[Concept] abstract enums

[AdamSpeight2008]

abstract enums

abstract enum E : int
{
  None = 0;
}

abstract enum E' : E
{
  None = 0, // Error: Already defined
  One  = 1
}

sealed enum E'' : E
{
  None = 2, // Error: Already defined
  One  = 1, // Error: Already defined
  Two  = 2
}

Inherting from a sealed enum

sealed enum F : E'' // Error: (E'') is sealed. Hence can not be inherited
{
 ...
} 

Generic Constraints
The CLR does allow constrained generics based on enums, just that it isn't (yet) expressible in C# or VB.

Foo <T : {enum, E} >( T e )
{

}

Existing enum construct

enum Foo { ... }

would be equivalent to

sealed enum Foo { ... }

Casts
All casting between abstract enum and a inherited enum,or vis-versa, must be explicit.

[HaloFour]

Seems mostly like a dupe of #9120