Why no abstract non-tagged types?

[Britt <britt.snodgrass@gmail.com>]

Recently I've been wishing I could declare some otherwise conventional enumeration types as "abstract" so they could only be used as a template for derived types.
Such abstract types couldn't be used directly for object declarations.  For example:

   type Valve_State_Base_Type is abstract (Unknown, Open, Closed);  -- in Ada 202X

   Valve_State : Valve_State_Base_Type;  -- illegal, type is abstract

   type Vent_Valve_State_Type is new Valve_State_Base_Type;  -- a legal derivation

   Vent_Valve_State : Vent_Valve_State_Type;  -- a legal object declaration

I think subtypes of such an abstract type should be implicitly abstract as well:

   subtype Valve_Cmd_Base_Type is Valve_State_Base_Type range Open .. Closed;

   Valve_Cmd : Valve_Cmd_Base_Type;  -- illegal,  base type is abstract

   subtype Vent_Valve_Cmd_Type is Vent_Valve_State_Type range Open .. Closed;

   Vent_Valve_Cmd : Vent_Valve_Cmd_Type;  -- legal

Perhaps this has been asked before but I couldn't find an earlier discussion. Is there any reason why the "abstract" concept couldn't be extended to enumeration and other non-tagged types in Ada 202X?

- Britt