Index: cfe/trunk/include/clang/Basic/VirtualFileSystem.h
===================================================================
--- cfe/trunk/include/clang/Basic/VirtualFileSystem.h
+++ cfe/trunk/include/clang/Basic/VirtualFileSystem.h
@@ -304,15 +304,20 @@
   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;
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override;
 
   using iterator = FileSystemList::reverse_iterator;
-  
+  using const_iterator = FileSystemList::const_reverse_iterator;
+
   /// Get an iterator pointing to the most recently added file system.
   iterator overlays_begin() { return FSList.rbegin(); }
+  const_iterator overlays_begin() const { return FSList.rbegin(); }
 
   /// Get an iterator pointing one-past the least recently added file
   /// system.
   iterator overlays_end() { return FSList.rend(); }
+  const_iterator overlays_end() const { return FSList.rend(); }
 };
 
 namespace detail {
Index: cfe/trunk/lib/Basic/VirtualFileSystem.cpp
===================================================================
--- cfe/trunk/lib/Basic/VirtualFileSystem.cpp
+++ cfe/trunk/lib/Basic/VirtualFileSystem.cpp
@@ -381,6 +381,15 @@
   return {};
 }
 
+std::error_code
+OverlayFileSystem::getRealPath(const Twine &Path,
+                               SmallVectorImpl<char> &Output) const {
+  for (auto &FS : FSList)
+    if (FS->exists(Path))
+      return FS->getRealPath(Path, Output);
+  return errc::no_such_file_or_directory;
+}
+
 clang::vfs::detail::DirIterImpl::~DirIterImpl() = default;
 
 namespace {
Index: cfe/trunk/unittests/Basic/VirtualFileSystemTest.cpp
===================================================================
--- cfe/trunk/unittests/Basic/VirtualFileSystemTest.cpp
+++ cfe/trunk/unittests/Basic/VirtualFileSystemTest.cpp
@@ -67,6 +67,21 @@
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override {
     return std::error_code();
   }
+  // Map any symlink to "/symlink".
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override {
+    auto I = FilesAndDirs.find(Path.str());
+    if (I == FilesAndDirs.end())
+      return make_error_code(llvm::errc::no_such_file_or_directory);
+    if (I->second.isSymlink()) {
+      Output.clear();
+      Twine("/symlink").toVector(Output);
+      return std::error_code();
+    }
+    Output.clear();
+    Path.toVector(Output);
+    return std::error_code();
+  }
 
   struct DirIterImpl : public clang::vfs::detail::DirIterImpl {
     std::map<std::string, vfs::Status> &FilesAndDirs;
@@ -196,6 +211,35 @@
   EXPECT_TRUE(Status->equivalent(*Status2));
 }
 
+TEST(VirtualFileSystemTest, GetRealPathInOverlay) {
+  IntrusiveRefCntPtr<DummyFileSystem> Lower(new DummyFileSystem());
+  Lower->addRegularFile("/foo");
+  Lower->addSymlink("/lower_link");
+  IntrusiveRefCntPtr<DummyFileSystem> Upper(new DummyFileSystem());
+
+  IntrusiveRefCntPtr<vfs::OverlayFileSystem> O(
+      new vfs::OverlayFileSystem(Lower));
+  O->pushOverlay(Upper);
+
+  // Regular file.
+  SmallString<16> RealPath;
+  EXPECT_FALSE(O->getRealPath("/foo", RealPath));
+  EXPECT_EQ(RealPath.str(), "/foo");
+
+  // Expect no error getting real path for symlink in lower overlay.
+  EXPECT_FALSE(O->getRealPath("/lower_link", RealPath));
+  EXPECT_EQ(RealPath.str(), "/symlink");
+
+  // Try a non-existing link.
+  EXPECT_EQ(O->getRealPath("/upper_link", RealPath),
+            errc::no_such_file_or_directory);
+
+  // Add a new symlink in upper.
+  Upper->addSymlink("/upper_link");
+  EXPECT_FALSE(O->getRealPath("/upper_link", RealPath));
+  EXPECT_EQ(RealPath.str(), "/symlink");
+}
+
 TEST(VirtualFileSystemTest, OverlayFiles) {
   IntrusiveRefCntPtr<DummyFileSystem> Base(new DummyFileSystem());
   IntrusiveRefCntPtr<DummyFileSystem> Middle(new DummyFileSystem());