draft proposal for Sum Types for D

[rikki cattermole]

Something else I want to talk a bit more on.

Sumtypes are not distinct in the type system. They are like alias, its 
just a collection of values/behavior that is given a name.

You would need to use features like templates, conditional compilation, 
UDA's ext. to make it distinct and even then they should be implicitly 
convertible as long as its a superset or equal in members.

This extends over to the AST implementation stuff, it means there would 
need to be a global table containing the actual instances and when you 
instantiate a templated sumtype the instance would effectively be just 
an alias into an entry in that table.

Like None, the non-distinctiveness of sumtypes is fundamental in the 
type theory and comes from set theory. So its important that we get this 
right, because it has a lot of flow on effects.