Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -547,6 +547,13 @@
     return SourceRange(mapLocationToPreamble(R.getBegin()),
                        mapLocationToPreamble(R.getEnd()));
   }
+
+  std::chrono::steady_clock::time_point getPreambleCreationTimePoint() const
+  {
+      if (Preamble)
+          return Preamble->getCreationTimePoint();
+      return std::chrono::steady_clock::time_point();
+  }
   
   // Retrieve the diagnostics associated with this AST
   typedef StoredDiagnostic *stored_diag_iterator;
Index: include/clang/Frontend/PrecompiledPreamble.h
===================================================================
--- include/clang/Frontend/PrecompiledPreamble.h
+++ include/clang/Frontend/PrecompiledPreamble.h
@@ -93,7 +93,7 @@
   /// MainFileBuffer) of the main file.
   bool CanReuse(const CompilerInvocation &Invocation,
                 const llvm::MemoryBuffer *MainFileBuffer, PreambleBounds Bounds,
-                vfs::FileSystem *VFS) const;
+                IntrusiveRefCntPtr<vfs::FileSystem> VFS) const;
 
   /// Changes options inside \p CI to use PCH from this preamble. Also remaps
   /// main file to \p MainFileBuffer and updates \p VFS to ensure the preamble
@@ -106,6 +106,10 @@
                            IntrusiveRefCntPtr<vfs::FileSystem> &VFS,
                            llvm::MemoryBuffer *MainFileBuffer) const;
 
+  std::chrono::steady_clock::time_point getCreationTimePoint() const {
+    return CreationTimePoint;
+  }
+
 private:
   PrecompiledPreamble(PCHStorage Storage, std::vector<char> PreambleBytes,
                       bool PreambleEndsAtStartOfLine,
@@ -237,6 +241,7 @@
   std::vector<char> PreambleBytes;
   /// See PreambleBounds::PreambleEndsAtStartOfLine
   bool PreambleEndsAtStartOfLine;
+  std::chrono::steady_clock::time_point CreationTimePoint;
 };
 
 /// A set of callbacks to gather useful information while building a preamble.
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -1244,7 +1244,7 @@
 
   if (Preamble) {
     if (Preamble->CanReuse(PreambleInvocationIn, MainFileBuffer.get(), Bounds,
-                           VFS.get())) {
+                           VFS)) {
       // Okay! We can re-use the precompiled preamble.
 
       // Set the state of the diagnostic object to mimic its state
Index: lib/Frontend/PrecompiledPreamble.cpp
===================================================================
--- lib/Frontend/PrecompiledPreamble.cpp
+++ lib/Frontend/PrecompiledPreamble.cpp
@@ -390,10 +390,10 @@
   return PreambleBounds(PreambleBytes.size(), PreambleEndsAtStartOfLine);
 }
 
-bool PrecompiledPreamble::CanReuse(const CompilerInvocation &Invocation,
-                                   const llvm::MemoryBuffer *MainFileBuffer,
-                                   PreambleBounds Bounds,
-                                   vfs::FileSystem *VFS) const {
+bool PrecompiledPreamble::CanReuse(
+    const CompilerInvocation &Invocation,
+    const llvm::MemoryBuffer *MainFileBuffer, PreambleBounds Bounds,
+    IntrusiveRefCntPtr<vfs::FileSystem> VFS) const {
 
   assert(
       Bounds.Size <= MainFileBuffer->getBufferSize() &&
@@ -434,11 +434,14 @@
         Status.getSize(), llvm::sys::toTimeT(Status.getLastModificationTime()));
   }
 
+  vfs::OverlayFileSystem OFS(VFS);
+  IntrusiveRefCntPtr<vfs::InMemoryFileSystem> IMFS(
+      new vfs::InMemoryFileSystem());
+  OFS.pushOverlay(IMFS);
   for (const auto &RB : PreprocessorOpts.RemappedFileBuffers) {
+    IMFS->addFile(RB.first, 0, llvm::MemoryBuffer::getMemBuffer(""));
     vfs::Status Status;
-    if (!moveOnNoError(VFS->status(RB.first), Status))
-      return false;
-
+    assert(moveOnNoError(IMFS->status(RB.first), Status));
     OverriddenFiles[Status.getUniqueID()] =
         PreambleFileHash::createForMemoryBuffer(RB.second);
   }
@@ -446,7 +449,7 @@
   // Check whether anything has changed.
   for (const auto &F : FilesInPreamble) {
     vfs::Status Status;
-    if (!moveOnNoError(VFS->status(F.first()), Status)) {
+    if (!moveOnNoError(OFS.status(F.first()), Status)) {
       // If we can't stat the file, assume that something horrible happened.
       return false;
     }
@@ -495,7 +498,8 @@
     llvm::StringMap<PreambleFileHash> FilesInPreamble)
     : Storage(std::move(Storage)), FilesInPreamble(std::move(FilesInPreamble)),
       PreambleBytes(std::move(PreambleBytes)),
-      PreambleEndsAtStartOfLine(PreambleEndsAtStartOfLine) {
+      PreambleEndsAtStartOfLine(PreambleEndsAtStartOfLine),
+      CreationTimePoint(std::chrono::steady_clock::now()){
   assert(this->Storage.getKind() != PCHStorage::Kind::Empty);
 }
 
Index: unittests/Frontend/PCHPreambleTest.cpp
===================================================================
--- unittests/Frontend/PCHPreambleTest.cpp
+++ unittests/Frontend/PCHPreambleTest.cpp
@@ -124,6 +124,23 @@
   }
 };
 
+TEST_F(PCHPreambleTest, ReparseDoesNotInvalidatePreambleDueToNotExistingUnsavedFile) {
+  std::string Header1 = "//./header1.h";
+  std::string MainName = "//./main.cpp";
+  AddFile(MainName, "#include \"//./header1.h\"\n"
+                    "int main() { return ZERO; }");
+
+  std::unique_ptr<ASTUnit> AST(ParseAST(MainName));
+  ASSERT_TRUE(AST.get());
+  auto InitialPreambleCreationTimePoint = AST->getPreambleCreationTimePoint();
+  ASSERT_NE(std::chrono::steady_clock::time_point(), InitialPreambleCreationTimePoint);
+
+  RemapFile(Header1, "#define ZERO 0\n");
+  ASSERT_TRUE(ReparseAST(AST));
+
+  ASSERT_EQ(InitialPreambleCreationTimePoint, AST->getPreambleCreationTimePoint());
+}
+
 TEST_F(PCHPreambleTest, ReparseWithOverriddenFileDoesNotInvalidatePreamble) {
   std::string Header1 = "//./header1.h";
   std::string Header2 = "//./header2.h";