diff --git a/src/option.h b/src/option.h
index ced4f3e..396c568 100644
--- a/src/option.h
+++ b/src/option.h
@@ -25,6 +25,7 @@
 				// the same.
 #define P_EXPAND	0x10	// environment expansion.  NOTE: P_EXPAND can
 				// never be used for local or hidden options!
+#define P_NO_CMD_EXPAND	0x20	// don't perform cmdline completions
 #define P_NODEFAULT	0x40	// don't set to default value
 #define P_DEF_ALLOCED	0x80	// default value is in allocated memory, must
 				//  use vim_free() when assigning new value
@@ -61,6 +62,10 @@
 #define P_MLE	     0x20000000L // under control of 'modelineexpr'
 #define P_FUNC	     0x40000000L // accept a function reference or a lambda
 #define P_COLON	     0x80000000L // values use colons to create sublists
+// Warning: Currently we have used all 32 bits for option flags. On some 32-bit
+//          systems, the flags are stored as a 32-bit integer, and adding more
+//          flags will overflow it. Adding another flag will need to change how
+//          it's stored first.
 
 // Returned by get_option_value().
 typedef enum {