Should constraints on stat types be more stringent?

[greenstack]

Is there a use case for allowing class objects to be used as a numeric stat, or should stats be restricted to struct value types? Alternatively, should we just add the notnull constraint? Worth considering is that notnull wouldn't break any existing code bases, so that wouldn't be a breaking change; restricting stats to only value types (structs) would be breaking from an API perspective.

The way I see it is that most of the time, the developer is going to want to wrap the stat around a value type anyway (such as int or float). Supporting classes would be more flexible, though. At the very least, I'd want to have notnull added.

Are there any other constraints that might be useful for stat types in general?