I don’t understand the type-speak stuff, but it does make sense that where isn’t working on types and values directly, so something like an isa isn’t possible. This question popped into my head because of the last two points in this comment; it was possible to jerry-rig a type-stability optimization by annotating a parametric method’s return type, but it seemed more natural to attempt to constrain the parameter based on the methods the parametric method would replace.