patch 7.4.786
Problem:    It is not possible for a plugin to adjust to a changed setting.
Solution:   Add the OptionSet autocommand event. (Christian Brabandt)
diff --git a/src/eval.c b/src/eval.c
index f163829..6a844e4 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -365,6 +365,9 @@
     {VV_NAME("windowid",	 VAR_NUMBER), VV_RO},
     {VV_NAME("progpath",	 VAR_STRING), VV_RO},
     {VV_NAME("completed_item",	 VAR_DICT), VV_RO},
+    {VV_NAME("option_new",	 VAR_STRING), VV_RO},
+    {VV_NAME("option_old",	 VAR_STRING), VV_RO},
+    {VV_NAME("option_type",	 VAR_STRING), VV_RO},
 };
 
 /* shorthand */
@@ -24720,6 +24723,16 @@
     }
 }
 
+/* reset v:option_new, v:option_old and v:option_type */
+    void
+reset_v_option_vars()
+{
+    set_vim_var_string(VV_OPTION_NEW,  NULL, -1);
+    set_vim_var_string(VV_OPTION_OLD,  NULL, -1);
+    set_vim_var_string(VV_OPTION_TYPE, NULL, -1);
+}
+
+
 #endif /* FEAT_EVAL */