Add shared storage to property wrappers

[hborla (Holly Borla)]

@Joe_Groff

However, it seems to me that, in order for any wrapper type to be able to have access to its shared properties while passed around, that would mean that the wrapper type with shared storage has to carry around an extra, possibly-refcounted, pointer to the shared instance, and that would in turn make it impossible for a wrapper with shared properties to be fully "zero-cost" in per-instance storage.

Implementation-wise, @John_McCall had a neat idea to break the property wrapper into two different types -- one with the instance storage and a "composite" type with the instance and shared storage -- and only piece together the composite type when needed, so most of the time you don't need to pay the cost of that pointer.

@Karl

Have generic value parameters been considered as an alternative?

This might be a good alternative solution for the Clamped case shown in the pitch, but not for other cases where the shared value isn't a compile-time constant. This was discussed a bit on the other thread that is linked in the pitch:

Future directions of Property Wrappers

The feature you mention would certainly address the @Clamped use case where we are working with constant values. However, it doesn't address the notion of having "per declared property" storage. I suspect there is significant overlap in the use cases of the two approaches, but that we can find cases where one or the other is better. This area could use more exploration for sure. Doug