Cannot use mutating member on immutable value of type

When you apply type casting to value types (such structs), if succeed, you receive immutable copy of requested value:

(self.data as! ModelOne) // this is copy of data

The only way (as known to me) how you can mutate values that need to be casted – reassign value (as @Sahil Beri pointed you need declare variable):

func someFunc() {
    if var data = data as? ModelOne {
        data.setSub(ModelOne.SubModelOne(someVar: 2, otherVar: 1))
        self.data = data // you can do this since ModelOne conforms to SuperModel
    }
}