diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
index b35f436e91b6..3b5ea9fa539b 100644
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -3206,6 +3206,8 @@ enum CXCallingConv {
   CXCallingConv_X86RegCall = 8,
   CXCallingConv_IntelOclBicc = 9,
   CXCallingConv_Win64 = 10,
+  /* Alias for compatibility with older versions of API. */
+  CXCallingConv_X86_64Win64 = CXCallingConv_Win64,
   CXCallingConv_X86_64SysV = 11,
   CXCallingConv_X86VectorCall = 12,
   CXCallingConv_Swift = 13,