I'm not sure what GP meant by real world issues, but they do have some specific constraints that they want their implementation of generics to achieve, one of t

I'm not sure what GP meant by real world issues, but they do have some specific constraints that they want their implementation of generics to achieve, one of the core team members tried to nail down the details over a period of years but there were issues with the previous proposals before the latest one. So I don't think the issue is parametric polymorphism or not, it's more about things like keeping fast compile times, avoiding boxing value types, etc.