Index: clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
===================================================================
--- clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
+++ clang/lib/DirectoryWatcher/windows/DirectoryWatcher-windows.cpp
@@ -6,18 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-// TODO: This is not yet an implementation, but it will make it so Windows
-//       builds don't fail.
-
 #include "DirectoryScanner.h"
 #include "clang/DirectoryWatcher/DirectoryWatcher.h"
-
 #include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/ScopeExit.h"
-#include "llvm/Support/AlignOf.h"
-#include "llvm/Support/Errno.h"
-#include "llvm/Support/Mutex.h"
+#include "llvm/Support/ConvertUTF.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Windows/WindowsSupport.h"
 #include <atomic>
 #include <condition_variable>
 #include <mutex>
@@ -28,23 +22,268 @@
 
 namespace {
 
+using DirectoryWatcherCallback =
+    std::function<void(llvm::ArrayRef<clang::DirectoryWatcher::Event>, bool)>;
+
 using namespace llvm;
 using namespace clang;
 
 class DirectoryWatcherWindows : public clang::DirectoryWatcher {
+  OVERLAPPED Overlapped;
+
+  std::vector<DWORD> Notifications;
+
+  std::thread WatcherThread;
+  std::thread HandlerThread;
+  std::function<void(ArrayRef<DirectoryWatcher::Event>, bool)> Callback;
+  SmallString<MAX_PATH> Path;
+  HANDLE Terminate;
+
+  class EventQueue {
+    std::mutex M;
+    std::queue<DirectoryWatcher::Event> Q;
+    std::condition_variable CV;
+
+  public:
+    void emplace(DirectoryWatcher::Event::EventKind Kind, StringRef Path) {
+      {
+        std::unique_lock<std::mutex> L(M);
+        Q.emplace(Kind, Path);
+      }
+      CV.notify_one();
+    }
+
+    DirectoryWatcher::Event pop_front() {
+      std::unique_lock<std::mutex> L(M);
+      while (true) {
+        if (!Q.empty()) {
+          DirectoryWatcher::Event E = Q.front();
+          Q.pop();
+          return E;
+        }
+        CV.wait(L, [this]() { return !Q.empty(); });
+      }
+    }
+  } Q;
+
 public:
-  ~DirectoryWatcherWindows() override { }
-  void InitialScan() { }
-  void EventReceivingLoop() { }
-  void StopWork() { }
+  DirectoryWatcherWindows(HANDLE DirectoryHandle, bool WaitForInitialSync,
+                          DirectoryWatcherCallback Receiver);
+
+  ~DirectoryWatcherWindows() override;
+
+  void InitialScan();
+  void WatcherThreadProc(HANDLE DirectoryHandle);
+  void NotifierThreadProc(bool WaitForInitialSync);
 };
+
+DirectoryWatcherWindows::DirectoryWatcherWindows(
+    HANDLE DirectoryHandle, bool WaitForInitialSync,
+    DirectoryWatcherCallback Receiver)
+    : Callback(Receiver), Terminate(INVALID_HANDLE_VALUE) {
+  // Pre-compute the real location as we will be handing over the directory
+  // handle to the watcher and performing synchronous operations.
+  {
+    DWORD Length = GetFinalPathNameByHandleW(DirectoryHandle, NULL, 0, 0);
+
+    std::vector<WCHAR> Buffer;
+    Buffer.resize(Length);
+
+    Length = GetFinalPathNameByHandleW(DirectoryHandle, Buffer.data(),
+                                       Buffer.size(), 0);
+    Buffer.resize(Length);
+
+    llvm::sys::windows::UTF16ToUTF8(Buffer.data(), Buffer.size(), Path);
+  }
+
+  Notifications.resize(4 * (sizeof(FILE_NOTIFY_INFORMATION) +
+                            MAX_PATH * sizeof(WCHAR)));
+
+  memset(&Overlapped, 0, sizeof(Overlapped));
+  Overlapped.hEvent =
+      CreateEventW(NULL, /*bManualReset=*/TRUE, /*bInitialState=*/FALSE, NULL);
+  assert(Overlapped.hEvent && "unable to create event");
+
+  Terminate = CreateEventW(NULL, /*bManualReset=*/TRUE,
+                           /*bInitialState=*/FALSE, NULL);
+
+  WatcherThread = std::thread([this, DirectoryHandle]() {
+    this->WatcherThreadProc(DirectoryHandle);
+  });
+
+  if (WaitForInitialSync)
+    InitialScan();
+
+  HandlerThread = std::thread([this, WaitForInitialSync]() {
+    this->NotifierThreadProc(WaitForInitialSync);
+  });
+}
+
+DirectoryWatcherWindows::~DirectoryWatcherWindows() {
+  // Signal the Watcher to exit.
+  SetEvent(Terminate);
+  HandlerThread.join();
+  WatcherThread.join();
+  CloseHandle(Terminate);
+  CloseHandle(Overlapped.hEvent);
+}
+
+void DirectoryWatcherWindows::InitialScan() {
+  Callback(getAsFileEvents(scanDirectory(Path.data())), /*IsInitial=*/true);
+}
+
+void DirectoryWatcherWindows::WatcherThreadProc(HANDLE DirectoryHandle) {
+  while (true) {
+    // We do not guarantee subdirectories, but macOS already provides
+    // subdirectories, might as well as ...
+    BOOL WatchSubtree = TRUE;
+    DWORD NotifyFilter = FILE_NOTIFY_CHANGE_FILE_NAME
+                       | FILE_NOTIFY_CHANGE_DIR_NAME
+                       | FILE_NOTIFY_CHANGE_SIZE
+                       | FILE_NOTIFY_CHANGE_LAST_ACCESS
+                       | FILE_NOTIFY_CHANGE_LAST_WRITE
+                       | FILE_NOTIFY_CHANGE_CREATION;
+
+    DWORD BytesTransferred;
+    if (!ReadDirectoryChangesW(DirectoryHandle, Notifications.data(),
+                               Notifications.size(), WatchSubtree,
+                               NotifyFilter, &BytesTransferred, &Overlapped,
+                               NULL)) {
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      break;
+    }
+
+    HANDLE Handles[2] = { Terminate, DirectoryHandle };
+    switch (WaitForMultipleObjects(2, Handles, FALSE, INFINITE)) {
+    case WAIT_OBJECT_0: // Terminate Request
+    case WAIT_FAILED:   // Failure
+      (void)CloseHandle(DirectoryHandle);
+      return;
+    case WAIT_TIMEOUT:  // Spurious wakeup?
+      continue;
+    case WAIT_OBJECT_0 + 1: // Directory change
+      break;
+    }
+
+    if (!GetOverlappedResult(DirectoryHandle, &Overlapped, &BytesTransferred,
+                             TRUE)) {
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      break;
+    }
+
+    // There was a buffer underrun on the kernel side.  We may have lost
+    // events, please re-synchronize.
+    if (BytesTransferred == 0) {
+      Q.emplace(DirectoryWatcher::Event::EventKind::WatcherGotInvalidated,
+                "");
+      break;
+    }
+
+    for (FILE_NOTIFY_INFORMATION *I =
+            (FILE_NOTIFY_INFORMATION *)Notifications.data();
+         I;
+         I = I->NextEntryOffset
+              ? (FILE_NOTIFY_INFORMATION *)((CHAR *)I + I->NextEntryOffset)
+              : NULL) {
+
+      DirectoryWatcher::Event::EventKind Kind =
+          DirectoryWatcher::Event::EventKind::WatcherGotInvalidated;
+      switch (I->Action) {
+      case FILE_ACTION_MODIFIED:
+        Kind = DirectoryWatcher::Event::EventKind::Modified;
+        break;
+      case FILE_ACTION_ADDED:
+        Kind = DirectoryWatcher::Event::EventKind::Modified;
+        break;
+      case FILE_ACTION_REMOVED:
+        Kind = DirectoryWatcher::Event::EventKind::Removed;
+        break;
+      case FILE_ACTION_RENAMED_OLD_NAME:
+        Kind = DirectoryWatcher::Event::EventKind::Removed;
+        break;
+      case FILE_ACTION_RENAMED_NEW_NAME:
+        Kind = DirectoryWatcher::Event::EventKind::Modified;
+        break;
+      }
+
+      SmallString<MAX_PATH> filename;
+      sys::windows::UTF16ToUTF8(I->FileName, I->FileNameLength / 2,
+                                filename);
+      Q.emplace(Kind, filename);
+    }
+  }
+
+  (void)CloseHandle(DirectoryHandle);
+}
+
+void DirectoryWatcherWindows::NotifierThreadProc(bool WaitForInitialSync) {
+  // If we did not wait for the initial sync, then we should perform the
+  // scan when we enter the thread.
+  if (!WaitForInitialSync)
+    this->InitialScan();
+
+  while (true) {
+    DirectoryWatcher::Event E = Q.pop_front();
+    Callback(E, /*IsInitial=*/false);
+    if (E.Kind == DirectoryWatcher::Event::EventKind::WatcherGotInvalidated)
+      break;
+  }
+}
+
+auto error(DWORD ErrorCode) {
+  DWORD Flags = FORMAT_MESSAGE_ALLOCATE_BUFFER
+              | FORMAT_MESSAGE_FROM_SYSTEM
+              | FORMAT_MESSAGE_IGNORE_INSERTS;
+
+  LPSTR Buffer;
+  if (!FormatMessageA(Flags, NULL, ErrorCode,
+                      MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&Buffer,
+                      0, NULL)) {
+    return make_error<llvm::StringError>("error " + utostr(ErrorCode),
+                                         inconvertibleErrorCode());
+  }
+  std::string Message{Buffer};
+  LocalFree(Buffer);
+  return make_error<llvm::StringError>(Message, inconvertibleErrorCode());
+}
+
 } // namespace
 
 llvm::Expected<std::unique_ptr<DirectoryWatcher>>
-clang::DirectoryWatcher::create(
-    StringRef Path,
-    std::function<void(llvm::ArrayRef<DirectoryWatcher::Event>, bool)> Receiver,
-    bool WaitForInitialSync) {
-  return llvm::Expected<std::unique_ptr<DirectoryWatcher>>(
-      llvm::errorCodeToError(std::make_error_code(std::errc::not_supported)));
+clang::DirectoryWatcher::create(StringRef Path,
+                                DirectoryWatcherCallback Receiver,
+                                bool WaitForInitialSync) {
+  if (Path.empty())
+    llvm::report_fatal_error(
+        "DirectoryWatcher::create can not accept an empty Path.");
+
+  if (!sys::fs::is_directory(Path))
+    llvm::report_fatal_error(
+        "DirectoryWatcher::create can not accept a filepath.");
+
+  SmallVector<wchar_t, MAX_PATH> WidePath;
+  if (sys::windows::UTF8ToUTF16(Path, WidePath))
+    return llvm::make_error<llvm::StringError>(
+        "unable to convert path to UTF-16", llvm::inconvertibleErrorCode());
+
+  DWORD DesiredAccess = FILE_LIST_DIRECTORY;
+  DWORD ShareMode = FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE;
+  DWORD CreationDisposition = OPEN_EXISTING;
+  DWORD FlagsAndAttributes = FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED;
+
+  HANDLE DirectoryHandle =
+      CreateFileW(WidePath.data(), DesiredAccess, ShareMode,
+                  /*lpSecurityAttributes=*/NULL, CreationDisposition,
+                  FlagsAndAttributes, NULL);
+  if (DirectoryHandle == INVALID_HANDLE_VALUE)
+    return error(GetLastError());
+
+  // NOTE: We use the watcher instance as a RAII object to discard the handles
+  // for the directory and the IOCP in case of an error.  Hence, this is early
+  // allocated, with the state being written directly to the watcher.
+  return std::make_unique<DirectoryWatcherWindows>(DirectoryHandle,
+                                                   WaitForInitialSync,
+                                                   Receiver);
 }
Index: clang/unittests/DirectoryWatcher/CMakeLists.txt
===================================================================
--- clang/unittests/DirectoryWatcher/CMakeLists.txt
+++ clang/unittests/DirectoryWatcher/CMakeLists.txt
@@ -1,4 +1,4 @@
-if(APPLE OR CMAKE_SYSTEM_NAME MATCHES "Linux")
+if(APPLE OR CMAKE_SYSTEM_NAME MATCHES "Linux" OR CMAKE_SYSTEM_NAME STREQUAL Windows)
 
   set(LLVM_LINK_COMPONENTS
     Support