diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -372,6 +372,8 @@
    * \see clang_createIndex()
    */
   unsigned DisplayDiagnostics : 1;
+  unsigned /*Reserved*/ : 14;
+
   /**
    * The path to a directory, in which to store temporary PCH files. If null or
    * empty, the default system temporary directory is used. These PCH files are
diff --git a/clang/tools/libclang/CIndex.cpp b/clang/tools/libclang/CIndex.cpp
--- a/clang/tools/libclang/CIndex.cpp
+++ b/clang/tools/libclang/CIndex.cpp
@@ -3724,6 +3724,13 @@
   // if (options->Size >= offsetof(CXIndexOptions, RecentlyAddedMember))
   //   do_something(options->RecentlyAddedMember);
 
+  // An exception: if a new option is small enough, it can be squeezed into the
+  // /*Reserved*/ bits in CXIndexOptions. Since the default value of each option
+  // is guaranteed to be 0 and the callers are advised to zero out the struct,
+  // programs built against older libclang versions would implicitly set the new
+  // options to default values, which should keep the behavior of previous
+  // libclang versions and thus be backward-compatible.
+
   // If options->Size > sizeof(CXIndexOptions), the user may have set an option
   // we can't handle, in which case we return nullptr to report failure.
   // Replace `!=` with `>` here to support older struct versions. `!=` has the