diff --git a/clang/include/clang/Basic/FileEntry.h b/clang/include/clang/Basic/FileEntry.h
--- a/clang/include/clang/Basic/FileEntry.h
+++ b/clang/include/clang/Basic/FileEntry.h
@@ -31,6 +31,22 @@
 
 namespace clang {
 
+class FileEntryRef;
+
+} // namespace clang
+
+namespace llvm {
+namespace optional_detail {
+
+/// Forward declare a template specialization for OptionalStorage.
+template <>
+class OptionalStorage<clang::FileEntryRef, /*is_trivially_copyable*/ true>;
+
+} // namespace optional_detail
+} // namespace llvm
+
+namespace clang {
+
 class DirectoryEntry;
 class FileEntry;
 
@@ -38,9 +54,9 @@
 /// accessed by the FileManager's client.
 class FileEntryRef {
 public:
-  const StringRef getName() const { return Entry->first(); }
+  StringRef getName() const { return ME->first(); }
   const FileEntry &getFileEntry() const {
-    return *Entry->second->V.get<FileEntry *>();
+    return *ME->second->V.get<FileEntry *>();
   }
 
   inline bool isValid() const;
@@ -49,12 +65,26 @@
   inline const llvm::sys::fs::UniqueID &getUniqueID() const;
   inline time_t getModificationTime() const;
 
+  /// Check if the underlying FileEntry is the same, intentially ignoring
+  /// whether the file was referenced with the same spelling of the filename.
   friend bool operator==(const FileEntryRef &LHS, const FileEntryRef &RHS) {
-    return LHS.Entry == RHS.Entry;
+    return &LHS.getFileEntry() == &RHS.getFileEntry();
+  }
+  friend bool operator==(const FileEntry *LHS, const FileEntryRef &RHS) {
+    return LHS == &RHS.getFileEntry();
+  }
+  friend bool operator==(const FileEntryRef &LHS, const FileEntry *RHS) {
+    return &LHS.getFileEntry() == RHS;
   }
   friend bool operator!=(const FileEntryRef &LHS, const FileEntryRef &RHS) {
     return !(LHS == RHS);
   }
+  friend bool operator!=(const FileEntry *LHS, const FileEntryRef &RHS) {
+    return !(LHS == RHS);
+  }
+  friend bool operator!=(const FileEntryRef &LHS, const FileEntry *RHS) {
+    return !(LHS == RHS);
+  }
 
   struct MapValue;
 
@@ -78,20 +108,190 @@
     MapValue(MapEntry &ME) : V(&ME) {}
   };
 
-private:
-  friend class FileManager;
+  /// Check if RHS referenced the file in exactly the same way.
+  bool isSameRef(const FileEntryRef &RHS) const { return ME == RHS.ME; }
+
+  /// Allow FileEntryRef to degrade into 'const FileEntry*' to facilitate
+  /// incremental adoption.
+  ///
+  /// The goal is to avoid code churn due to dances like the following:
+  /// \code
+  /// // Old code.
+  /// lvalue = rvalue;
+  ///
+  /// // Temporary code from an incremental patch.
+  /// lvalue = &rvalue.getFileEntry();
+  ///
+  /// // Final code.
+  /// lvalue = rvalue;
+  /// \endcode
+  ///
+  /// FIXME: Once FileEntryRef is "everywhere" and FileEntry::LastRef and
+  /// FileEntry::getName have been deleted, delete this implicit conversion.
+  operator const FileEntry *() const { return &getFileEntry(); }
 
   FileEntryRef() = delete;
-  explicit FileEntryRef(const MapEntry &Entry)
-      : Entry(&Entry) {
-    assert(Entry.second && "Expected payload");
-    assert(Entry.second->V && "Expected non-null");
-    assert(Entry.second->V.is<FileEntry *>() && "Expected FileEntry");
+  explicit FileEntryRef(const MapEntry &ME) : ME(&ME) {
+    assert(ME.second && "Expected payload");
+    assert(ME.second->V && "Expected non-null");
+    assert(ME.second->V.is<FileEntry *>() && "Expected FileEntry");
+  }
+
+  /// Expose the underlying MapEntry to simplify packing in a PointerIntPair or
+  /// PointerUnion and allow construction in Optional.
+  const clang::FileEntryRef::MapEntry &getMapEntry() const { return *ME; }
+
+private:
+  friend class llvm::optional_detail::OptionalStorage<
+      FileEntryRef, /*is_trivially_copyable*/ true>;
+  struct optional_none_tag {};
+
+  // Private constructor for use by OptionalStorage.
+  FileEntryRef(optional_none_tag) : ME(nullptr) {}
+  bool hasOptionalValue() const { return ME; }
+
+  const MapEntry *ME;
+};
+
+static_assert(sizeof(FileEntryRef) == sizeof(const FileEntry *),
+              "FileEntryRef must avoid size overhead");
+
+static_assert(std::is_trivially_copyable<FileEntryRef>::value,
+              "FileEntryRef must be trivially copyable");
+
+} // end namespace clang
+
+namespace llvm {
+namespace optional_detail {
+
+/// Customize OptionalStorage<FileEntryRef> to use FileEntryRef and its
+/// optional_none_tag to keep it the size of a single pointer.
+template <> class OptionalStorage<clang::FileEntryRef> {
+  clang::FileEntryRef MaybeRef = clang::FileEntryRef::optional_none_tag{};
+
+public:
+  ~OptionalStorage() = default;
+  constexpr OptionalStorage() noexcept = default;
+  constexpr OptionalStorage(OptionalStorage const &Other) = default;
+  constexpr OptionalStorage(OptionalStorage &&Other) = default;
+  OptionalStorage &operator=(OptionalStorage const &Other) = default;
+  OptionalStorage &operator=(OptionalStorage &&Other) = default;
+
+  template <class... ArgTypes>
+  constexpr explicit OptionalStorage(in_place_t, ArgTypes &&...Args)
+      : MaybeRef(std::forward<ArgTypes>(Args)...) {}
+
+  void reset() noexcept { MaybeRef = clang::FileEntryRef::optional_none_tag(); }
+
+  constexpr bool hasValue() const noexcept {
+    return MaybeRef.hasOptionalValue();
+  }
+
+  clang::FileEntryRef &getValue() LLVM_LVALUE_FUNCTION noexcept {
+    assert(hasValue());
+    return MaybeRef;
+  }
+  constexpr clang::FileEntryRef const &
+  getValue() const LLVM_LVALUE_FUNCTION noexcept {
+    assert(hasValue());
+    return MaybeRef;
+  }
+#if LLVM_HAS_RVALUE_REFERENCE_THIS
+  clang::FileEntryRef &&getValue() &&noexcept {
+    assert(hasValue());
+    return std::move(MaybeRef);
+  }
+#endif
+
+  template <class... Args> void emplace(Args &&...args) {
+    MaybeRef = clang::FileEntryRef(std::forward<Args>(args)...);
   }
 
-  const MapEntry *Entry;
+  OptionalStorage &operator=(clang::FileEntryRef Ref) {
+    MaybeRef = Ref;
+    return *this;
+  }
 };
 
+static_assert(sizeof(Optional<clang::FileEntryRef>) ==
+                  sizeof(clang::FileEntryRef),
+              "Optional<FileEntryRef> must avoid size overhead");
+
+static_assert(std::is_trivially_copyable<Optional<clang::FileEntryRef>>::value,
+              "Optional<FileEntryRef> should be trivially copyable");
+
+} // end namespace optional_detail
+} // end namespace llvm
+
+namespace clang {
+
+/// Wrapper around Optional<FileEntryRef> that degrades to 'const FileEntry*',
+/// facilitating incremental patches to propagate FileEntryRef.
+///
+/// This class can be used as return value or field where it's convenient for
+/// an Optional<FileEntryRef> to degrade to a 'const FileEntry*'. The purpose
+/// is to avoid code churn due to dances like the following:
+/// \code
+/// // Old code.
+/// lvalue = rvalue;
+///
+/// // Temporary code from an incremental patch.
+/// Optional<FileEntryRef> MaybeF = rvalue;
+/// lvalue = MaybeF ? &MaybeF.getFileEntry() : nullptr;
+///
+/// // Final code.
+/// lvalue = rvalue;
+/// \endcode
+///
+/// FIXME: Once FileEntryRef is "everywhere" and FileEntry::LastRef and
+/// FileEntry::getName have been deleted, delete this class and replace
+/// instances with Optional<FileEntryRef>.
+class OptionalFileEntryRefDegradesToFileEntryPtr
+    : public Optional<FileEntryRef> {
+public:
+  constexpr OptionalFileEntryRefDegradesToFileEntryPtr() noexcept = default;
+  constexpr OptionalFileEntryRefDegradesToFileEntryPtr(
+      OptionalFileEntryRefDegradesToFileEntryPtr &&) = default;
+  constexpr OptionalFileEntryRefDegradesToFileEntryPtr(
+      const OptionalFileEntryRefDegradesToFileEntryPtr &) = default;
+  OptionalFileEntryRefDegradesToFileEntryPtr &
+  operator=(OptionalFileEntryRefDegradesToFileEntryPtr &&) = default;
+  OptionalFileEntryRefDegradesToFileEntryPtr &
+  operator=(const OptionalFileEntryRefDegradesToFileEntryPtr &) = default;
+
+  OptionalFileEntryRefDegradesToFileEntryPtr(llvm::NoneType) {}
+  OptionalFileEntryRefDegradesToFileEntryPtr(FileEntryRef Ref)
+      : Optional<FileEntryRef>(Ref) {}
+  OptionalFileEntryRefDegradesToFileEntryPtr(Optional<FileEntryRef> MaybeRef)
+      : Optional<FileEntryRef>(MaybeRef) {}
+
+  OptionalFileEntryRefDegradesToFileEntryPtr &operator=(llvm::NoneType) {
+    Optional<FileEntryRef>::operator=(None);
+    return *this;
+  }
+  OptionalFileEntryRefDegradesToFileEntryPtr &operator=(FileEntryRef Ref) {
+    Optional<FileEntryRef>::operator=(Ref);
+    return *this;
+  }
+  OptionalFileEntryRefDegradesToFileEntryPtr &
+  operator=(Optional<FileEntryRef> MaybeRef) {
+    Optional<FileEntryRef>::operator=(MaybeRef);
+    return *this;
+  }
+
+  /// Degrade to 'const FileEntry *' to allow  FileEntry::LastRef and
+  /// FileEntry::getName have been deleted, delete this class and replace
+  /// instances with Optional<FileEntryRef>
+  operator const FileEntry *() const {
+    return hasValue() ? &getValue().getFileEntry() : nullptr;
+  }
+};
+
+static_assert(
+    std::is_trivially_copyable<
+        OptionalFileEntryRefDegradesToFileEntryPtr>::value,
+    "OptionalFileEntryRefDegradesToFileEntryPtr should be trivially copyable");
+
 /// Cached information about one file (either on disk
 /// or in the virtual file system).
 ///
@@ -147,10 +347,6 @@
   bool isNamedPipe() const { return IsNamedPipe; }
 
   void closeFile() const;
-
-  // Only for use in tests to see if deferred opens are happening, rather than
-  // relying on RealPathName being empty.
-  bool isOpenForTests() const { return File != nullptr; }
 };
 
 bool FileEntryRef::isValid() const { return getFileEntry().isValid(); }
diff --git a/clang/unittests/Basic/CMakeLists.txt b/clang/unittests/Basic/CMakeLists.txt
--- a/clang/unittests/Basic/CMakeLists.txt
+++ b/clang/unittests/Basic/CMakeLists.txt
@@ -5,6 +5,7 @@
 add_clang_unittest(BasicTests
   CharInfoTest.cpp
   DiagnosticTest.cpp
+  FileEntryTest.cpp
   FileManagerTest.cpp
   LineOffsetMappingTest.cpp
   SourceManagerTest.cpp
diff --git a/clang/unittests/Basic/FileEntryTest.cpp b/clang/unittests/Basic/FileEntryTest.cpp
new file mode 100644
--- /dev/null
+++ b/clang/unittests/Basic/FileEntryTest.cpp
@@ -0,0 +1,104 @@
+//===- unittests/Basic/FileEntryTest.cpp - Test FileEntry/FileEntryRef ----===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Basic/FileEntry.h"
+#include "llvm/ADT/StringMap.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace clang;
+
+namespace {
+
+using MapEntry = FileEntryRef::MapEntry;
+using MapValue = FileEntryRef::MapValue;
+using MapType = StringMap<llvm::ErrorOr<MapValue>>;
+
+FileEntryRef addRef(MapType &M, StringRef Name, FileEntry &E) {
+  return FileEntryRef(*M.insert({Name, MapValue(E)}).first);
+}
+
+TEST(FileEntryTest, FileEntryRef) {
+  MapType Refs;
+  FileEntry E1, E2;
+  FileEntryRef R1 = addRef(Refs, "1", E1);
+  FileEntryRef R2 = addRef(Refs, "2", E2);
+  FileEntryRef R1Also = addRef(Refs, "1-also", E1);
+
+  EXPECT_EQ("1", R1.getName());
+  EXPECT_EQ("2", R2.getName());
+  EXPECT_EQ("1-also", R1Also.getName());
+
+  EXPECT_EQ(&E1, &R1.getFileEntry());
+  EXPECT_EQ(&E2, &R2.getFileEntry());
+  EXPECT_EQ(&E1, &R1Also.getFileEntry());
+
+  const FileEntry *CE1 = R1;
+  EXPECT_EQ(CE1, &E1);
+}
+
+TEST(FileEntryTest, OptionalFileEntryRefDegradesToFileEntryPtr) {
+  MapType Refs;
+  FileEntry E1, E2;
+  OptionalFileEntryRefDegradesToFileEntryPtr M0;
+  OptionalFileEntryRefDegradesToFileEntryPtr M1 = addRef(Refs, "1", E1);
+  OptionalFileEntryRefDegradesToFileEntryPtr M2 = addRef(Refs, "2", E2);
+  OptionalFileEntryRefDegradesToFileEntryPtr M0Also = None;
+  OptionalFileEntryRefDegradesToFileEntryPtr M1Also =
+      addRef(Refs, "1-also", E1);
+
+  EXPECT_EQ(M0, M0Also);
+  EXPECT_EQ(StringRef("1"), M1->getName());
+  EXPECT_EQ(StringRef("2"), M2->getName());
+  EXPECT_EQ(StringRef("1-also"), M1Also->getName());
+
+  EXPECT_EQ(&E1, &M1->getFileEntry());
+  EXPECT_EQ(&E2, &M2->getFileEntry());
+  EXPECT_EQ(&E1, &M1Also->getFileEntry());
+
+  const FileEntry *CE1 = M1;
+  EXPECT_EQ(CE1, &E1);
+}
+
+TEST(FileEntryTest, equals) {
+  MapType Refs;
+  FileEntry E1, E2;
+  FileEntryRef R1 = addRef(Refs, "1", E1);
+  FileEntryRef R2 = addRef(Refs, "2", E2);
+  FileEntryRef R1Also = addRef(Refs, "1-also", E1);
+
+  EXPECT_EQ(R1, &E1);
+  EXPECT_EQ(&E1, R1);
+  EXPECT_EQ(R1, R1Also);
+  EXPECT_NE(R1, &E2);
+  EXPECT_NE(&E2, R1);
+  EXPECT_NE(R1, R2);
+
+  OptionalFileEntryRefDegradesToFileEntryPtr M0;
+  OptionalFileEntryRefDegradesToFileEntryPtr M1 = R1;
+
+  EXPECT_EQ(M1, &E1);
+  EXPECT_EQ(&E1, M1);
+  EXPECT_NE(M1, &E2);
+  EXPECT_NE(&E2, M1);
+}
+
+TEST(FileEntryTest, isSameRef) {
+  MapType Refs;
+  FileEntry E1, E2;
+  FileEntryRef R1 = addRef(Refs, "1", E1);
+  FileEntryRef R2 = addRef(Refs, "2", E2);
+  FileEntryRef R1Also = addRef(Refs, "1-also", E1);
+
+  EXPECT_TRUE(R1.isSameRef(FileEntryRef(R1)));
+  EXPECT_TRUE(R1.isSameRef(FileEntryRef(R1.getMapEntry())));
+  EXPECT_FALSE(R1.isSameRef(R2));
+  EXPECT_FALSE(R1.isSameRef(R1Also));
+}
+
+} // end namespace
diff --git a/clang/unittests/Basic/FileManagerTest.cpp b/clang/unittests/Basic/FileManagerTest.cpp
--- a/clang/unittests/Basic/FileManagerTest.cpp
+++ b/clang/unittests/Basic/FileManagerTest.cpp
@@ -350,6 +350,58 @@
             f2 ? *f2 : nullptr);
 }
 
+TEST_F(FileManagerTest, getFileRefEquality) {
+  auto StatCache = std::make_unique<FakeStatCache>();
+  StatCache->InjectDirectory("dir", 40);
+  StatCache->InjectFile("dir/f1.cpp", 41);
+  StatCache->InjectFile("dir/f1-also.cpp", 41);
+  StatCache->InjectFile("dir/f1-redirect.cpp", 41, "dir/f1.cpp");
+  StatCache->InjectFile("dir/f2.cpp", 42);
+  manager.setStatCache(std::move(StatCache));
+
+  auto F1 = manager.getFileRef("dir/f1.cpp");
+  auto F1Again = manager.getFileRef("dir/f1.cpp");
+  auto F1Also = manager.getFileRef("dir/f1-also.cpp");
+  auto F1Redirect = manager.getFileRef("dir/f1-redirect.cpp");
+  auto F2 = manager.getFileRef("dir/f2.cpp");
+
+  // Check Expected<FileEntryRef> for error.
+  ASSERT_FALSE(!F1);
+  ASSERT_FALSE(!F1Also);
+  ASSERT_FALSE(!F1Again);
+  ASSERT_FALSE(!F1Redirect);
+  ASSERT_FALSE(!F2);
+
+  // Check names.
+  EXPECT_EQ("dir/f1.cpp", F1->getName());
+  EXPECT_EQ("dir/f1.cpp", F1Again->getName());
+  EXPECT_EQ("dir/f1-also.cpp", F1Also->getName());
+  EXPECT_EQ("dir/f1.cpp", F1Redirect->getName());
+  EXPECT_EQ("dir/f2.cpp", F2->getName());
+
+  // Compare against FileEntry*.
+  EXPECT_EQ(&F1->getFileEntry(), *F1);
+  EXPECT_EQ(*F1, &F1->getFileEntry());
+  EXPECT_NE(&F2->getFileEntry(), *F1);
+  EXPECT_NE(*F1, &F2->getFileEntry());
+
+  // Compare using ==.
+  EXPECT_EQ(*F1, *F1Also);
+  EXPECT_EQ(*F1, *F1Again);
+  EXPECT_EQ(*F1, *F1Redirect);
+  EXPECT_EQ(*F1Also, *F1Redirect);
+  EXPECT_NE(*F2, *F1);
+  EXPECT_NE(*F2, *F1Also);
+  EXPECT_NE(*F2, *F1Again);
+  EXPECT_NE(*F2, *F1Redirect);
+
+  // Compare using isSameRef.
+  EXPECT_TRUE(F1->isSameRef(*F1Again));
+  EXPECT_TRUE(F1->isSameRef(*F1Redirect));
+  EXPECT_FALSE(F1->isSameRef(*F1Also));
+  EXPECT_FALSE(F1->isSameRef(*F2));
+}
+
 // getFile() Should return the same entry as getVirtualFile if the file actually
 // is a virtual file, even if the name is not exactly the same (but is after
 // normalisation done by the file system, like on Windows). This can be checked