In Swift, how to cast to protocol with associated type?

Question

In the following code, I want to test if x is a SpecialController. If it is, I want to get the currentValue as a SpecialValue

Answer 1

[Edited to fix: : SpecialValue, not = SpecialValue]

This is not possible. SpecialValueController is an "incomplete type" conceptually so the compiler cannot know. SpecialValueType, although it is constrained by SpecialValue, it is not known until it is determined by any adopting class. So it is a really placeholder with inadequate information. as?-ness cannot be checked.

You could have a base class that adopts SpecialController with a concrete type for SpecialValueController, and have multiple child classes that inherit from the adopting class, if you're still seeking a degree of polymorphism.