Index: libcxx/include/filesystem
===================================================================
--- libcxx/include/filesystem
+++ libcxx/include/filesystem
@@ -1117,7 +1117,12 @@
   _LIBCPP_INLINE_VISIBILITY
   void clear() noexcept { __pn_.clear(); }
 
-  path& make_preferred() { return *this; }
+  path& make_preferred() {
+#if defined(_LIBCPP_WIN32API)
+    std::replace(__pn_.begin(), __pn_.end(), '/', '\\');
+#endif
+    return *this;
+  }
 
   _LIBCPP_INLINE_VISIBILITY
   path& remove_filename() {
Index: libcxx/src/filesystem/operations.cpp
===================================================================
--- libcxx/src/filesystem/operations.cpp
+++ libcxx/src/filesystem/operations.cpp
@@ -49,6 +49,17 @@
 _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM
 
 namespace {
+
+static bool isSeparator(path::value_type C) {
+  if (C == '/')
+    return true;
+#if defined(_LIBCPP_WIN32API)
+  if (C == '\\')
+    return true;
+#endif
+  return false;
+}
+
 namespace parser {
 
 using string_view_t = path::__string_view;
@@ -266,21 +277,21 @@
   }
 
   PosPtr consumeSeparator(PosPtr P, PosPtr End) const noexcept {
-    if (P == End || *P != '/')
+    if (P == End || !isSeparator(*P))
       return nullptr;
     const int Inc = P < End ? 1 : -1;
     P += Inc;
-    while (P != End && *P == '/')
+    while (P != End && isSeparator(*P))
       P += Inc;
     return P;
   }
 
   PosPtr consumeName(PosPtr P, PosPtr End) const noexcept {
-    if (P == End || *P == '/')
+    if (P == End || isSeparator(*P))
       return nullptr;
     const int Inc = P < End ? 1 : -1;
     P += Inc;
-    while (P != End && *P != '/')
+    while (P != End && !isSeparator(*P))
       P += Inc;
     return P;
   }
@@ -1303,7 +1314,7 @@
   auto PP = PathParser::CreateBegin(__pn_);
   if (PP.State == PathParser::PS_InRootName) {
     auto NextCh = PP.peek();
-    if (NextCh && *NextCh == '/') {
+    if (NextCh && isSeparator(*NextCh)) {
       ++PP;
       return createView(__pn_.data(), &PP.RawEntry.back());
     }
@@ -1401,6 +1412,10 @@
     return PK_DotDot;
   if (Part == PS("/"))
     return PK_RootSep;
+#if defined(_LIBCPP_WIN32API)
+  if (Part == PS("\\"))
+    return PK_RootSep;
+#endif
   return PK_Filename;
 }