Index: libcxx/src/filesystem/operations.cpp
===================================================================
--- libcxx/src/filesystem/operations.cpp
+++ libcxx/src/filesystem/operations.cpp
@@ -92,6 +92,10 @@
   return false;
 }
 
+static bool isDriveLetter(path::value_type C) {
+  return (C >= 'a' && C <= 'z') || (C >= 'A' && C <= 'Z');
+}
+
 namespace parser {
 
 using string_view_t = path::__string_view;
@@ -148,6 +152,12 @@
 
     switch (State) {
     case PS_BeforeBegin: {
+      PosPtr TkEnd = consumeRootName(Start, End);
+      if (TkEnd)
+        return makeState(PS_InRootName, Start, TkEnd);
+    }
+      _LIBCPP_FALLTHROUGH();
+    case PS_InRootName: {
       PosPtr TkEnd = consumeSeparator(Start, End);
       if (TkEnd)
         return makeState(PS_InRootDir, Start, TkEnd);
@@ -170,7 +180,6 @@
     case PS_InTrailingSep:
       return makeState(PS_AtEnd);
 
-    case PS_InRootName:
     case PS_AtEnd:
       _LIBCPP_UNREACHABLE();
     }
@@ -188,9 +197,15 @@
       if (PosPtr SepEnd = consumeSeparator(RStart, REnd)) {
         if (SepEnd == REnd)
           return makeState(PS_InRootDir, Path.data(), RStart + 1);
+        PosPtr TkStart = consumeRootName(SepEnd, REnd);
+        if (TkStart == REnd)
+          return makeState(PS_InRootDir, RStart, RStart + 1);
         return makeState(PS_InTrailingSep, SepEnd + 1, RStart + 1);
       } else {
-        PosPtr TkStart = consumeName(RStart, REnd);
+        PosPtr TkStart = consumeRootName(RStart, REnd);
+        if (TkStart == REnd)
+          return makeState(PS_InRootName, TkStart + 1, RStart + 1);
+        TkStart = consumeName(RStart, REnd);
         return makeState(PS_InFilenames, TkStart + 1, RStart + 1);
       }
     }
@@ -201,11 +216,17 @@
       PosPtr SepEnd = consumeSeparator(RStart, REnd);
       if (SepEnd == REnd)
         return makeState(PS_InRootDir, Path.data(), RStart + 1);
-      PosPtr TkEnd = consumeName(SepEnd, REnd);
-      return makeState(PS_InFilenames, TkEnd + 1, SepEnd + 1);
+      PosPtr TkStart = consumeRootName(SepEnd ? SepEnd : RStart, REnd);
+      if (TkStart == REnd) {
+        if (SepEnd)
+          return makeState(PS_InRootDir, SepEnd + 1, RStart + 1);
+        return makeState(PS_InRootName, TkStart + 1, RStart + 1);
+      }
+      TkStart = consumeName(SepEnd, REnd);
+      return makeState(PS_InFilenames, TkStart + 1, SepEnd + 1);
     }
     case PS_InRootDir:
-      // return makeState(PS_InRootName, Path.data(), RStart + 1);
+      return makeState(PS_InRootName, Path.data(), RStart + 1);
     case PS_InRootName:
     case PS_BeforeBegin:
       _LIBCPP_UNREACHABLE();
@@ -220,7 +241,10 @@
     case PS_AtEnd:
       return PS("");
     case PS_InRootDir:
-      return PS("/");
+      if (RawEntry[0] == '\\')
+        return PS("\\");
+      else
+        return PS("/");
     case PS_InTrailingSep:
       return PS("");
     case PS_InRootName:
@@ -309,7 +333,7 @@
   }
 
   PosPtr consumeSeparator(PosPtr P, PosPtr End) const noexcept {
-    if (P == End || !isSeparator(*P))
+    if (P == nullptr || P == End || !isSeparator(*P))
       return nullptr;
     const int Inc = P < End ? 1 : -1;
     P += Inc;
@@ -318,15 +342,72 @@
     return P;
   }
 
+  PosPtr consumeSeparators(PosPtr P, PosPtr End, int N) const noexcept {
+    PosPtr Ret = consumeSeparator(P, End);
+    if (Ret == nullptr)
+      return nullptr;
+    if (P < End) {
+      if (Ret == P + N)
+        return Ret;
+    } else {
+      if (Ret == P - N)
+        return Ret;
+    }
+    return nullptr;
+  }
+
   PosPtr consumeName(PosPtr P, PosPtr End) const noexcept {
-    if (P == End || isSeparator(*P))
+    PosPtr Start = P;
+    if (P == nullptr || P == End || isSeparator(*P))
       return nullptr;
     const int Inc = P < End ? 1 : -1;
     P += Inc;
     while (P != End && !isSeparator(*P))
       P += Inc;
+    if (P == End && Inc < 0) {
+      // Iterating backwards and consumed all the rest of the input.
+      // Check if the start of the string would have been considered
+      // a root name.
+      PosPtr RootEnd = consumeRootName(End + 1, Start);
+      if (RootEnd)
+        return RootEnd - 1;
+    }
     return P;
   }
+
+  PosPtr consumeDriveLetter(PosPtr P, PosPtr End) const noexcept {
+    if (P == End)
+      return nullptr;
+    if (P < End) {
+      if (P + 1 == End || !isDriveLetter(P[0]) || P[1] != ':')
+        return nullptr;
+      return P + 2;
+    } else {
+      if (P - 1 == End || !isDriveLetter(P[-1]) || P[0] != ':')
+        return nullptr;
+      return P - 2;
+    }
+  }
+
+  PosPtr consumeNetworkRoot(PosPtr P, PosPtr End) const noexcept {
+    if (P == End)
+      return nullptr;
+    if (P < End) {
+      return consumeName(consumeSeparators(P, End, 2), End);
+    } else {
+      return consumeSeparators(consumeName(P, End), End, 2);
+    }
+  }
+
+  PosPtr consumeRootName(PosPtr P, PosPtr End) const noexcept {
+#if defined(_LIBCPP_WIN32API)
+    if (PosPtr Ret = consumeDriveLetter(P, End))
+      return Ret;
+    if (PosPtr Ret = consumeNetworkRoot(P, End))
+      return Ret;
+#endif
+    return nullptr;
+  }
 };
 
 string_view_pair separate_filename(string_view_t const& s) {