue(Result.VfsOverlay, Other.VfsOverlay);
 
   for (const auto &KeyValue : Other.CheckOptions)
     Result.CheckOptions[KeyValue.first] = KeyValue.second;
Index: clang-tidy/tool/ClangTidyMain.cpp
===================================================================
--- clang-tidy/tool/ClangTidyMain.cpp
+++ clang-tidy/tool/ClangTidyMain.cpp
@@ -209,6 +209,12 @@
                            cl::init(false),
                            cl::cat(ClangTidyCategory));
 
+static cl::opt<std::string> VfsOverlay("vfsoverlay", cl::desc(R"(
+Overlay the virtual filesystem described by file over the real file system.
+)"),
+                           cl::value_desc("filename"),
+                           cl::cat(ClangTidyCategory));
+
 namespace clang {
 namespace tidy {
 
@@ -295,6 +301,7 @@
   DefaultOptions.AnalyzeTemporaryDtors = AnalyzeTemporaryDtors;
   DefaultOptions.FormatStyle = FormatStyle;
   DefaultOptions.User = llvm::sys::Process::GetEnv("USER");
+  DefaultOptions.VfsOverlay = VfsOverlay;
   // USERNAME is used on Windows.
   if (!DefaultOptions.User)
     DefaultOptions.User = llvm::sys::Process::GetEnv("USERNAME");
@@ -312,6 +319,8 @@
     OverrideOptions.AnalyzeTemporaryDtors = AnalyzeTemporaryDtors;
   if (FormatStyle.getNumOccurrences() > 0)
     OverrideOptions.FormatStyle = FormatStyle;
+  if (VfsOverlay.getNumOccurrences() > 0)
+    OverrideOptions.VfsOverlay = VfsOverlay;
 
   if (!Config.empty()) {
     if (llvm::ErrorOr<ClangTidyOptions> ParsedConfig =