Index: llvm/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/lib/Support/VirtualFileSystem.cpp
+++ llvm/lib/Support/VirtualFileSystem.cpp
@@ -1682,14 +1682,23 @@
   if (std::error_code EC = makeAbsolute(Path))
     return EC;
 
+  // Now that we have an absolute path we can examine it to determine the
+  // path style.
+  sys::path::Style path_style = sys::path::Style::native;
+  if (sys::path::is_absolute(Path, sys::path::Style::posix)) {
+    path_style = sys::path::Style::posix;
+  } else if (sys::path::is_absolute(Path, sys::path::Style::windows)) {
+    path_style = sys::path::Style::windows;
+  }
+
   // Canonicalize path by removing ".", "..", "./", components. This is
   // a VFS request, do not bother about symlinks in the path components
   // but canonicalize in order to perform the correct entry search.
-  Path = canonicalize(Path);
+  llvm::sys::path::remove_dots(Path, /*remove_dot_dot=*/true, path_style);
   if (Path.empty())
     return make_error_code(llvm::errc::invalid_argument);
 
-  sys::path::const_iterator Start = sys::path::begin(Path);
+  sys::path::const_iterator Start = sys::path::begin(Path, path_style);
   sys::path::const_iterator End = sys::path::end(Path);
   for (const auto &Root : Roots) {
     ErrorOr<RedirectingFileSystem::Entry *> Result =
Index: llvm/unittests/Support/VirtualFileSystemTest.cpp
===================================================================
--- llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -2188,3 +2188,93 @@
   Status = FS->status("foo/a");
   ASSERT_TRUE(Status.getError());
 }
+
+TEST_F(VFSFromYAMLTest, BackslashWindowsPath) {
+  IntrusiveRefCntPtr<DummyFileSystem> Lower(new DummyFileSystem());
+  Lower->addRegularFile("c:\\root\\foo\\bar\\a");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(
+      "{ 'roots': [\n"
+      "{\n"
+      "  'type': 'directory',\n"
+      "  'name': 'c:\\root\\',\n"
+      "  'contents': [ {\n"
+      "                  'type': 'file',\n"
+      "                  'name': 'file1',\n"
+      "                  'external-contents': 'c:\\root\\foo\\bar\\a'\n"
+      "                }\n"
+      "              ]\n"
+      "}\n"
+      "]\n"
+      "}",
+      Lower);
+  ASSERT_TRUE(FS.get() != nullptr);
+
+  IntrusiveRefCntPtr<vfs::OverlayFileSystem> O(
+      new vfs::OverlayFileSystem(Lower));
+  O->pushOverlay(FS);
+
+  ErrorOr<vfs::Status> S = O->status("c:\\root\\foo\\..\\file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:\\root\\foo\\bar\\a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+
+  S = O->status("c:/root/foo/../file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:\\root\\foo\\bar\\a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+
+  S = O->status("c:/root/foo\\..\\file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:\\root\\foo\\bar\\a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+
+  S = O->status("c:\\root\\foo/../file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:\\root\\foo\\bar\\a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+}
+
+TEST_F(VFSFromYAMLTest, ForwardslashWindowsPath) {
+  IntrusiveRefCntPtr<DummyFileSystem> Lower(new DummyFileSystem());
+  Lower->addRegularFile("c:/root/foo/bar/a");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(
+      "{ 'roots': [\n"
+      "{\n"
+      "  'type': 'directory',\n"
+      "  'name': 'c:/root/',\n"
+      "  'contents': [ {\n"
+      "                  'type': 'file',\n"
+      "                  'name': 'file1',\n"
+      "                  'external-contents': 'c:/root/foo/bar/a'\n"
+      "                }\n"
+      "              ]\n"
+      "}\n"
+      "]\n"
+      "}",
+      Lower);
+  ASSERT_TRUE(FS.get() != nullptr);
+
+  IntrusiveRefCntPtr<vfs::OverlayFileSystem> O(
+      new vfs::OverlayFileSystem(Lower));
+  O->pushOverlay(FS);
+
+  ErrorOr<vfs::Status> S = O->status("c:\\root\\foo\\..\\file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:/root/foo/bar/a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+
+  S = O->status("c:/root/foo/../file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:/root/foo/bar/a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+
+  S = O->status("c:/root/foo\\..\\file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:/root/foo/bar/a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+
+  S = O->status("c:\\root\\foo/../file1");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("c:/root/foo/bar/a", S->getName());
+  EXPECT_TRUE(S->IsVFSMapped);
+}