Index: lib/Support/Windows/Path.inc
===================================================================
--- lib/Support/Windows/Path.inc
+++ lib/Support/Windows/Path.inc
@@ -94,13 +94,16 @@
         return EC;
       FullPath.append(CurPath);
     }
-    // Traverse the requested path, canonicalizing . and .. as we go (because
-    // the \\?\ prefix is documented to treat them as real components).
-    // The iterators don't report separators and append() always attaches
-    // preferred_separator so we don't need to call native() on the result.
+    // Traverse the requested path, canonicalizing . and .. (because the \\?\
+    // prefix is documented to treat them as real components).  Ignore
+    // separators, which can be returned from the iterator if the path has a
+    // drive name.  We don't need to call native() on the result since append()
+    // always attaches preferred_separator.
     for (llvm::sys::path::const_iterator I = llvm::sys::path::begin(Path8Str),
                                          E = llvm::sys::path::end(Path8Str);
                                          I != E; ++I) {
+      if (I->size() == 1 && is_separator((*I)[0]))
+        continue;
       if (I->size() == 1 && *I == ".")
         continue;
       if (I->size() == 2 && *I == "..")
Index: unittests/Support/Path.cpp
===================================================================
--- unittests/Support/Path.cpp
+++ unittests/Support/Path.cpp
@@ -699,6 +699,13 @@
     ThisDir = path::parent_path(ThisDir);
   }
 
+  // Also verify that paths with Unix separators are handled correctly.
+  SmallString<270> LongPathWithUnixSeparators("C:");
+  while (LongPathWithUnixSeparators.size() < 260) {
+    LongPathWithUnixSeparators.append("/DirNameWith19Charss");
+  }
+  ASSERT_NO_ERROR(fs::create_directories(Twine(LongPathWithUnixSeparators)));
+
   // Similarly for a relative pathname.  Need to set the current directory to
   // TestDirectory so that the one we create ends up in the right place.
   char PreviousDir[260];