diff --git a/libcxx/src/filesystem/file_descriptor.h b/libcxx/src/filesystem/file_descriptor.h
--- a/libcxx/src/filesystem/file_descriptor.h
+++ b/libcxx/src/filesystem/file_descriptor.h
@@ -129,7 +129,25 @@
   static FileDescriptor create(const path* p, error_code& ec, Args... args) {
     ec.clear();
     int fd;
-    if ((fd = detail::open(p->c_str(), args...)) == -1) {
+#ifdef _LIBCPP_WIN32API
+    // TODO: most of the filesystem implementation uses native Win32 calls
+    // (mostly via posix_compat.h). However, here we use the C-runtime APIs to
+    // open a file, because we subsequently pass the C-runtime fd to
+    // `std::[io]fstream::__open(int fd)` in order to implement copy_file.
+    //
+    // Because we're calling the windows C-runtime, win32 error codes are
+    // translated into C error numbers by the C runtime, and returned in errno,
+    // rather than being accessible directly via GetLastError.
+    //
+    // Ideally copy_file should be calling the Win32 CopyFile2 function, which
+    // works on paths, not open files -- at which point this FileDescriptor type
+    // will no longer be needed on windows at all.
+    fd = ::_wopen(p->c_str(), args...);
+#else
+    fd = open(p->c_str(), args...);
+#endif
+
+    if (fd == -1) {
       ec = capture_errno();
       return FileDescriptor{p};
     }
@@ -154,8 +172,14 @@
   file_status refresh_status(error_code& ec);
 
   void close() noexcept {
-    if (fd != -1)
-      detail::close(fd);
+    if (fd != -1) {
+#ifdef _LIBCPP_WIN32API
+      ::_close(fd);
+#else
+      ::close(fd);
+#endif
+      // FIXME: shouldn't this return an error_code?
+    }
     fd = -1;
   }
 
diff --git a/libcxx/src/filesystem/posix_compat.h b/libcxx/src/filesystem/posix_compat.h
--- a/libcxx/src/filesystem/posix_compat.h
+++ b/libcxx/src/filesystem/posix_compat.h
@@ -273,7 +273,9 @@
 
 int mkdir(const wchar_t *path, int permissions) {
   (void)permissions;
-  return _wmkdir(path);
+  if (!CreateDirectoryW(path, nullptr))
+    return set_errno();
+  return 0;
 }
 
 int symlink_file_dir(const wchar_t *oldname, const wchar_t *newname,
@@ -348,11 +350,11 @@
   return 0;
 }
 
-template <class... Args> int open(const wchar_t *filename, Args... args) {
-  return _wopen(filename, args...);
+int chdir(const wchar_t* path) {
+  if (!SetCurrentDirectoryW(path))
+    return set_errno();
+  return 0;
 }
-int close(int fd) { return _close(fd); }
-int chdir(const wchar_t *path) { return _wchdir(path); }
 
 struct StatVFS {
   uint64_t f_frsize;
@@ -387,7 +389,25 @@
   return 0;
 }
 
-wchar_t *getcwd(wchar_t *buff, size_t size) { return _wgetcwd(buff, size); }
+wchar_t* getcwd(wchar_t* in_buf, size_t in_size) {
+  // Only expected to be used with us allocating the buffer.
+  _LIBCPP_ASSERT(in_buf == nullptr, "Windows getcwd() assumes in_buf==nullptr");
+  _LIBCPP_ASSERT(in_size == 0, "Windows getcwd() assumes in_size==0");
+
+  size_t buff_size = MAX_PATH + 10;
+  std::unique_ptr<wchar_t, decltype(&::free)> buff(static_cast<wchar_t*>(malloc(buff_size * sizeof(wchar_t))), &::free);
+  DWORD retval = GetCurrentDirectoryW(buff_size, buff.get());
+  if (retval > buff_size) {
+    buff_size = retval;
+    buff.reset(static_cast<wchar_t*>(malloc(buff_size * sizeof(wchar_t))));
+    retval = GetCurrentDirectoryW(buff_size, buff.get());
+  }
+  if (!retval) {
+    set_errno();
+    return nullptr;
+  }
+  return buff.release();
+}
 
 wchar_t *realpath(const wchar_t *path, wchar_t *resolved_name) {
   // Only expected to be used with us allocating the buffer.
@@ -530,7 +550,6 @@
   return ::symlink(oldname, newname);
 }
 using ::chdir;
-using ::close;
 using ::fchmod;
 #if defined(AT_SYMLINK_NOFOLLOW) && defined(AT_FDCWD)
 using ::fchmodat;
@@ -541,7 +560,6 @@
 using ::link;
 using ::lstat;
 using ::mkdir;
-using ::open;
 using ::readlink;
 using ::realpath;
 using ::remove;