> As a positive example of what I'm talking about with respect to an 80/20 type system, consider a type system that supported covariant generics + function type

> As a positive example of what I'm talking about with respect to an 80/20 type system, consider a type system that supported covariant generics + function types that were contravariant wrt their argument types and covariant wrt their return types. Java and C# are that with respect to array types. Arrays in both are covariant , which is unsound since arrays are mutable, and erroneous uses are checked at compile time. Dart is exactly the language you describe: All generic type parameters are treated covariantly and we use runtime checks to preserve soundness. It's... OK. It works out mostly OK in practice because the generic types that users happen to use variantly are conveniently used in a safely covariant way: Reading stuff out of Iterables and Maps. But there is a significant performance cost to the runtime checks needed to preserve soundness. And when you using a type in a covariant way incorrectly, it is deeply confusing to users. I think the right 80/20 approach is to do what C# 2.0 and just make all generics invariant.