Comment by pron

Regardless of questions of what should or shouldn't be done, there is a serious issue with reifying generic types with type parameters that support subtyping, as the JVM does. It requires deciding on a variance policy (to determine the subtyping relationships among the generic types), and different languages choose different variance policies to fit with their overall design. Reifying generic types in this case would bake a single variance policy into the runtime, which would hinder the ability of different languages to share data structures. If the type parameters do not support subtyping, this is not a problem, and, indeed, when generic types will be instantiated with parameters of Java's upcoming value types (that cannot be subtyped), they will likely be reified and specialized.