The concern I initially raised is just that retrofitting parametric polymorphism into a statically typed language with subtype polymorphism and a sizeable codeb

The concern I initially raised is just that retrofitting parametric polymorphism into a statically typed language with subtype polymorphism and a sizeable codebase worth not breaking... has a history of not going so well. Although zero values are inelegant, I don't see why they would interact so badly with parametric polymorphism. In Rust terms, it's as if every type implicitly implemented the Default trait. (EDIT: You're right, steveklabnik. It's closer to Default than to Zero. Thanks!) As for nil, I'd rather not comment, since I'm not familiar with the specifics of how it works in Go. (But a previous conversation with pcwalton made it perfectly clear that it doesn't work the same as in Java or .NET.)