draft proposal for Sum Types for D

[Timon Gehr]

On 11/29/22 21:50, H. S. Teoh wrote:
>> What's even more important than the memory layout is the value of the
>> tag. Something needs to be specified in regards to the value of the
>> tag. One needs to be able to draw some conclusions about what happens
>> with the value if members are added, removed or reordered. Ideally the
>> tag value of existing members should not change. These are important
>> things when using sumtypes at ABI boundaries.
> You can't dynamically change a sumtype (add/remove/reorder its
> constituents) after the fact, because that may change their size, which
> implies an ABI change.  I think it's pretty much a given that you have
> to recompile after any change to a sumtype's definition.  (Just like you
> have to recompile when you change a struct's definition -- since its
> size may no longer be compatible with its old definition.)

I guess the point is that in some contrast to structs, it may well be 
compatible in some cases, and it is important to know in which cases it 
remains compatible.