diff --git a/clang/include/clang/Lex/HeaderSearch.h b/clang/include/clang/Lex/HeaderSearch.h
--- a/clang/include/clang/Lex/HeaderSearch.h
+++ b/clang/include/clang/Lex/HeaderSearch.h
@@ -706,16 +706,18 @@
   /// Retrieve a uniqued framework name.
   StringRef getUniqueFrameworkName(StringRef Framework);
 
-  /// Suggest a path by which the specified file could be found, for
-  /// use in diagnostics to suggest a #include.
+  /// Suggest a path by which the specified file could be found, for use in
+  /// diagnostics to suggest a #include. Returned path will only contain forward
+  /// slashes as separators.
   ///
   /// \param IsSystem If non-null, filled in to indicate whether the suggested
   ///        path is relative to a system header directory.
   std::string suggestPathToFileForDiagnostics(const FileEntry *File,
                                               bool *IsSystem = nullptr);
 
-  /// Suggest a path by which the specified file could be found, for
-  /// use in diagnostics to suggest a #include.
+  /// Suggest a path by which the specified file could be found, for use in
+  /// diagnostics to suggest a #include. Returned path will only contain forward
+  /// slashes as separators.
   ///
   /// \param WorkingDir If non-empty, this will be prepended to search directory
   /// paths that are relative.
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