diff --git a/llvm/include/llvm/Support/VirtualFileSystem.h b/llvm/include/llvm/Support/VirtualFileSystem.h
--- a/llvm/include/llvm/Support/VirtualFileSystem.h
+++ b/llvm/include/llvm/Support/VirtualFileSystem.h
@@ -306,6 +306,12 @@
   /// \returns success if \a path has been made absolute, otherwise a
   ///          platform-specific error_code.
   virtual std::error_code makeAbsolute(SmallVectorImpl<char> &Path) const;
+
+  virtual void dump(raw_ostream &OS) const {}
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+  LLVM_DUMP_METHOD void dump() const;
+#endif
 };
 
 /// Gets an \p vfs::FileSystem for the 'real' file system, as seen by
@@ -357,6 +363,8 @@
   using const_iterator = FileSystemList::const_reverse_iterator;
   using reverse_iterator = FileSystemList::iterator;
   using const_reverse_iterator = FileSystemList::const_iterator;
+  using range = iterator_range<iterator>;
+  using const_range = iterator_range<const_iterator>;
 
   /// Get an iterator pointing to the most recently added file system.
   iterator overlays_begin() { return FSList.rbegin(); }
@@ -373,6 +381,12 @@
   /// Get an iterator pointing one-past the most recently added file system.
   reverse_iterator overlays_rend() { return FSList.end(); }
   const_reverse_iterator overlays_rend() const { return FSList.end(); }
+
+  range overlays_range() { return llvm::reverse(FSList); }
+
+  const_range overlays_range() const { return llvm::reverse(FSList); }
+
+  void dump(raw_ostream &OS) const override;
 };
 
 /// By default, this delegates all calls to the underlying file system. This
@@ -520,6 +534,8 @@
                               SmallVectorImpl<char> &Output) const override;
   std::error_code isLocal(const Twine &Path, bool &Result) override;
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
+
+  void dump(raw_ostream &OS) const override;
 };
 
 /// Get a globally unique ID for a virtual file or directory.
@@ -910,11 +926,8 @@
 
   std::vector<llvm::StringRef> getRoots() const;
 
-  void dump(raw_ostream &OS) const;
+  void dump(raw_ostream &OS) const override;
   void dumpEntry(raw_ostream &OS, Entry *E, int NumSpaces = 0) const;
-#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
-  LLVM_DUMP_METHOD void dump() const;
-#endif
 };
 
 /// Collect all pairs of <virtual path, real path> entries from the
diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -151,6 +151,10 @@
   return Status && Status->exists();
 }
 
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+LLVM_DUMP_METHOD void FileSystem::dump() const { dump(dbgs()); }
+#endif
+
 #ifndef NDEBUG
 static bool isTraversalComponent(StringRef Component) {
   return Component.equals("..") || Component.equals(".");
@@ -273,6 +277,8 @@
   std::error_code getRealPath(const Twine &Path,
                               SmallVectorImpl<char> &Output) const override;
 
+  virtual void dump(raw_ostream &OS) const override;
+
 private:
   // If this FS has its own working dir, use it to make Path absolute.
   // The returned twine is safe to use as long as both Storage and Path live.
@@ -354,6 +360,15 @@
   return llvm::sys::fs::real_path(adjustPath(Path, Storage), Output);
 }
 
+void RealFileSystem::dump(raw_ostream &OS) const {
+  OS << "RealFileSystem using ";
+  if (WD)
+    OS << "own";
+  else
+    OS << "process";
+  OS << " CWD\n";
+}
+
 IntrusiveRefCntPtr<FileSystem> vfs::getRealFileSystem() {
   static IntrusiveRefCntPtr<FileSystem> FS(new RealFileSystem(true));
   return FS;
@@ -459,6 +474,15 @@
   return errc::no_such_file_or_directory;
 }
 
+void OverlayFileSystem::dump(raw_ostream &OS) const {
+  OS << "OverlayFileSystem running operations on: \n";
+  for (auto FS : enumerate(overlays_range())) {
+    OS << FS.index() << ") ";
+    FS.value()->dump(OS);
+    OS << "\n";
+  }
+}
+
 llvm::vfs::detail::DirIterImpl::~DirIterImpl() = default;
 
 namespace {
@@ -1047,6 +1071,10 @@
   return {};
 }
 
+void InMemoryFileSystem::dump(raw_ostream &OS) const {
+  OS << "InMemoryFileSystem\n";
+}
+
 } // namespace vfs
 } // namespace llvm
 
@@ -1382,6 +1410,7 @@
 }
 
 void RedirectingFileSystem::dump(raw_ostream &OS) const {
+  OS << "RedirectingFileSystem\n";
   for (const auto &Root : Roots)
     dumpEntry(OS, Root.get());
 }
@@ -1392,23 +1421,29 @@
   StringRef Name = E->getName();
   for (int i = 0, e = NumSpaces; i < e; ++i)
     OS << " ";
-  OS << "'" << Name.str().c_str() << "'"
-     << "\n";
+  OS << "'" << Name.str().c_str() << "'";
 
-  if (E->getKind() == RedirectingFileSystem::EK_Directory) {
-    auto *DE = dyn_cast<RedirectingFileSystem::DirectoryEntry>(E);
-    assert(DE && "Should be a directory");
+  switch (E->getKind()) {
+  case EK_Directory: {
+    auto *DE = cast<RedirectingFileSystem::DirectoryEntry>(E);
 
+    OS << "\n";
     for (std::unique_ptr<Entry> &SubEntry :
          llvm::make_range(DE->contents_begin(), DE->contents_end()))
       dumpEntry(OS, SubEntry.get(), NumSpaces + 2);
+    break;
+  }
+  case EK_DirectoryRemap: {
+    auto *DRE = cast<RedirectingFileSystem::DirectoryRemapEntry>(E);
+    OS << " -> '" << DRE->getExternalContentsPath() << "'\n";
+    break;
+  }
+  case EK_File:
+    OS << "\n";
+    break;
   }
 }
 
-#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
-LLVM_DUMP_METHOD void RedirectingFileSystem::dump() const { dump(dbgs()); }
-#endif
-
 /// A helper class to hold the common YAML parsing state.
 class llvm::vfs::RedirectingFileSystemParser {
   yaml::Stream &Stream;
diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -171,6 +171,8 @@
                   sys::fs::file_type::symlink_file, sys::fs::all_all);
     addEntry(Path, S);
   }
+
+  void dump(raw_ostream &OS) const override { OS << "DummyFileSystem\n"; }
 };
 
 class ErrorDummyFileSystem : public DummyFileSystem {