diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1720,5 +1720,5 @@
 
   if (IsSystem)
     *IsSystem = BestPrefixLength ? BestSearchDir >= SystemDirIdx : false;
-  return File.drop_front(BestPrefixLength);
+  return path::convert_to_slash(File.drop_front(BestPrefixLength));
 }
diff --git a/clang/unittests/Lex/HeaderSearchTest.cpp b/clang/unittests/Lex/HeaderSearchTest.cpp
--- a/clang/unittests/Lex/HeaderSearchTest.cpp
+++ b/clang/unittests/Lex/HeaderSearchTest.cpp
@@ -91,5 +91,14 @@
             "z");
 }
 
+#ifdef _WIN32
+TEST_F(HeaderSearchTest, BackSlash) {
+  addSearchDir("C:\\x\\y\\");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("C:\\x\\y\\z\\t",
+                                                   /*WorkingDir=*/""),
+            "z/t");
+}
+#endif
+
 } // namespace
 } // namespace clang