Cannot use mutating member on immutable value of type

Question

I have following struct:

public protocol SuperModel {
    // empty protocol
}
struct ModelOne: SuperModel {
    struct SubModelOne {
        var someVar: Dou         


        

Answer 1

First downcast the self.data to ModelOne then call setSub function

 if var data = self.data as? ModelOne {
   data.setSub(ModelOne.SubModelOne(someVar: 2, othervar: 1))
 }