diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index bc52100..28a6a69 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -1034,7 +1034,12 @@
   Args.AddLastArg(CmdArgs, options::OPT_femit_all_decls);
   Args.AddLastArg(CmdArgs, options::OPT_ffreestanding);
   Args.AddLastArg(CmdArgs, options::OPT_fheinous_gnu_extensions);
-  Args.AddLastArg(CmdArgs, options::OPT_flax_vector_conversions);
+
+  // -flax-vector-conversions is default.
+  if (!Args.hasFlag(options::OPT_flax_vector_conversions,
+                    options::OPT_fno_lax_vector_conversions))
+    CmdArgs.push_back("-fno-lax-vector-conversions");
+
   Args.AddLastArg(CmdArgs, options::OPT_fno_caret_diagnostics);
   Args.AddLastArg(CmdArgs, options::OPT_fno_show_column);
   Args.AddLastArg(CmdArgs, options::OPT_fobjc_gc_only);