> Well, your own link proves it, doesn't it?
> I ported that code directly to Kotlin
> They're not using Kennedy and Pierce's system, but it's closely related. See Tate (2013) [1], in particular, "In general, since declaration-site variance can easily be encoded into use-site variance..."
Well, Kotlin has both declaration-site and use-site variance. So surely that result applies.
> Can you share your translation of Eric's code? Maybe we can get it to work.
Ah I didn't mean that code, I meant the earlier C# one from SO. https://pl.kotl.in/4BrbZAmZG
Well, your own link proves it, doesn't it? Kotlin has nominal subtyping, contravariance, and generics, therefore its type system is undecidable.
> I think it needs to be a generic type in Kotlin for this to work, because otherwise it will dispatch to a single method.
That's the whole point - the type system has to decide which overload to dispatch it to at compile time, which it can only do by performing an arbitrarily complex computation. I ported that code directly to Kotlin and it doesn't compile but it "should", which, well, I don't know what you were expecting or what you'd accept as proof when we've already got an academic paper that proves the point.