Generics is not really the right cutting point: parametric polymorphism is perfectly decidable. However, few languages stop at just parametric polymorphism. For

Generics is not really the right cutting point: parametric polymorphism is perfectly decidable. However, few languages stop at just parametric polymorphism. For instance, the OCaml issue in the linked blog post happens at the module level, which mixes parametric polymorphism and subtyping. And typically, while SML has parametric polymorphism, it lacks the OCaml extension that makes the module type system undecidable.