Index: libcxx/include/filesystem
===================================================================
--- libcxx/include/filesystem
+++ libcxx/include/filesystem
@@ -1003,13 +1003,51 @@
   }
 
 private:
+#if !defined(_LIBCPP_WIN32API)
   template <class _ECharT>
   static bool __source_is_absolute(_ECharT __first_or_null) {
     return __is_separator(__first_or_null);
   }
+#endif
 
 public:
   // appends
+#if defined(_LIBCPP_WIN32API)
+  path& operator/=(const path& __p) {
+    auto __p_root_name = __p.__root_name();
+    auto __p_root_name_size = __p_root_name.size();
+    if (__p.is_absolute() ||
+        (!__p_root_name.empty() && __p_root_name != root_name())) {
+      __pn_ = __p.__pn_;
+      return *this;
+    }
+    if (__p.has_root_directory()) {
+      path __root_name_str = root_name();
+      __pn_ = __root_name_str.native();
+      __pn_ += __p.__pn_.substr(__p_root_name_size);
+      return *this;
+    }
+    if (has_filename() || (!has_root_directory() && is_absolute()))
+      __pn_ += preferred_separator;
+    __pn_ += __p.__pn_.substr(__p_root_name_size);
+    return *this;
+  }
+  template <class _Source>
+  _LIBCPP_INLINE_VISIBILITY _EnableIfPathable<_Source>
+  operator/=(const _Source& __src) {
+    return operator/=(path(__src));
+  }
+
+  template <class _Source>
+  _EnableIfPathable<_Source> append(const _Source& __src) {
+    return operator/=(path(__src));
+  }
+
+  template <class _InputIt>
+  path& append(_InputIt __first, _InputIt __last) {
+    return operator/=(path(__first, __last));
+  }
+#else
   path& operator/=(const path& __p) {
     if (__p.is_absolute()) {
       __pn_ = __p.__pn_;
@@ -1054,6 +1092,7 @@
     _CVT::__append_range(__pn_, __first, __last);
     return *this;
   }
+#endif
 
   // concatenation
   _LIBCPP_INLINE_VISIBILITY
@@ -1292,7 +1331,11 @@
     return string_type(__root_directory());
   }
   _LIBCPP_INLINE_VISIBILITY path root_path() const {
+#if defined(_LIBCPP_WIN32API)
+    return string_type(__root_path_raw());
+#else
     return root_name().append(string_type(__root_directory()));
+#endif
   }
   _LIBCPP_INLINE_VISIBILITY path relative_path() const {
     return string_type(__relative_path());
Index: libcxx/test/std/input.output/filesystems/class.path/path.member/path.append.pass.cpp
===================================================================
--- libcxx/test/std/input.output/filesystems/class.path/path.member/path.append.pass.cpp
+++ libcxx/test/std/input.output/filesystems/class.path/path.member/path.append.pass.cpp
@@ -65,6 +65,58 @@
       , {S("/p1"), S("/p2/"), S("/p2/")}
       , {S("p1"),   S(""),      S("p1/")}
       , {S("p1/"),  S(""),      S("p1/")}
+
+#ifdef _WIN32
+      , {S("//host"),  S("foo"),      S("//host/foo")}
+      , {S("//host/"), S("foo"),      S("//host/foo")}
+      , {S("//host"),  S(""),         S("//host/")}
+
+      , {S("foo"),     S("C:/bar"),   S("C:/bar")}
+      , {S("foo"),     S("C:"),       S("C:")}
+
+      , {S("C:"),      S(""),         S("C:")}
+      , {S("C:foo"),   S("/bar"),     S("C:/bar")}
+      , {S("C:foo"),   S("bar"),      S("C:foo/bar")}
+      , {S("C:/foo"),  S("bar"),      S("C:/foo/bar")}
+      , {S("C:/foo"),  S("/bar"),     S("C:/bar")}
+
+      , {S("C:foo"),   S("C:/bar"),   S("C:/bar")}
+      , {S("C:foo"),   S("C:bar"),    S("C:foo/bar")}
+      , {S("C:/foo"),  S("C:/bar"),   S("C:/bar")}
+      , {S("C:/foo"),  S("C:bar"),    S("C:/foo/bar")}
+
+      , {S("C:foo"),   S("c:/bar"),   S("c:/bar")}
+      , {S("C:foo"),   S("c:bar"),    S("c:bar")}
+      , {S("C:/foo"),  S("c:/bar"),   S("c:/bar")}
+      , {S("C:/foo"),  S("c:bar"),    S("c:bar")}
+
+      , {S("C:/foo"),  S("D:bar"),    S("D:bar")}
+#else
+      , {S("//host"),  S("foo"),      S("//host/foo")}
+      , {S("//host/"), S("foo"),      S("//host/foo")}
+      , {S("//host"),  S(""),         S("//host/")}
+
+      , {S("foo"),     S("C:/bar"),   S("foo/C:/bar")}
+      , {S("foo"),     S("C:"),       S("foo/C:")}
+
+      , {S("C:"),      S(""),         S("C:/")}
+      , {S("C:foo"),   S("/bar"),     S("/bar")}
+      , {S("C:foo"),   S("bar"),      S("C:foo/bar")}
+      , {S("C:/foo"),  S("bar"),      S("C:/foo/bar")}
+      , {S("C:/foo"),  S("/bar"),     S("/bar")}
+
+      , {S("C:foo"),   S("C:/bar"),   S("C:foo/C:/bar")}
+      , {S("C:foo"),   S("C:bar"),    S("C:foo/C:bar")}
+      , {S("C:/foo"),  S("C:/bar"),   S("C:/foo/C:/bar")}
+      , {S("C:/foo"),  S("C:bar"),    S("C:/foo/C:bar")}
+
+      , {S("C:foo"),   S("c:/bar"),   S("C:foo/c:/bar")}
+      , {S("C:foo"),   S("c:bar"),    S("C:foo/c:bar")}
+      , {S("C:/foo"),  S("c:/bar"),   S("C:/foo/c:/bar")}
+      , {S("C:/foo"),  S("c:bar"),    S("C:/foo/c:bar")}
+
+      , {S("C:/foo"),  S("D:bar"),    S("C:/foo/D:bar")}
+#endif
     };