Index: include/llvm/Support/VirtualFileSystem.h
===================================================================
--- include/llvm/Support/VirtualFileSystem.h
+++ include/llvm/Support/VirtualFileSystem.h
@@ -258,6 +258,8 @@
   getBufferForFile(const Twine &Name, int64_t FileSize = -1,
                    bool RequiresNullTerminator = true, bool IsVolatile = false);
 
+  virtual llvm::ErrorOr<std::string> getExternalPath(const Twine &Path);
+
   /// Get a directory_iterator for \p Dir.
   /// \note The 'end' iterator is directory_iterator().
   virtual directory_iterator dir_begin(const Twine &Dir,
@@ -326,6 +328,7 @@
   llvm::ErrorOr<Status> status(const Twine &Path) override;
   llvm::ErrorOr<std::unique_ptr<File>>
   openFileForRead(const Twine &Path) override;
+  llvm::ErrorOr<std::string> getExternalPath(const Twine &Path) override;
   directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
@@ -357,6 +360,9 @@
   llvm::ErrorOr<Status> status(const Twine &Path) override {
     return FS->status(Path);
   }
+  llvm::ErrorOr<std::string> getExternalPath(const Twine &Path) override {
+    return FS->getExternalPath(Path);
+  }
   llvm::ErrorOr<std::unique_ptr<File>>
   openFileForRead(const Twine &Path) override {
     return FS->openFileForRead(Path);
Index: lib/Support/VirtualFileSystem.cpp
===================================================================
--- lib/Support/VirtualFileSystem.cpp
+++ lib/Support/VirtualFileSystem.cpp
@@ -120,6 +120,10 @@
   return (*F)->getBuffer(Name, FileSize, RequiresNullTerminator, IsVolatile);
 }
 
+ErrorOr<std::string> FileSystem::getExternalPath(const Twine &path) {
+  return make_error_code(llvm::errc::not_supported);
+}
+
 std::error_code FileSystem::makeAbsolute(SmallVectorImpl<char> &Path) const {
   if (llvm::sys::path::is_absolute(Path))
     return {};
@@ -235,6 +239,7 @@
   ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
   directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
 
+  llvm::ErrorOr<std::string> getExternalPath(const Twine &Path) override;
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
   std::error_code isLocal(const Twine &Path, bool &Result) override;
@@ -303,6 +308,10 @@
   return llvm::sys::fs::real_path(Path, Output);
 }
 
+llvm::ErrorOr<std::string> RealFileSystem::getExternalPath(const Twine &Path) {
+  return Path.str();
+}
+
 IntrusiveRefCntPtr<FileSystem> vfs::getRealFileSystem() {
   static IntrusiveRefCntPtr<FileSystem> FS = new RealFileSystem();
   return FS;
@@ -372,6 +381,13 @@
   return make_error_code(llvm::errc::no_such_file_or_directory);
 }
 
+ErrorOr<std::string> OverlayFileSystem::getExternalPath(const Twine &Path) {
+  for (auto &FS : FSList)
+    if (FS->exists(Path))
+      return FS->getExternalPath(Path);
+  return make_error_code(llvm::errc::not_supported);
+}
+
 llvm::ErrorOr<std::string>
 OverlayFileSystem::getCurrentWorkingDirectory() const {
   // All file systems are synchronized, just take the first working directory.
@@ -1182,6 +1198,7 @@
 
   ErrorOr<Status> status(const Twine &Path) override;
   ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
+  ErrorOr<std::string> getExternalPath(const Twine &Path) override;
 
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override {
     return ExternalFS->getCurrentWorkingDirectory();
@@ -1890,6 +1907,23 @@
       llvm::make_unique<FileWithFixedStatus>(std::move(*Result), S));
 }
 
+ErrorOr<std::string> RedirectingFileSystem::getExternalPath(const Twine &Path) {
+  ErrorOr<Entry *> E = lookupPath(Path);
+  if (!E) {
+    if (IsFallthrough &&
+        E.getError() == llvm::errc::no_such_file_or_directory) {
+      return Path.str();
+    }
+    return E.getError();
+  }
+
+  auto *F = dyn_cast<RedirectingFileEntry>(*E);
+  if (!F)
+    return make_error_code(llvm::errc::not_supported);
+
+  return ExternalFS->getExternalPath(F->getExternalContentsPath());
+}
+
 IntrusiveRefCntPtr<FileSystem>
 vfs::getVFSFromYAML(std::unique_ptr<MemoryBuffer> Buffer,
                     SourceMgr::DiagHandlerTy DiagHandler,
Index: unittests/Support/VirtualFileSystemTest.cpp
===================================================================
--- unittests/Support/VirtualFileSystemTest.cpp
+++ unittests/Support/VirtualFileSystemTest.cpp
@@ -1775,3 +1775,44 @@
   checkContents(FS->dir_begin("//root/foo", EC),
                 {"//root/foo/a", "//root/foo/b"});
 }
+
+TEST_F(VFSFromYAMLTest, ExternalPath) {
+  ScopedDir TestDirectory("virtual-file-system-test", /*Unique*/ true);
+  ScopedDir _a(TestDirectory + "/a");
+  ScopedDir _b(TestDirectory + "/b");
+
+  IntrusiveRefCntPtr<vfs::FileSystem> RealFS = vfs::getRealFileSystem();
+
+  std::string Root = StringRef(TestDirectory).str();
+  // clang-format off
+  std::string Yaml = "{ 'roots': [\n"
+                     "{\n"
+                     "  'type': 'directory',\n"
+                     "  'name': '/',\n"
+                     "  'contents': [ {\n"
+                     "                  'type': 'file',\n"
+                     "                  'name': 'foo',\n"
+                     "                  'external-contents': '"+ Root +"/a'\n"
+                     "                },\n"
+                     "                {\n"
+                     "                  'type': 'file',\n"
+                     "                  'name': 'bar',\n"
+                     "                  'external-contents': '"+ Root +"/b'\n"
+                     "                }\n"
+                     "              ]\n"
+                     "}\n"
+                     "]\n"
+                     "}";
+  // clang-format on
+
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(Yaml, RealFS);
+  ASSERT_TRUE(FS.get() != nullptr);
+
+  auto A = FS->getExternalPath("/foo");
+  ASSERT_TRUE(A);
+  ASSERT_EQ(Root + "/a", *A);
+
+  auto B = FS->getExternalPath("/bar");
+  ASSERT_TRUE(B);
+  ASSERT_EQ(Root + "/b", *B);
+}