How can covariance in pragmatical class methods be made statically type-safe?

Abstract

Binary class methods are often more suitable for modelling binary operations than binary instance methods. Allowing covariant parameter type adaptations in class methods can make them even more attractive. We discuss the implications of covariant class methods on type safety. We discuss two examples, one type-safe at run-time and the other not, and formulate type rules that make it possible to identify the difference at compile-time.