duplicates:

• SR-5499 Incorrect dynamic dispatch behaviour for nested class overridden across modules when using whole module optimization.

Issue Description:

Suppose in an external module we have these defs:

public struct Woof {
    open class Qux {
        open var name: String? {
            return nil
        }
        public func printName() {
            print(name ?? "no name")
        }
        public init() {}
    }
}

And then in the main module we have this:

class Foo: Woof.Qux {
    override var name: String {
        return "zorg"
    }    
}

let test = Foo()
test.printName()

Then if run with a release build config, we get "no name".
We can make it print "zorg" using any of the following changes:

Turn off "whole module optimization" for the external module.
Make Qux a non-nested type by moving it out of Woof.
Put all the code in the same module.